OXPO Working papers

The (changing) idea of a university

The famous manifesto by John Henry Newman in 1852 not only renewed the terms of the debate on the social role of universities but also remained a long-standing reference, still in use today. Indeed, what is largely conceived as the fundamental dual role of a universitydeveloping knowledge for its own sake and diffusing it throughout societyhas persistently been at stake. In the United Kingdom, since the 1950s, and concomitantly with the massification of higher education, this debate has taken an upswing with the charge against the "elitist" mission of British universities of educating people in accordance with liberal ideals and not endorsing the mission of training professionals for the labour market. Since about the 1990s, most universities have incorporated this new prerogative. Throughout the 20 th century, increasing emphasis on research has translated into a substantial growth of research activity in British universities.

The focus of the debate has further shifted as universities have progressively adopted the characteristics of other formal organisations [START_REF] Musselin | Are universities specific organisations ?[END_REF][START_REF] Krücken | Turning the University into an Organizational Actor[END_REF], intensified their ties with the economic sector in accordance with the so-called "third mission"

and developed new businesses in the campus. Hence, from the quasi-monastic scholarship and apprenticeship of medieval universities like Oxbridge that prevailed since the 13 th century; to Newman"s ideals; to the adoption of the "job placement" prerogative; to the model of the "multiversity" [START_REF] Kerr | The Uses of the University[END_REF] pursuing an increasing variety of missions, the idea of a university has considerably evolved.

The teaching-research nexus disentangled?

Right at the centre of these debates lays the intricate issue of the teaching-research nexus. The differentiation of research and teaching takes a paramount importance in the debates and the relationship between these two activities is subject to fierce arguments. On the one hand, following Humboldt's vision, advocates of the teaching-research nexus stress out the symbiotic relationship of these activities. Opponents to that vision, epitomised by Newman but also Condorcet, argue that teaching and research are, by essence, too different to be performed by the same individuals or by the same institutions [START_REF] Gingras | Idées d'universités : Enseignement, recherche et innovation[END_REF].

These confronting views take a new importance in England as the academic professions are undergoing a trend of job specialisation. A recent study reports that the division of labour between these two activities has increased: there are 25% of "teaching only academics" and 22% of "research only academics" (Locke & Bennion, 2009, p. 5).

This tendency is subject to contrasting opinions. On the one hand, some scholars have pointed to a threat of "deprofessionalisation" or even "proletarianisation"of the academic profession [START_REF] Fulton | Profession or Proletariat: Academic Staff in the United Kingdom After Two Decades of Change[END_REF][START_REF] Halsey | Decline of donnish dominion: the British academic professions in the twentieth century[END_REF]. On the other, the development of academic career management by universities is seen as a gain in professionalism in academic activities: increase of "teaching excellence", "good research" and "academic leadership". This view is particularly supported by publications intended for an audience of higher education practitioners 1 . In-between these two rather polar stances there is a wide array of intermediate interpretations, that combine in different ways declinist and optimistic elements attributable to these two stances.

Given such antagonism in the way the literature covers the issue, the "fate" of the teachingresearch nexus in an era of occupational specialisation is a crucial empirical question. It is also unclear how its possible disintegration relates to the internal dynamics of the academic professions.

How to study occupational specialisation between teaching and research?

Since the pioneering works of [START_REF] Halsey | The British Academics[END_REF] and [START_REF] Williams | The academic labour market: Economic and social aspects of a profession[END_REF], this phenomenon has mostly been studied through synchronic labour market analyses based on 1 See for example the reviews "Perspectives: Policy and Practice in Higher Education" published by the Association of University Administrators or the "Journal of Higher Education Policy and Management". large-scale surveys on academic careers 2 or on secondary analysis of official statistics 3 . The limit of these methods lays in their approach in terms of stocks, that undermine their capacity to reconstitute careers as longitudinal processes.

The drawbacks of any "stock approach" are threefold. Firstly, it doesn"t account accurately for the point(s) in the career in which the state of the teaching-research balance changes.

Taking the example of a teaching-only staff, assuming that she holds a PhD and therefore has practised research, it is impossible to know when she abandoned the practice. Therefore, using the stock approach, one can only know whether a change has happened between the PhD award and the situation in the date of the survey filling. Whether this change results from one or various state transitions and when such transitions happened remain unsolved questions. Thus, by overlooking the temporal dimension of careers, the risk incurred by a "stock approach" is to reify general categories such as "teaching career" or "research career", whereas it is highly questionable that people follow exclusive paths all the way through their career. This remark leads to the second drawback: the possibility to interpret phenomena of segmentation 4 . Increased division of labour between teaching and research doesn't mechanically introduce segmentation in the labour market. Such interpretation dangerously amalgamates task differentiation and patterns of mobility.

Thirdly, approaches focusing on stocks don"t provide any information on the mechanisms that produce increased division of labour. There is limited knowledge on how and why certain individuals end up in teaching posts, others in research posts and others on more traditional posts, performing both. These drawbacks can be seen as a consequence of the fact that labour markets are still too often studied independently from careers, and vice versa 5 . The approach proposed here is intended to bridge this gap by analysing labour market and careers together. It consists in 2 See for example Brennan, Locke & Naidoo (2007). 3 See for example Sastry (2005). 4 Segmentation is a polysemic term. It is used in sociology of labour markets referring to the boundary between primary and secondary [START_REF] Piore | Notes for a theory of labor market stratification[END_REF], internal and external [START_REF] Doeringer | Internal Labor Markets and Manpower Analysis[END_REF]) and sometimes occupational job markets, as well as gendered, class-based or race-based labour markets [START_REF] Reich | A Theory of Labor Market Segmentation[END_REF]. In the sociology of occupations, segments refer to ensembles of individuals in competition to each other, represented by different institutions statuses and carrying organised identities (Bucher & Strauss, 1992, pp. 68-69).

5

Just as studies on academic labour markets haven't contributed much to scholarly knowledge of academic careers, the rare studies that analysed academic careers as longitudinal processes have not informed much about the functioning of academic labour markets. Illustration of mutual contribution can nevertheless be found when leaving the empirical terrain of academia, for example in transitional labour market theories [START_REF] Gautié | Les "marchés transitionnels du travail": à quel paradigme appartiennent-ils?[END_REF], or in lifecourse perspectives of labour markets [START_REF] Blossfeld | Labor Market Segmentation in the Federal Republic of Germany: An Empirical Study of Segmentation Theories from a Life Course Perspective[END_REF].

unveiling the patterns of the labour market by looking through the lens of careers. In other words, this "flow" approach consists in throwing new light on the functioning of the labour market by apprehending it as a space of professional circulation.

The method: longitudinal qualitative research

It is suggested here to seize labour market mechanisms through the analysis of individual retrospective accounts of their own career 6 . It is therefore a method drawing on longitudinal qualitative research, inspired by interactionist approaches to careers and life histories [START_REF] Becker | Outsiders[END_REF]Stebbins, 1971). The style of the interviews for this research has been in some points similar to what French sociologists call "entretien biographique" [START_REF] Demazière | Analyser les entretiens biographiques. L'exemple des récits d'insertion[END_REF] and to the approach of the sociology of careers pleaded by Hermanowicz (2007) 7 .

The overarching research question for which these interviews were designed was: "What shapes the professional profile of academics in the course of their career and how?" I asked them as initial directive to relate their career and to cover as much as possible professional and non-professional events. I didn't impose questions and only interfered when the interviewee was taking the conversation towards an uninteresting route. The duration of the interviews ranged from 45 minutes to 2 hours and 20 minutes and most of them lasted well over an hour.

The data was gathered during interviews with 20 academics of different grades, among which there were 8 political scientists, 2 philosophers, 5 sociologists, 4 historians and 1 psychologist. The graph below shows the distribution per grade of the interviewees: 6 All the results discussed below are part of a wider research geared to shed light on how academic careers have evolved in their organisational and professional contexts. This research focuses more particularly on the local dynamics of the academic labour market, on change in the local professional order, and on conflicts between the profession and its hosting organisation (or between academia and management). The empirical material was gathered during fieldwork performed in social sciences and humanities departments in two British universities: one ex-polytechnic and one research-led university founded in the 1960s. In University A, senior academics are slightly over-represented, whereas academics on temporary contracts are less represented in both institutions as they have been less responsive to my request and also harder to localise.

When ranged by date of Bachelor (or equivalent diploma) award and classified in three periods, interviewees are evenly distributed across the two universities:

The analysis of these qualitative interviews is intended to unveil both subjective and objective aspects of people's career that, in turn, inform how the labour market works 8 . It is therefore an attempt to study academic careers otherwise than using an approach based on "predictors" as 8

Interviewing academics on their own lives can be seen as an intricate task, for the boundaries between the "indigenous" and the "scientist" roles can substantially blur during and beyond the interaction. For a reflection on the methodological problems of such a research configuration, see Paye (2011, forthcoming). is usually the case 9 . Therefore, the guiding principle is to examine careers and labour markets not in terms of determination but in terms of differentiation.

Research question

Most interviewees insisted on the fact that their career was "atypical". Particularly striking was the fact that no one identified its own career as "typical". This statement involves a comparison: one considers its own trajectory as unusual in relation to a more or less tacit norm. But if the majority claims to have an atypical career, does it mean that there is no norm at all? Or is the norm just an unachievable ideal that people hold in their mind as a theoretical reference and to which they compare their own trajectory? Alternatively, could there be various norms, so that the persons who could possibly fall into someone's norm don't qualify their career as normal because they have in mind another reference? These questions are intricate ones and will not be addressed here in detail. But they shed light on how people position themselves within a given social group. Does this emphasis on singularity reflect a high level of heterogeneity in the "career space" of their professional group?

The analysis of the career paths shows that there is indeed a great variety of professional trajectories (as well as social, political, domestic, conjugal, health and psychological trajectories). Of the 20 persons interviewed, there is no case of significant coincidence between two career paths. One important element of differentiation is the orientation of the professional career in terms of core activities of academic work, namely research, teaching, and, to keep it simple here, the so-called "third-leg" stream of tasks, which includes administration duties, management, meetings, service to the profession, etc.

Leaving out right away the idea of building a typology of careers from a small sample of interviews, one can nevertheless identify roughly four main orientations that the mix of work of a given individual may take permanently or temporarily: a research orientation, a teaching orientation, a managerial orientation, or no orientation at all, which means keeping in balance the core academic tasks, which is sometimes referred to as the idea of a "generic" academic career. Using this rough categorisation for the modest purpose of describing the current professional profile of the 20 interviewees shows that they are dispersed as follows: 3 9 Especially in American sociology: see for example Scott [START_REF] Long | Productivity and Academic Position in the Scientific Career[END_REF], [START_REF] Kulis | Organizations, Labor Markets, and Gender Integration in Academic Sociology[END_REF], and more recently [START_REF] Xie | Sex Differences in Research Productivity: New Evidence about an Old Puzzle[END_REF] and [START_REF] Perna | Sex and Race Differences in Faculty Tenure and Promotion[END_REF], but also in British sociology or higher education studies, for example [START_REF] Over | Early Career Patterns of Men and Women in British Universities[END_REF].

"research-oriented", 6 "teaching-oriented", 3 "management-oriented" and 8 "generic". This noticeable variety necessarily underlies a process of differentiation in people's career.

To understand why careers are not isomorphic, I propose to explore the following research question: How can people deviate from both the canonical model of the "generic" profile and the increasingly promoted "research-oriented" profile? This question can be re-framed more generally using the form of the well-known research question in interactionist sociology: How does one become teacher, researcher, manager?

The answer to this question would be fallacious if consideration would only be given to differentiation. In the flow of individuals through the academic labour market, there are particular moments in which profiles have to stick to certain requirements. The most significant one seems to be the pivotal "passage" from a temporary situation in academia to a permanent position 10 . Let's see first what happens before it occurs.

Convergence towards a professional norm: the teaching-research balance

In British academia, this key event in the career happens generallythough not exclusivelyduring the process of securing a position of permanent lecturer. Even though all lecturer positions do not strictly refer to the same competences, and the same mix of activities, they do most of the time carry the same set of basic requirements. These latter act as coercive criteria that reduce the possibility space of careers, in the same fashion as a bottleneck. Put in practical terms, this means holding a PhD, having a certain number of scientific publications, demonstrating teaching experience, etc., a process of accumulation of proofs that can be depicted as an academisation of the professional profile. Most, if not all, academics have at one point in their life had to conform to a strong professional norm, despite the fact that they were coming from various horizons, carrying different life histories and professional experiences.

The great diversity of starting points in our interviewees sample shows that even though academisation leads to a common outcome, it takes different routes towards it. For example, 6 out of the 20 interviewees have had another professional career before becoming academics.

Of the 14 others, not all of them have followed a study programme with a strong social 10 In terms of internal labour markets, this movement takes place in the "entry port" [START_REF] Doeringer | Internal Labor Markets and Manpower Analysis[END_REF], the boundary between secondary and primary labour markets. In interactionist sociology, it can be depicted as a "status passage" [START_REF] Glaser | Status passage[END_REF].

science component and a clear orientation towards research and scholarship. But however distant are the social and cognitive origins of the trajectories, all of them have gone through a kind of "funnel" as they have tailored their profile to a prevalent norm. In the discourse of the actors, this "funnelling" is expressed in terms of "ticking the boxes" or "jump through the hoop". As the interviewees have got closer to the possibility to enter into the primary labour market, their endeavours to conform to the norm have increased. This clearly appears in the account of a politics teacher in an ex-Polytechnic. During his undergraduate studies, he is not very engaged in getting good academic results: "I haven"t been attending any lectures to be honest, I was always doing demonstrations, or going down to London, attending meetings, speaking at meetings… I was very surprised to pass my first year at university considering that I was partly present. But in those days you could do it."

Later on, when having to choose between two offers for his PhD studies, he considers the opportunity to teach as a positive criterion:

"So I went for the money and not the status… Manchester University has much better status; I could have gone there. I chose Nottingham, because there was more money on the table and there was guaranteed teaching experience."

And only a couple of years after, he volunteered to offer unpaid teaching work in order to complement his academic profile:

"I happened to go and knock on people"s door and ask for teaching. And they didn"t pay me for it; it was just to make my CV look better." Such a process of profile adaptation in the early professional career illuminates the convergence of the trajectories in the social space constituted by academic jobs. But without subsequent divergence, there wouldn't be such a variety of professional profiles as observed in our sample, in the large-scale surveys or in public statistical data on academic staff cited above. The next part of the paper is geared to illuminate some of the main mechanisms that produce this divergence, or, in other words, career differentiation.

Career differentiation

The observation that individuals are dispersed unequally within a single labour market is a common fact in the sociological literature and in heterodox economics. This dispersion, however, could be interpreted in many ways, according to whether the "entrance" into the profession involves coercive criteria or not. Manifestly, the entrance into labour markets like poetry or catering is wider than the "entry port" (to use Doeringer & Piore's expression) of those constituted by the medical or law professions. Therefore, the dispersion in the professional space can be interpreted in the first case as a natural continuation of initial individual differences, or instead as the outcome of a divergence of career paths through the appearance of constructed individual differences, which is more likely to occur in law and medicine.

This part is geared to suggest that observed dispersion in the professional space in British academia is more likely to be an outcome of a divergence of career paths rather than a natural consequence of initial differences. As said above, the great majority of professional academics have gone through a narrow "funnel" that has had an effect of homogenisation of their profile. In the subsequent course of the career, individual profiles increasingly diverge from the "generic" model, thereby exhibiting constructed differences. These features are consistent with a process of career differentiation which needs to be explained.

-Models of career routes 11

A first way to explain career differentiation is to consider the structuring effect of existing models of career routes. These can be apprehended both as cognitive references of professional trajectories that impinge on people's representations and as embodied rules of mobility, often formulated at the university level, which organise career advancement. Both cognitive and regulative facets of career routes play a substantial role in the differentiation of careers 12 .

The rules of career advancement within a given university are often erected on the idea that there can be a bifurcation between teaching and research. In England, the standard academic career ladder (i.e. the most common) is structured in three main levels of seniority: the grades of lecturer, senior lecturer (which roughly corresponds to principal lecturer in ex-Polytechnics) and professor. Regarding the grade of reader, most interviewees have offered a view consistent with the idea that "the status of "reader" is the equivalent of senior lecturer in terms of remuneration but has more of a research focus and hence higher status" (Brennan, Locke & Naidoo, 2007, p.164). Several universities have implemented local reforms in the management of academic careers, recasting more or less radically their career ladder on a 11 To simplify the argument, we will only concentrate on what is called the "teaching route" and the "research route", thereby leaving out other models of career route, including what is generally referred as to the "management route". For further information on this latter, see for example [START_REF] Warner | An academic and/or an administrative career?[END_REF] or Henkel (2000, pp. 235-249).

12 Introducing this distinction between embodied and cognitive forms is intended to clarify the mechanisms at stake, but it is relevant to point out that interviewees tend to mix those forms in their accounts. two-track model 13 . This has been recently the case in university B. Even though most academics are not aware of the recent reform led by the human resource department, there is a tacit agreement among them that career advancement can follow two different logics:

"There is a teaching promotion route. But not many people take that. (…) For most of us, you"re promoted on your research, not on your teaching." (reader in history)

In university A, no recent reform has been implemented but the issue is under debate:

"I"ve written internal memos to my senior management colleagues exactly on this issue. Because, the question is: is it that one has to make a choice as an individual between going for the reader route or going for the principal lecturer route? And what skills and competencies and priorities does that mean? Or does it potentially rule out; are they mutually exclusive? And I certainly failed, that could be. But I was assured by the head of school here that they aren"t mutually exclusive." (senior lecturer in politics)

However, the two career routes are already nested in the current career system: the two intermediary grades between lecturer and professor, ("reader" and "principal lecturer") are connoted in terms of teaching/research orientation. Individuals' career strategies are therefore formulated in those terms:

"the idea is that if one is going down the full scale research route, one will apply for a readership.

And because I have never even published a book I would not really be able to apply for a readership. What I"m saying is I have started so late that I have not built up a sufficient record of publications. So my view was that: "If I do want to advance, it would be more down this route [meaning: the principal lecturer route]"." (principal lecturer in history, university A)

Whether they are implicit or explicit, local rules of mobility are crucial elements of a more or less subtle channelling of careers. Career routes are at least equally influential when they operate as cognitive references. Asking academics to relate crucial choices related to career orientation allows examining their representations of the possible alternatives, which frequently involve an exclusive choice between a research and a teaching stream, like in this example:

"So I had two options. Either to go into a kind of research-led career, you know, a contractresearch, work as a research associate or eventually get my own grants and so on; or go for a lecturership. (…) I don"t do well with that kind of insecurity. You know, of having to apply, year after year, after year for money. I don"t have the personality for it. I don"t like risk. So I wanted a lecturership." (assistant professor in sociology, university B)

Here, the research route is perceived as insecure and risky as compared to a more stable teaching career. This view is not shared by everyone. Some academics on research positions claim to prefer their condition as they are freed from the "burden" of teaching and can therefore devote more time on research and publications. For a vast majority of interviewees, abandoning research activities is perceived as a failure. Reciprocally, the idea of a "successful career" most of the time involves a reference to research activities and publications. Whatever the normativity attached to them, models of career routes widely circulate in academic spheres and provide cognitive frames for the formulation of individuals" career strategies 14 .

Their influence cannot be neglected: they indeed reveal to be as important as the formal rules of job mobility.

-Loose coupling between appraisal, time management and career orientation

The fact that people tend to think about career decisions using mutually-exclusive categories is not sufficient to explain how their trajectory effectively differentiates itself from others.

The main mechanism of career differentiation is rather to be found in a range of practical actions regulating promotions and work allocation at both department and university levels 15 .

It involves two main organisational regulative actions: appraisal and time allocation.

Appraisal

In the two universities studied, there was a formal appraisal process, which was not directly coupled with the promotion process. As put by a former head of department of politics:

"Appraisal had never really been important to University B. It"s always been a relatively formalistic process. (…) [People here] could not see any demonstrable benefit that came from it, and I think they were probably right. It wasn"t leading to any change in anything; it was just a kind of box-ticking exercise."

Most interviewees agreed on such a view and appraisal was almost unanimously perceived as of little relevance on career progression. Another evaluation process, not labelled as appraisal and not performed at the individual level, has nevertheless a pivotal role on individual careers: the Research Assessment Exercise (RAE) 16 . Originally, the RAE was intended to set 14 It is no surprise that the most emblematic discourses on career strategy, the guides for academic careers, also tend to frame career choices in those terms. See for example the two most famous British guides for academic careers, both emanating from university B [START_REF] Grant | Managing your academic career[END_REF][START_REF] Blaxter | The academic career handbook[END_REF].

15

Even though they can be considered as the core mechanism, they would hardly operate without the cognitive references and the rules of mobility above discussed.

16

The RAE is led by the Higher Education Funding Councils of the four constituent countries of the UK. It is a cycle of evaluation of the research activity of university departments. The outcome of the RAE is taken as the main criterion for public funding for research.

the basis for research funding allocation, based on the assessment of research at the department level. But it also turned out to operate implicitly as an individual evaluation.

When preparing the RAE submission, heads of departments, sometimes sharing responsibility with a RAE committee, make a selection of the academic staff from which the publication record is to be submitted. This selection has the effect of labelling de facto academics whose research is valued and those whose research is less valued 17 . In both cases, the RAE acts as a signal. For the former, it signals that they stay on a competitive track and can potentially be used for career advancement, as is the case with this senior lecturer in politics at University B:

"I"m kind of a banker for the RAE.

-What do you mean by "banker"? -(…) You"re seen as an asset in RAE terms. And if you"re an asset in RAE terms, it"s easier to get employed and you get well thought of in your department."

For the latter, a non-submission often suggests an idea of failure, and can translate into a sanction, as it happened with this lecturer in politics at University B:

"When it came to filling out the RAE requirements, I was obviously in difficulty. I was not able to provide the number of items that was expected. So that was a formal indication that I was not meeting the basic requirements. (…) And in later rounds, the department decided that I wouldn"t be submitted. …/...

recently [in 2006], because my research had come to a holdalmost to a holdthe department made me an offer of becoming purely teaching as opposed to research. So they have a special status whereby, with no loss of salary, you could accept an expanded teaching responsibility

without being expected to perform in terms of research (you weren"t going to be submitted on the RAE, etc.) (…) for quite a long time, I sort of went on with a normal status trying to revive my research activity but in practice not being very successful in doing so. So that eventually led to this offer."

The examination of the whole set of interviews indicates that the RAE has more influence than the formal yearly appraisal in terms of career orientation. Yet, "offers" of career reorientation such as the example cited above are relatively rare. To understand how people's careers diverge, it is needed to enquire on how division of teaching and research labour is held within departments. 17 Hence the emergence and subsequent wide use of the expressions "research-active" and "researchpassive" among academic staff and university administrators.

Work time allocation

The distribution of teaching load and administrative duties is taken very seriously by most interviewees. Indeed, the way academic work is divided within academic departments directly relates to the time allocated to each individual for core academic tasks. Again, decisions on this issue are taken by the head of department, who is sometimes seconded by purposely appointed academics 18 . What is mostly at stakeand this resonates with the idea of a "successful career" exposed above -is what is called "research time". As put by a principal lecturer in history:

"We all want to research [laughs]. So there is no problem with that. The problem is getting the time to do it."

Whether it relates to a willingness to stick to a "successful career", to the greatest autonomy characterising research work or to individual preferences, time given for research is more valued. Therefore, teaching load, administrative duties and research leaves concentrate an important amount of individual concern. It can pave the way to genuine struggle around the issue of time, for example when power relations prevail over perceived merit. As a response to these problems, a number of universities in the UK have implemented systems of workload models 19 . This is the case of the two universities studied in this research 20 . In both contexts, their implementation was justified by a principle of transparency: by affecting to each member of staff an individual workload model, it was hoped that unfairness and inequalities would reduce. Yet, the interviews indicate that power relations have not totally disappeared:

"where the problems occur in my experience, is that some people are better at lobbying for themselves.

…/… Some people are very good at protecting their own time." (associate professor in sociology, University B)

"Protecting one's own time" implies providing convincing arguments to the time-keepers.

Hence the diversity of strategies used by individuals to negotiate. At this point, the outcomes of the annual appraisal and the RAE submission are used by staff as arguments providing competitive advantage and by the time-keepers as implicit criteria of differentiation.

18

In one of the two universities studied, they are called "teaching and learning coordinators".

19

Workload models are time allocation schedules. They generally take the form of spreadsheets filled in by each academic staff indicating the amount of time to be devoted to a range of activities. For example, the workload model of a senior lecturer in politics in university A indicates: 70% teaching; 8% support activities; 16% scholarship and professional development; 2% post-graduate research supervision; 4% general management and administration. 20 To be perfectly accurate, one of these universities is currently implementing it and in the other, some departments have developed their own local system.

"Protecting one's own time" can be achieved through other strategies. An important one is the practice of "buy-out" 21 . To take a simple instance:

"There was a buy-out attached to the ESRC grant (…) and I wasn"t given the buy-out that I"d got through the grant, which made doing the research very difficult. (…) I was quite cross that I didn"t get the time, I was quite angry about that. (…) I"m actually on research leave now. And I had two terms of a crude research leave. And I managed to trade in the buy-out that I never got for another term of leave." (associate professor in sociology, University B) She was denied the possibility to outsource teaching hours but has been able to negotiate, as a counterpart, an extra term of leave. This example shows that time devoted to teaching is far from being strictly determined in advance and is subject to modifications at the discretion of management.

Thus, division of work and time allocation constitute a key principle of career differentiation.

Even though they are not clearly coupled with appraisal, they implicitly rely on it.

Career transitions

Allowance of research time is therefore contingent on negotiations, which in turn depend on the outcome of appraisal, on the RAE submission, or on other "arguments", such as the control of research grants as a transferable asset. Extra research time can also be obtained through the practice of buy-out. In all these cases, negotiation power mostly depends on research production 22 . If one's research production is judged as unsatisfactory, it is likely that at the end of the loop, one ends up with limited time to conduct research. On the contrary, if one's research is regarded as successful, one is more likely to be granted a high amount of research time. In other words, it can reasonably be expected that, all things being equal, produced research volume affects the production of research volume, according to a cumulative logic 23 .

The accounts of interviewees on their careerespecially those who have dropped research activityoffer a large number of examples. A rather extreme case is that of this young 21

The expression of "buy-out" is the one commonly used in British academia. It refers to a specific form of outsourcing, in which a permanent academic staff controlling transferable assets (for example a research grant) convinces its department to take on this money to recruit a temporary lecturer who delivers the teaching originally attributed to him. If the operation concludes, he ends up with substantial gains in research time.

22

Except perhaps for annual appraisal, where research is supposed to be assessed to the same extent than teaching and administrative responsibilities.

23

This resonates with Robert Merton's characterisation of the Matthew effect: "the rich get richer at a rate that makes the poor become relatively poorer" (1968, p.7). Another way to qualify this phenomenon is to talk in terms of vicious and virtuous circles, but it entails attaching normative content to career orientations.

teacher in politics at University B. He declares to be unable to pursue research, because of his high teaching load of 22 hours per week: Less extreme but equally relevant is the case of this principal lecturer in politics at University A, who is involved in faculty administration and labour union activism:

"Most of my energy, most of my time these days is in my director of studies role and my vice-chair of the UCU role. I do a small amount of teaching, and my research is dormant, I"m afraid."

This principle of cumulative advantage can also be embodied in local rules. Such is the case with a measure implemented a few years ago in University A to induce publication endeavours: the faculty of social sciences has a policy of giving 30 days of research time to each member of staff considered as "research-active".

Career differences are not only amplified by cumulative logics, but also by phenomena of irreversibility 24 . Small events in one point of the career can reveal to be of considerable significance in its subsequent development. For example, accepting a research-only job in the beginning of the academic career can entail unforeseen consequences, such as in the case of this research-only academic politics in University A: "I didn"t realise the implication at that time that I kind of placed myself into a research path (…) Now that I think about it I didn"t realise that I was saying to the world without knowing it that I had decided to be a researcher rather than track myself into a teaching career, which is more stable" She finds it difficult to be recruited on a lecturer position, because recruiters consider her as a researcher. The contrary can happen too:

"What I had found was that I signed up for teaching." (Emeritus fellow, former senior lecturer in philosophy, University B)

Just as for the lecturer in politics at University B who was "offered" a teaching-only position after his non-submission to the RAE, this philosopher would have had to spend an enormous amount of energy to reach back a level of research activity comparable to that of his fellows.

Any transition people experience in their occupational career involves a "cost". The variability of these costs can be significant, depending on the nature of the transition.

Considering different cases can help seize this variability. Career accounts of the interviewees 24 Irreversibility is the term used here, although the concept of "path-dependency" could have been chosen. The latter has the slight inconvenient, however, to suggest that careers follow a unique path, something that will precisely be questioned in the next section. The recent French book Bifurcations offers stimulating insights on the heuristics of these two concepts (2010).

seem consistent with the idea that a transition towards a higher teaching component is "cheaper" than a transition from a teaching-oriented profile to a more research-oriented one. It appears for example that the progressive placement of individuals on teaching positions often followed a logic of abandonment of research activity. No single case of return to research activity after a desertion has been observed in our sample, which seems consistent with the idea that leaving research is a fairly irreversible move.

The "forging" of one's academic profile thus involves different sources of irreversibility attached to specific events throughout the career. events can be decisions to accept a job offer, outcomes of teaching load negotiations, or even continuous or punctual micro interactions between colleagues bestowing specific meanings and diffusing them around 25 .

A striking feature common to almost all the career accounts is the paucity of multiple reorientations between the access to a permanent job and the interview date26 . Change between different professional orientations occurs seldom more than once over the course of employment. This suggests that as people get more involved in one core academic task, their possibility to reach back former positions decreases. In other words, when placement in one of the channels occurs, the subsequent course of employment is modified. This narrowing of the possibility space involves irreversibility, so that getting back to a former position involves significant endeavours or is seen as too "costly" to be envisaged 27 .

To conclude, appraisal practices have little direct influence on the differentiation of careers.

But their outcome performs a segregative function in negotiations during the subsequent stage of work time allocation. This allocation, in turn, has dramatic effects on the subsequent trajectory, amplifying career reorientations through cumulative and irreversible logics. It is worth noting that models of career routes and main mechanisms of career differentiation have been analysed separately for the sole sake of clarity. Empirically, it is their articulation that ensures the function of differentiation.

25

An example is the practice of informal labelling in conversations. One who is labelled as "researcher", whatever one's working contract says, will most likely be regarded as a researcher, hence as someone who will not be considered for a lecturing job, whether one wishes it or not. These features correspond to Grossetti's definition of "bifurcation" (2010), and to a lesser extend to that of "turning point" as introduced by Hughes and further formalised by [START_REF] Abbott | On the Concept of Turning Point[END_REF].

So far we have only analysed collective sources of disparity in the labour market. But differences in individual career paths do not only result from organisational and professional mechanisms.

-Multiple commitments and intertwined interests

Many research endeavours have striven to explore the entanglement of work and other spheres of life, such as health, family, hobbies, politics or housing. The concept of commitment, as theorised by [START_REF] Becker | Notes on the Concept of Commitment[END_REF] and subsequently by [START_REF] Stebbins | On misunderstanding the concept of commitment: A theoretical clarification[END_REF], offers an analytical heuristics to understand how investments in different spheres of life interact, and how people integrate them into a "consistent line of activity" (Becker, 1960, p.32). Being committed, for example to one's family, means acting consistently with past actions performed in the various "portfolios" of life. For example, commitment can account for the reasons why most people don't suddenly change their career orientation, leave their organisation or their occupation (ibid, p.33): Commitment can also explain why people are bound to decrease their involvement in research work following childbirth. This is a common feature in the careers of the interviewees.

Parenthood constitutes a new source of commitment that can affect commitment in academic activities. The arrival of the child can be related to what Becker calls a "side bet". Side bets occur when someone "has staked something of value to him, something originally unrelated to his present line of action, on being consistent in his present behavior." (ibid, p.35). For example, a principal lecturer in politics at University A now at the doorstep of retirement recalls how he progressively disengaged from research. He invokes the birth of his child, the refurbishing of his house, his time-consuming divorce and his progressive involvement in union activism. All these involvements have acted against sustaining a sufficient research activity so as to keep up on a "generic" academic route, and have entailed a drift towards a more teaching-oriented professional profile.

Another source of individual differences in career strategies can stem from one's sense of what a "successful career" is. Interview material shows that there is a wide variety of criteria.

In fact, almost each individual formulates a unique description. For example, for a professor in criminology at university A, success means becoming a professor and "buying out" as much teaching as possible to concentrate on research. For one of her colleagues, a principal lecturer in sociology, a successful career implies a healthy equilibrium between private and professional lives. For two other colleagues in political sciences, the overarching criterion is being politically-involved in teaching and scholarship. Other conceptions of career success can rely on teaching excellence, or good administrative service to the department. Career success can also be defined negatively: "I"m not working with promotion in mind. Because often most promotions also come with a large amount of administrative work that I don"t particularly enjoy. I would rather not be promoted than have a large component in my working life that I don"t like." (associate professor in sociology,

University B)

Hence, personal definitions of professional success also explain inter-individual differences, particularly because they underlie strategic career decisions. It would be misleading however to argue that the idea of career success comes first, followed by career strategy and finally a resulting set of commitments. Interviewees have good reasons to formulate their idea of career success in accordance with the number and the colour of the balls they are juggling with. It is therefore likely that career success be thought in relation to one's repertoire of commitments. Both have been depicted here as sources of individual differences. Not accounting for these would have overweighted the coercive effect of organisational mechanisms and the weight of professional norms. This part constitutes a first attempt to analytically disaggregate the main causes of divergence in career paths. The three main sources of career differentiation offer a first explanation of why, after being tailored to a rather homogeneous professional standard of recruitment, individual profiles increasingly diverge from the "generic" model of an academic career. This indicates that the dispersion of individuals in the academic labour market is mainly the outcome of a divergence of career paths.

Conclusion

The considerable stakes attached to the splitting of the teaching-research nexus raised the question of whether increased job specialisation was to be attributed to a genuine labour market balkanisation or to a process of career differentiation. The treatment of this question has been done with a life course perspective focusing on flows of individuals rather than on stocks of occupied job positions. The longitudinal qualitative method used allowed concluding that observed variety of professional profiles results from career differentiation.

We identified a tendency to convergence operating before the access to the first permanent position, and a subsequent diverging trend. This divergence is a puzzling phenomenon. There is no clear or systematic social allocation principle for teaching-oriented or research-oriented posts. Nevertheless, the career accounts of the interviewees have provided with sufficient empirical material to suggest a variety of sources of individual differences: models of career routes, acting as both cognitive models and embodied rules of mobility; mechanisms operating through appraisal, work time allocation and career reorientation; and phenomena of commitment at a more individual level.

In other words, it can be said: "one is not born a teacher (or a researcher), but becomes one".

The perhaps excessive existentialist tone of this statement has the great advantage of stressing the fact that teaching-only and research-only jobs are not there by essence, but are filled by people who have diverged from the norm imposed by the requirements of the entry jobs in the profession. This suggests potential areas for further research.

First, this paper lays a first set of arguments to make the broad prediction that job specialisation is more likely to occur after the passage to permanentship. Therefore, the heterogeneity of the labour market is to be put in relation to career stages.

Second, there is an urge to move from the narrator to the statistical unit. A quantitative approach is needed to test the depth of the phenomena identified from the biographical interviews. The 20 interviewees of this study are part of a larger sample from which a dataset containing the career histories of 140 individuals has been built. Quantification will be used to validate or contradict the phenomenon of career convergencedivergence and to accurately characterise, through the use of sequence analysis, the variety of trajectories followed by academics. The quantitative analysis also shall allow exploring more determinants of individual trajectories by gender, age, education, etc. Indeed, historical, gender-based, institutional and disciplinary variations have not been addressed here. For example, comparing different generations can help assess the depth of the idea of transformation of academic careers, particularly the question of whether there is a trend towards differentiation or normalisation of careers. Differences between the two universities and across disciplines will also be the object of greater attention in the next stages of research. However, independently from disciplinary, individual or organisational variations, the convergence and divergence of career paths is expected to be the most relevant account of diversity.

Finally, this double movement of convergencedivergence is not exclusive to the academic career and could potentially be observed in a range of occupational groups characterised by closed labour markets [START_REF] Paradeise | Les professions comme marchés du travail fermés[END_REF]. Those labour markets which are "protected" by a professional coalition (occupational labour market), an administrative unit (internal labour market), or by more peculiar configurations (such as the French "corps d'Etat") are particularly expected to exhibit the same feature, even if perhaps in a less "pure" form than in British academia. Studying the process of organisational and individual forging of professional profiles in a variety of labour markets would therefore allow clarifying whether observed division of labour is interpretable to segmentation or to career differentiation.

  Convergence and Divergence of Career Paths in the Labour Market: the Case of British Academia Simon Paye, Centre de Sociologie des Organisations, Sciences Po, et Centre Maurice Halbwachs

7

  Distribution per grade (n=20)

"

  So I work between 70 and 85 hours a week. (…) That of course closes out the space of research for obvious reasons."

"

  -Do you think about leaving the institution or the job? -Not really, because of family reason. My ex-wife and my younger son live locally. (…) I have to stay locally so that I could have regular weekly contact with my son." (Principal lecturer in politics, University A)

See for example[START_REF] Strike | Evolution of academic career structures in English universities[END_REF] on this issue.

Only one of them has effectively gone through transitions between teaching, research and administrative orientations, even though the sequence of the positions he occupied do not well reflect them.

Abstract:

The proliferation of "research-only" and "teaching-only" posts in British universities can be interpreted as a signal that academics evolve in a segmented labour market. This view, however, is debatable. This paper proposes to go beyond static approaches of the labour market and to adopt a "career perspective" that emphasises on flow instead of stocks. The analysis of work histories gathered during in-depth interviews suggests that the labour market is not strictly segmented in terms of teaching and research, but that division of academic labour results from a process of individual differentiation. First, a convergence of early career paths leads to the crucial stage of securing a first permanent position. Subsequently, professional profiles tend to diverge through a sequence of career transitions. What is therefore put forward is an explanation of heterogeneity of the academic professions in terms of career differentiation.


Thomas Méline

Murielle Muzard

Magali Deleu

Harivony Rakotoarivonina

Richard Plantier-Royon

Caroline Rémond
email: caroline.remond@univ-reims.fr

D-xylose and L-arabinose laurate esters : enzymatic synthesis, characterization and physico-chemical properties

Keywords: D-xylose-laurate esters, L-arabinose laurate esters, lipase, surfactants, biorefinery

Efficient enzymatic synthesis of D-xylose and L-arabinose lauryl mono-and diesters has been achieved by transesterification reactions catalyzed by immobilized Candida antarctica lipase B as biocatalyst, in organic medium in the presence of D-xylose or Larabinose and vinyl laurate at 50 °C. In case of L-arabinose, one monoester and one diester were obtained in a 57 % overall yield. A more complex mixture was produced for D-xylose as two monoesters and two diesters were synthesized in a 74.9 % global yield. The structures of all these pentose laurate esters was solved. Results demonstrated that the esterification first occurred regioselectively onto the primary hydroxyl groups. Pentose laurate esters exhibited interesting features such as low critical aggregation concentrations values all inferior to 25 µM. Our study demonstrates that the enzymatic production of L-arabinose and D-xylose-based esters represents an interesting approach for the production of green surfactants from lignocellulosic biomass-derived pentoses.

Introduction

For several years, there has been an increasing interest for bio-based surfactants derived from annually renewable resources [START_REF] Foley | Derivation and synthesis of renewable surfactants[END_REF]. Among bio-based surfactants, alkyl glycosides and sugar fatty esters are non-ionic surface active compounds which present numerous advantages such as no toxicity for humans and for the environment, biodegradability, absence of odor and color [START_REF] Baker | Sugar fatty acid ester surfactants: Structure and ultimate aerobic biodegradability[END_REF][START_REF] De | A review on natural surfactants[END_REF][START_REF] Rybinski | Alkyl Polyglycosides-Properties and Applications of a new Class of Surfactants[END_REF]. The main fields of application of these non-ionic surfactants are related to personal care, cosmetics and pharmaceutical applications as well as food emulsification in case of sugar fatty esters [START_REF] Rybinski | Alkyl Polyglycosides-Properties and Applications of a new Class of Surfactants[END_REF][START_REF] Santos | Biosurfactants: Multifunctional Biomolecules of the 21st Century[END_REF][START_REF] Hill | Sugar-based surfactants for consumer products and technical applications[END_REF]. The main alkyl glycosides and sugar fatty esters industrially produced or described in literature were generally obtained from hexoses, especially D-glucose, or hexose-based oligosaccharides such as sucrose, maltose and maltodextrins [START_REF] Foley | Derivation and synthesis of renewable surfactants[END_REF][START_REF] Deutschmann | From plant biomass to bio-based chemicals: Latest developments in xylan research[END_REF]. D-xylose and L-arabinose are both pentoses abundantly present in lignocellulosic plant cell walls and are main components of xylans [START_REF] Ebringerova | Xylan and xylan derivatives-biopolymers with valuable properties, 1. Naturally occurring xylans structures, isolation procedures and properties[END_REF]. The production of new added-value molecules from pentoses represents a challenge for the valorisation of lignocelluloses in the context of development of biorefineries [START_REF] Foley | Derivation and synthesis of renewable surfactants[END_REF][START_REF] Deutschmann | From plant biomass to bio-based chemicals: Latest developments in xylan research[END_REF].

Xylose is currently reduced into xylitol, converted into furfural or fermented into ethanol [START_REF] Deutschmann | From plant biomass to bio-based chemicals: Latest developments in xylan research[END_REF]. Although few pentose-based surfactants were previously developed, some recent studies described the chemical synthesis of alkyl pentosides and pentose-based fatty esters [START_REF] Bouxin | Direct conversion of xylan into alkyl pentosides[END_REF][START_REF] Deleu | Synthesis and physicochemical characterization of bolaamphiphiles derived from alkenyl d-xylosides[END_REF][START_REF] Deleu | d-xylose-based bolaamphiphiles: Synthesis and influence of the spacer nature on their interfacial and membrane properties[END_REF][START_REF] Klai | d-Xylose-based surfactants: Synthesis, characterization and molecular modeling studies[END_REF][START_REF] Martel | Development of Agriculture Left-Overs: Fine Organic Chemicals from Wheat Hemicellulose-Derived Pentoses[END_REF]. These syntheses often require high energy and various catalysts that might not be compatible with green chemistry processes.

Classical chemical routes to the formation of fatty acid esters generally require esterification or transesterification reactions and the use of polar solvents (DMF, DMSO), a basic catalyst and high reaction temperatures (80-120°C) leading to complex mixtures of monoesters, di-and higher esters as different regioisomers and unreacted sugar [START_REF] Polat | Syntheses and applications of sucrose-based esters[END_REF]. The use of fatty esters or acyl chlorides in the presence of an organic solvent and pyridine can improve the reaction yields but again with various degree of substitution [START_REF] Chortyk | Syntheses and Characterizations of Insecticidal Sucrose Esters[END_REF]. Selective protection of the hydroxyl groups of the carbohydrate (acetyl, benzyl, isopropylidene) can orient the position of esterification reaction and also sometimes to control the pyranose/furanose structure of the products [START_REF] Guo | Protecting groups in carbohydrate chemistry: influence on stereoselectivity of Glycosylations[END_REF].

The use of enzymes represents an interesting alternative for the preparation of surfactants from biomass [START_REF] Deutschmann | From plant biomass to bio-based chemicals: Latest developments in xylan research[END_REF][START_REF] Dumon | Progress and future prospects for pentose-specific biocatalysts in biorefining[END_REF]. For example, we previously described the enzymatic synthesis and the surfactant properties of alkyl xylosides and alkyl oligoxylosides from xylans and pretreated wheat bran using a transglycosylation approach with a xylosidase (EC 3.2.1.37) or xylanases (EC 3.2.1.8) [START_REF] Muzard | Evaluation of the transglycosylation activities of a GH 39 β-d-xylosidase for the synthesis of xylose-based glycosides[END_REF][START_REF] Ochs | Enzymatic synthesis of alkyl [small beta]-d-xylosides and oligoxylosides from xylans and from hydrothermally pretreated wheat bran[END_REF]. Among well-known biocatalysts, lipases (EC 3.1.1.3) were widely used to catalyze the ester bond formation of sugar fatty esters [START_REF] Chang | Biocatalysis for the production of carbohydrate esters[END_REF][START_REF] Kennedy | Enzymecatalyzed regioselective synthesis of sugar esters and related compounds[END_REF]. Enzymatic synthesis of sugar fatty esters is usually achieved by esterification reaction from a carbohydrate and a fatty acid or by a transesterification reaction from a carbohydrate and a fatty acid ester. Enzymatic synthesis of sugar fatty esters with lipases represents a green alternative compared to the conventional chemical approach [START_REF] Chang | Biocatalysis for the production of carbohydrate esters[END_REF]. Moreover, lipases can be used in immobilized form, then allowing a recyclability and reusability of the biocatalyst thus reinforcing the green alternative. Lipases display high regioselectivity compared to chemical acylation decreasing the complexity of mixtures of regioisomers produced [START_REF] Cauglia | The enzymatic synthesis of glucosylmyristate as a reaction model for general considerations on 'sugar esters' production[END_REF][START_REF] Degn | Lipase-catalysed synthesis of glucose fatty acid esters in tert-butanol[END_REF][START_REF] Pérez-Victoria | Complementary regioselective esterification of nonreducing oligosaccharides catalyzed by different hydrolases[END_REF]. During esterification, the amount of water present in reaction mixtures and formed during reaction must be highly controlled as water induces hydrolysis of esters products [START_REF] Klibanov | Improving enzymes by using them in organic solvents[END_REF]. Hence, most of the lipase-based syntheses were performed in organic media as water quantity can be controlled by the use of salts or molecular sieves as desiccating agents [START_REF] Chamouleau | Influence of water activity and water content on sugar esters lipase-catalyzed synthesis in organic media[END_REF][START_REF] Yadav | Immobilized lipase-catalysed esterification and transesterification reactions in non-aqueous media for the synthesis of tetrahydrofurfuryl butyrate: comparison and kinetic modeling[END_REF]. Transesterification catalyzed by lipases in presence of fatty acid esters, especially vinyl esters, represents an interesting strategy to overcome the water production during reaction and to induce better reactions yields. The main disadvantage in this latter case is the production of acetaldehyde as a by-product but the most widely employed lipases seem to be quite stable in the presence of acetaldehyde [START_REF] Weber | Sensitivity of microbial lipases to acetaldehyde formed by acyl-transfer reactions from vinyl esters[END_REF].

Although enzymatic synthesis of hexose-derived fatty esters was extensively described in the last twenty years, studies dealing with pentose-based fatty esters were less reported in literature [START_REF] Cauglia | The enzymatic synthesis of glucosylmyristate as a reaction model for general considerations on 'sugar esters' production[END_REF][START_REF] Gumel | Lipase mediated synthesis of sugar fatty acid esters[END_REF][START_REF] Fregapane | Enzymatic synthesis of monosaccharide fatty acid esters and their comparison with conventional products[END_REF][START_REF] Fregapane | Enzymic solvent-free synthesis of sugar acetal fatty acid esters[END_REF][START_REF] Tsitsimpikou | Microbial Lipases in the BiocatalysisComparative studies on the sugar esters synthesis catalysed by Candida antarctica and Candida rugosa lipases in hexane[END_REF][START_REF] Bidjou-Haiour | Lipase catalyzed synthesis of fatty acid xylose esters and their surfactant properties[END_REF][START_REF] Pappalardo | Synthesis and characterization of arabinose-palmitic acid esters by enzymatic esterification[END_REF]. Moreover, in most of the cases, structural data related to these molecules are not described.

In the present paper, we report the successful enzymatic synthesis of laurate pentose esters from D-xylose and L-arabinose catalyzed by the lipase B from Candida antarctica (Novozym 435). Our strategy was based on transesterification reactions with vinyl laurate and allowed producing different mono-and diesters from D-xylose and L-arabinose. The structural features of these sugar esters were analysed by NMR and mass spectrometry and their surface-active properties were evaluated.

2.

Experimental section Reactions were stopped by incubating samples at 100 °C for 10 min and reaction mixtures were centrifuged at 500 x g for 5 min in order to pellet molecular sieves and enzymes. Supernatant was used to monitor, sugar fatty esters production by thin layer chromatography and HPLC. Molecular sieves and enzymes were washed twice with ultrapure water in order to collect residual pentoses (HPLC quantification).

Kinetic studies were achieved at 50 °C with 20 mL reaction mix and sampling occurred at 1, 2, 4, 8, 24 and 48 hours of incubation, 1 mL of reaction mixture was taken each time. These reactions were performed in triplicates.

Higher volume syntheses (100 mL) occurred in similar conditions in order to produce sufficient quantities of products for purification and characterization.

Reactions were stopped after 4 hours of incubation at 50 °C.

Recycling of the lipase was assessed in presence of D-xylose or L-arabinose (50 mM), vinyllaurate (150 mM), molecular sieves (10 % w/v) and Novozym 435 (1 % w/v).

Reaction was conducted during 4 h at 50°C under magnetic stirring (400 x rpm). After 4h, reaction was centrifugated (45 x g) and pellets containing the lipase and the molecular sieves were further incubated with fresh D-xylose or L-arabinose and vinyllaurate. A total of 6 cycles of 4 h were performed.

Purification, characterization and quantification of the transesterification products

The production of pentose fatty esters was investigated by TLC, using precoated TLC-sheets ALUGRAM® Xtra SIL G/UV 254 (Macherey-Nagel Gmbh & Co., Düren, Germany) and BuOH : AcOH : water (2/1/1) as the mobile phase. Products were revealed using 0.2 % (w/v) orcinol in H 2 SO 4 (20 % v/v in water) and heating at 250 °C.

After removal of enzymes and molecular sieves, 2M2B was evaporated using a rotary evaporator (Büchi Labortechnik AG, Flawil, Switzerland) and crude products with remaining reactants were collected. Two hexane washings were then performed to eliminate the remaining vinyllaurate and a white paste was collected for both Dxylose and L-arabinose-based esters. Vinyllaurate removal was qualitatively assessed by HPLC. Finally, the residual pentose was precipitated in THF (100 mL) leading to soluble fractions containing mono-and diesters collected for further purification.

Residual pentose was finally solubilized in water (50 mL) and quantified by HPLC.

The purification of D-xylose or L-arabinose laurate esters was performed by silica gel chromatography (9385 Merck Kieselgel 60, 230-400 mesh, 40-63 µm). Diesters were eluted using PE / EA (7/3, v/v) and monoesters were eluted using pure EA. All the products were obtained as a white crystalline powder.

NMR spectra were recorded on Bruker spectrometers (500 or 600 MHz for 1

Determination of D-xylose and L-arabinose laurate esters surface-active properties

Adsorption to an air-aqueous medium interface was analysed at 25.0 ± 0.2 °C with an automated Langmuir Balance system equipped with a Wilhelmy plate (KSV minitrough, KSV instruments Ltd., Helsinki, Finland). Purified sugar esters, or mixtures of esters (extracted from crude reactions of 4 hours at 50 °C with no further purification of mono-and diesters) were solubilized in DMSO and injected (20 µL) into the subphase (Tris HCl 10 mM pH 7) to a range of final concentrations (C). Injections were done using a Hamilton syringe and two homemade devices allowing the injection of the product without disturbing the air-water interface. These devices were placed at two fixed positions on the trough to ensure a reproducible injection process. The subphase was stirred, during the whole experimentation, using two cylindrical micromagnetic rods (8 ± 1.5 mm 2 ) and two electronic stirrer heads located beneath the trough (model 300, Rank Brothers, Bottisham, U.K.). Stirring was performed at 100

x rpm with an auto-reverse movement. The increasing of the surface pressure was recorded right after the injection until its value reaches equilibrium (eq). CAC was determined from the plot eq = f (C) at the intersection between the linear regression of the ascendant and plateau parts

pH stability investigation

The pH stability of purified molecules was investigated by performing incubation at room temperature of 5 mM aqueous solutions of purified products at pH varying from 4 to 9. Citrate phosphate buffer 50 mM was used to prepare pH solutions from 4 to 8 and borate buffer 50 mM was used to prepare pH 9 solution. Stability was monitored, during 72 hours with regular sampling (1, 2, 4, 8, 24, 48 and 72 hours), qualitatively by TLC and quantitatively by HPLC.

Results

Transesterification reactions between L-arabinose or D-xylose and vinyllaurate

were performed with immobilized C. antarctica lipase B, commercially known as Novozym 435. Various conditions were tested (substrates and lipase loading, organic solvents) and led to the choice of reaction parameters (data not presented) to carry out the enzymatic catalysis: L-arabinose or D-xylose (50 mM) and vinyllaurate (150 mM) with molecular sieves (10 % w/v) in screwed glass bottles for various incubation times with 2M2B as solvent and magnetic stirring (400 x rpm).

Enzymatic synthesis of L-arabinose laurate esters

Products of transesterification reactions at 50 °C were first visualized by TLC, indicating that a mixture of monoester 1 and diester 2 of L-arabinose was obtained.

Those esters were purified from 100 mL reactions performed during 4 hours with conditions described above.

After purification by flash chromatography over silica gel and a careful NMR study, the structure of monoester 1 was attributed to 5-O-lauryl-L-arabinofuranose and the structure of diester 2 to 3,5-di-O-lauryl-L-arabinofuranose as mixtures of / anomers (Figure 1A). Global conversion of L-arabinose was evaluated by HPLC by measuring Larabinose remaining after 4 hours in reaction media. From a 100 mL reaction at 50 °C, 320 and 756 mg of L-arabinose lauryl monoester 1 and diester 2 were respectively recovered corresponding to a conversion of 49.0 % of introduced L-arabinose and a yield of 48.6 % (19.2 % of monoester 1 and 29.4 % of diester 2).

Kinetic of formation of L-arabinose lauryl esters obtained at 50°C was evaluated for 48 hours by HPLC with 20 mL reactions (Figure 1B). During acylation of L-arabinose, both monoester 1 and diester 2 were formed during the first hour of the reaction (Figure 2)

and were produced at 2.8 ± 0.1 mg.mL -1 and 3.2 ± 0.3 mg.mL -1 respectively. Maximal quantities of L-arabinose lauryl esters were detected after 4 hours of catalysis and were equivalent to 4.0 ± 0.1 and 9.1 ± 0.3 mg.mL -1 for 1 and 2 respectively, corresponding to a conversion of 59.2 ± 1.5 % of introduced L-arabinose and a 56.8 % yield (22.8 % of monoester 1 and 34.0 % of diester 2). After 4 hours of reaction, a slight decrease in concentration of diester 2 was observed maybe due to a slight hydrolysis.

Enzymatic synthesis of D-xylose laurate esters

For the transesterification reactions of D-xylose with vinyllaurate at 50 °C during 4h, two sets of spots were detected by TLC indicating the formation of a complex mixture of both monoesters and diesters. From 100 mL reactions, one major were obtained for recycling experiments in presence of L-arabinose.

pH stability of L-arabinose and D-xylose laurate esters

The pH stability of the purified sugar esters produced at 50 °C was investigated by incubating aqueous solutions with pH range from pH 4 to 9. L-arabinofuranose and D-xylofuranose laurate esters solutions were incubated at room temperature and samples were taken during 72 hours. The evaluation of the stability was studied by HPLC (data not shown). The L-arabinofuranose and D-xylofuranose laurate esters were stable in solution within a pH range from 4 to 9 as no liberation of pentose indicating a hydrolysis of the ester bonds was detectable.

Surface-active properties of xylose and arabinose laurate esters

Surface-active properties were investigated for pentose esters produced at 50 °C. The ability of pentose esters to adsorb to an air-aqueous medium interface was studied by measuring the surface pressure increase further to the injection of the esters in the aqueous subphase. Figure 3 shows as an example the adsorption kinetics of monoester 3a. Similar results were obtained for the other esters. For concentration below 2 µM, no surface pressure increase was observed, suggesting that at very low concentration in the subphase, the adsorption of monoester 3a to the air-water interface was too low to exert an effect on the surface pressure. At concentration ≥ at 3 µM, a surface pressure increase can be observed indicating that the monoester was able to adsorb at the air-aqueous medium interface. Before the detection of the surface pressure increase, there was a lag time, depending on the concentration (Figure 3). This suggests that the adsorption to the air-aqueous medium interface is increased according to monoester 3a concentration, indicating a higher probability of interactions between the ester molecules at the interface [START_REF] Nasir | Interactions of the natural antimicrobial mycosubtilin with phospholipid membrane models[END_REF]. The higher number of molecules at the interface also explains the higher equilibrium surface pressure reached at the plateau.

Critical aggregation concentrations (CAC) were determined for purified D-xylofuranose and L-arabinofuranose monoesters and for mixtures of D-xylofuranose or Larabinofuranose mono-and diesters (Table 1). For pure compounds, the surface pressure was measured for a range of concentrations going from 1 µM to 35 µM in the aqueous subphase (pH 7, room temperature) (Figure 4). As pure D-xylofuranose diesters 4 and 5 or L-arabinofuranose diester 2 showed an extreme hydrophobic behavior (no solubility in aqueous media) for concentrations superior to 2 µM, no measurement of CAC was possible for these molecules. D-xylofuranose monoester 3a and L-arabinofuranose monoester 1 presented respectively a CAC of 11.5 ± 1.6 µM and 8.4 ± 0.9 µM (Figure 4A and4B). In case of xylose or arabinose esters mixtures, CAC were estimated to 23.4 ± 2.3 µM and 11.8 ± 0.7 µM respectively (Figure 4C and4D).

Discussion

Numerous studies from literature deal with esterification and transesterification reactions catalysed by lipases for hexoses acylation, mainly Dglucose. In this context, the selective acylation of the primary hydroxyl group for hexoses and the formation of 6-O acylhexopyranoses is well known [START_REF] Kennedy | Enzymecatalyzed regioselective synthesis of sugar esters and related compounds[END_REF][START_REF] Cauglia | The enzymatic synthesis of glucosylmyristate as a reaction model for general considerations on 'sugar esters' production[END_REF][START_REF] Gumel | Thermo-kinetics of lipase-catalyzed synthesis of 6-O-glucosyldecanoate[END_REF].

The enzymatic transesterification of L-arabinose was previously described with a lipase from Pseudomonas cepiaca using oxime esters as acyl donors [START_REF] Pulido | Enzymatic regioselective acylation of hexoses and pentoses using oxime esters[END_REF]. The reaction resulted in the selective formation of the 5-O monoacylated Larabinofuranose in 45-70 % yield according to the length of the acyl chain [START_REF] Pulido | Enzymatic regioselective acylation of hexoses and pentoses using oxime esters[END_REF]. In a recent study, the synthesis of 5-O-palmitoyl-L-arabinofuranose monoester was obtained by esterification reaction with lipase N435 [START_REF] Pappalardo | Synthesis and characterization of arabinose-palmitic acid esters by enzymatic esterification[END_REF]. The regioselective acylation onto the most reactive primary hydroxyl position of the furanose isomer of L-arabinose was also obtained in the present study. However, in our reaction conditions, the diester 2 was also produced as the major compound in addition to the monoester 1.

In case of D-xylose, lipase-catalysed esterification is described to yield complex mixtures of products with immobilized lipase of C. antarctica and lipase of Candida rugosa [START_REF] Tsitsimpikou | Microbial Lipases in the BiocatalysisComparative studies on the sugar esters synthesis catalysed by Candida antarctica and Candida rugosa lipases in hexane[END_REF]. In previous studies, lipase-catalysed synthesis of fatty acid xylose esters was investigated but no information concerning neither the regioselectivity of the reaction nor the structures of the esters were available [START_REF] Tsitsimpikou | Microbial Lipases in the BiocatalysisComparative studies on the sugar esters synthesis catalysed by Candida antarctica and Candida rugosa lipases in hexane[END_REF][START_REF] Bidjou-Haiour | Lipase catalyzed synthesis of fatty acid xylose esters and their surfactant properties[END_REF][START_REF] Abdulmalek | Optimization and characterization of lipase catalysed synthesis of xylose caproate ester in organic solvents[END_REF]. Lipase-catalysed esterification of 1,2-O-isopropylidene--D-xylofuranose with various fatty acids was studied. After deprotection of the acetal, selective synthesis of 5-O-acyl D-xylofuranose occurred [START_REF] Fregapane | Enzymatic synthesis of monosaccharide fatty acid esters and their comparison with conventional products[END_REF][START_REF] Fregapane | Enzymic solvent-free synthesis of sugar acetal fatty acid esters[END_REF][START_REF] Ward | Lipase-catalyzed synthesis of a sugar ester containing arachidonic acid[END_REF]. Solvent-free esterification reactions at 75 °C with Rhizomucor miehei lipase and 1,2-O-isopropylidene-α-D-xylofuranose as acyl acceptor were carried out with fatty acids (lauric to arachidonic acid) and N435 as biocatalyst at 50 °C [START_REF] Ward | Lipase-catalyzed synthesis of a sugar ester containing arachidonic acid[END_REF].

Conversion rate of the protected xylose reached 50 % after 24 hours with arachidonic acid.

In the present study, esters produced from D-xylose and L-arabinose at 50 °C were present as furanose isomers except a minor monoester 3b for D-xylose as a pyranose isomer. No diesters in pyranosic form were detected and no acylation on the anomeric position occurred. This indicates that esterification first took place regioselectively onto the primary hydroxyl group. The pyranose / furanose equilibrium that should exist in the reaction mixture is displaced by the better reactivity of the primary hydroxyl group. Further esterification on a secondary hydroxyl group occurred with monoesters as substrates for the lipase. Indeed, experiments conducted with monoester 3a as acyl acceptor at 50 °C (in the conditions described in section 2.1.1)

showed that, after 2 hours of reactions, diesters 4 and 5 were obtained in the same ratio (data not shown). For both pentoses, kinetic studies revealed that maximum

Conclusions

This study dedicated to the lipase-catalysed synthesis of fatty esters from two Our study allowed developing a green route for the one step functionalization of pentoses from lignocellulosic biomass to produce sugar esters that could be useful as surfactants for various applications (cosmetics, phytochemistry, food …). Table 1. CAC values were measured for pure esters 1 and 3a whereas in case of mixtures CAC values were determined with an average molecular weight of each compound.

Figures Captions

  H, 125 or 150 MHz for 13 C). Chemical shifts are expressed in parts per million (ppm) using tetramethylsilane as an internal standard. NMR spectra are presented as supplementary data. Mass spectra (ESI-MS) and high resolution mass spectra (ESI-HRMS) were performed on Q-TOF Micro micromass positive ESI (CV = 30 V). 5-O-lauryl-L-arabinofuranose 1. White solid, mp 130 °C. 1 H NMR (500 MHz, CDCl 3 ): / = undetermined ratio,  5.30-5.35 (m, H-1, H-1), 4.20-4.35 (m, 2 H-5, 2 H-5H-4 or ), 4.05-4.15 (m, H-2, H-2H-4 or ), 2.37 (t, J = 7 Hz, 2H), 1.60-1.66 (m, 2H), 1.25-1.35 (m, 16H), 0.90 (t, J = 7 Hz, 3H); 13 C NMR (125 MHz, CDCl 3 ):  174.8 and 174.4 (C=O and ), 102.5 (C-1, 95.9 (C-1), 82.5 and 81.2 (C-4 and ), 79.5 (C-3), 77.6 (C-2), 77.4 (C-3), 76.1 (C-2), 65.9 and 64.3 (C-5 and ), 2D experiment (HMBC): correlations between C=O and H-5; ESI-MS: 355.2 (M + Na) + ; ESI-HRMS : m/z calcd for C 17 H 32 O 6 Na 355.2097, found 355.2089. 3,5-di-O-lauryl-L-arabinofuranose 2. White solid, mp 69 °C, 1 H NMR (500 MHz, CDCl 3 ) : / =1/1.56,  5.35-5.40 (m, H-1, H-1), 4.94 (t, J = 5 Hz, H-3), 4.70 (dd, J = 5 Hz, J = 2 Hz, H-3), 4.29-4.44 (m, H-5, H-5H-4), 4.23 (dd, J = 12 Hz, J = 5 Hz, H-5), 4.13-4.19 (m, H-2, H-2), 4.07-4.11 (m, H-4), 2.38 (m, 2H), 1.59-1.65 (m, 2H), 1.20-1.38 (m, 16H), 0.90 (t, J = 7 Hz, 3H); 13 C NMR (125 MHz, CDCl 3 ):  174.6, 174.4, 173.9 and 173.5 (2 C=O  and ), 102.9 (C-1), 96.9 (C-1), 81.2 (C-2), 81.0 (C-3), 79.9 and 79.8 (C-4 and C-3), 79.1 (C-4), 76.3 (C-2 and ), 65.0 and 63.4 (C-5  and ), 2D experiments (HMBC): correlations between C=O 174.6 and 174.4 and H-3, C=O 173.9 and 173.5 and H-5; ESI-MS: 537.5 (M + Na) + ; ESI-HRMS : m/z calcd for C 29 H 54 O 7 Na 537.3767, found 537.3773. 5-O-lauryl-D-xylofuranose 3a. White solid, mp 95 °C (lit. 93-95 °C [35]), 1 H NMR (600 MHz, CD 3 OD): / = 1/1,  5.35 (d, J = 4 Hz, H-1), 5.10 (d, J = 1 Hz, H-1), 4.40 (dd, J = 11 Hz, J = 4 Hz, H-54.29-4.35 (m, H-4 and ), 4.27 (d, J = 4 Hz, H-5), 4.25 (d, J = 4 Hz, H-5), 4.13-4.18 (m, H-3, H-5), 4.05 (dd, J = 4 Hz, J = 2 Hz, H-33.96-3.98 (m, H-2, 3.94 (t, J = 4 Hz, H-2), 2.34 (q, J = 7 Hz, 2H), 1.59-1.65 (m, 2H), 1.25-1.35 (m, 16H), 0.90 (t, J = 7 Hz, 3H); 13 C NMR (150 MHz, CD 3 OD): 175.5 and 175.4 (C=O  and ), 104.3 (C-1), 97.8 (C-1), 82.2 (C-2), 80.9 (C-4), 78.2 (C-2), 77.7 (C-4), 74.8 and 74.6 (C-3and ), 65.4 (C-5), 64.7 (C-5), 2D experiment (HMBC): correlations between C=O and H-5; ESI-MS: 355.3 (M + Na) + ; ESI-HRMS : m/z calcd for C 17 H 32 O 6 Na 355.2097, found 355.2092. 4-O-lauryl-D-xylopyranose 3b. White solid, mp 96 °C, 1 H NMR (500 MHz, CD 3 OD): / = 2/1,  5.23 (d, J = 3 Hz, H-1), 4.85 (dd, J = 8 Hz, J = 3 Hz, H-4), 4.79 (td, J = 8 Hz, J = 5.5 Hz, H-44.72 (d, J = 6 Hz, H-1), 4.14 (dd, J = 12 Hz, J = 5 Hz, H-5), 3.94 (t, J = 8.5 Hz, H-3), 3.72-3.83 (m, H-3, 2xH-5), 3.60 (d, J = 8.5 Hz, H-23.47 (t, J = 7 Hz, H-2), 3.38 (dd, J = 12 Hz, J = 8.5 Hz, H-5), 2.34 (q, J = 6 Hz, 2H), 1.58-1.65 (m, 2H), 1.25-1.35 (m, 16H), 0.87 (t, J = 7 Hz, 3H); 13 C NMR (150 MHz, CD 3 OD): 173.7 (C=O  and ), 96.8 (C-1), 92.4 (C-1), 73.7 (C-2), 72.9 (C-3), 72.6 (C-2), 71.7 (C-3), 71.2 (C-4C-2), 62.0 (C-5), 59.

3, 5 -

 5 di-O-lauryl-D-xylofuranose 5. White solid, mp 77 °C, 1 H NMR (500 MHz, CDCl 3 ): / = 1/0.2,  5.47 (d, J = 4 Hz, H-1), 5.28 (d, J = 1 Hz, H-1), 5.17 (dd, J = 5 Hz, J = 3 Hz, H-3), 5.11 (dd, J = 5 Hz, J = 2 Hz, H-3), 4.54-4.60 (m, H-4 H-4), 4.25-4.33 (m, 2 H-5), 4.10-4.22 (m,2 H-5, H-2H-2), 2.25-2.35 (m, 4H), 1.55-1.64 (m, 4H), 1.20-1.28 (m, 32H), 0.80 (t, J = 7 Hz, 6H). 13 C NMR (125 MHz, CDCl 3 ): 173.80 (C=O ), 173.7 (C=O 173.5 (C=O and), 103.1 (C-1), 96.0 (C-1), 80.6 (C-2), 78.6 (C-3), 78.3 (C-3), 77.8 (C-4), 75.7 (C-2), 75.0 (C-4), 63.1 (C-5), 62.2 (C-5), 2D experiment (HMBC): correlations between C=O (173.8 and 173.5) and H-3, and between C=O (173.7 and 173.5) and H-5; ESI-MS: 537.5 (M + Na) + ; ESI-HRMS : m/z calcd for C 29 H 54 O 7 Na 537.3767, found 537.3777. The quantification of sugar fatty esters was performed by HPLC using a NUCLEOSHELL® RP 18 ec, 5 µm 250 x 4.6 mm (Macherey Nagel) column at 40 °C. Purified products were used as standards. Mono-and diesters were eluted at 0.6 mL.min -1 , at 40 °C and with an acetonitrile (A)/water (B) mobile phase, 0-5 minutes 80 % A and 20 % B, 5-20 minutes 80 to 100 % A and 20-30 minutes 100 % A. The detection was performed with a dynamic light scattering detector (PL-ELSD 1000, Polymer Laboratories) at 40 °C under 350 kPa azote pressure. Retention times were determined for each ester 1: 6.7 min, 2: 31.5 min, 3a-b (not separated in these conditons) : 6.2 min, 5: 28.9 min and 4: 30.4 min. The quantification of residual sugars was also performed by HPLC using a NUCLEODUR® 100-5 NH 2 -RP, 5 µm 250 x 4 mm (Macherey Nagel) column at 40 °C. Pure sugars were used as standards. Sugars were eluted at 1 mL.min -1 , at 40 °C and with an acetonitrile (A)/water (B) mobile phase 75/25 % in isocratic flow. The detection was performed with a dynamic light scattering detector (PL-ELSD 1000, Polymer Laboratories) at 40 °C under 350 kPa azote pressure. Retention time of D-xylose and Larabinose were 4.5 and 4.2 minutes respectively.

monoester 1 for 4

 14 3a was separated by flash chromatography and NMR data assigned its structure to the 5-O-lauryl-D-xylofuranose, as a mixture of anomers. A minor monoester 3b was also detected and its structure was attributed to 4-O-lauryl-Dxylopyranose. Two diesters 4 and 5 were purified by flash chromatography over silica gel. The less polar diester 4 was identified as 2,5-di-O-lauryl-D-xylofuranose and the more polar diester 5 as 3,5-di-O-lauryl-D-xylofuranose as mixtures of anomers (Figure 2A). From 100 mL reactions, a global conversion of 53 % of initial D-xylose was reached. After separation by chromatography, 340 mg of monoesters (20.5 %) and 557 mg of diesters 4 and 5 (25.5 %) were purified and a global yield of 45.5 % was obtained. Kinetic studies were performed during 48 hours in 20 mL reactions. During acylation reactions of D-xylose, the first products formed were monoesters 3a-b 3.3 ± 0.3 mg.mL -1 during the first two hours of the reaction (Figure 2B). After the third hour, the two diesters appeared with quantities of 4.8 ± 0.1 mg.mL -1 for 5 and 4.0 ± 0.1 mg.mL -whereas the production of 3 reached 6.2 ± 0.1 mg.mL -1 . The maximal production of xylose lauryl monoesters occurred in 4 hours while the maximal yield of diester was attained after 24 hours. Global conversion of D-xylose was evaluated by HPLC by measuring D-xylose remaining after 4 hours in reaction media and attained 77.1 ± 2.0 % and a 74.8 % global overall yield (4h) (38.4% of monoesters 3, 19.4% of diester 4 and 17.4 % of diester 5). Conversion of D-xylose was higher compared to this obtained with the 100 mL reaction probably due to a better mass transfer. Assays of recycling the enzyme were conducted in the same conditions, by performing transesterification of D-xylose and vinyllaurate during 4 hours at 50 °C. Every 4 hours, the reaction medium and the immobilized enzyme were separated. Reaction medium was analysed by HPLC and the remaining enzyme was used to perform another round of transesterification reactions with fresh D-xylose (50 mM) and vinyllaurate (150 mM). HPLC analysis showed that the Novozym 435 was re-usable for 5 cycles without losing significant efficiency. During the sixth cycle, the synthesis of total D-xylose laurate esters reached 5 % of synthesis occurring during each previous cycle indicating a drastic loss of lipase efficiency (data not presented). Similar results

  yields were obtained at 50 °C after 4 hours of reaction for xylose monoesters 3 and arabinose monoester 1 and diester 2 and after 24 hours for xylose diesters 4, 5, with a higher conversion rate for D-xylose. However, D-xylose-based esters production was slower during the first two hours of reaction compared to L-arabinose. This could be explained by the lower ratio of the furanose isomers for D-xylose compared to Larabinose in the reaction mixture as it is the case in aqueous solution[START_REF] Drew | 13 C-labeled aldopentoses: detection and quantitation of cyclic and acyclic forms by heteronuclear 1D and 2D NMR spectroscopy[END_REF] or by a difference in dissolution kinetics as described for D-glucose and D-fructose in 2M2B[START_REF] Engasser | Kinetic modeling of glucose and fructose dissolution in 2-methyl 2-butanol[END_REF]. Monoesters 1 and 3a can undergo a second acylation reaction. For L-arabinose, only diester 2 resulting from a transesterification reaction on the hydroxyl group at C-3 was synthesized and was already detected in the first hour of the reaction at 50°C. No evolution of the monoester / diester ratio was observed during the reaction course even after 48 hours. In comparison, the reaction was more complex for D-xylose, as two diesters 4 and 5 were obtained from monoester 3. The second acylation proved to be less selective and slower than for L-arabinose. These results could be rationalized by an enhanced reactivity of the secondary hydroxyl groups on C-3 compared to the C-2 position for both monoesters 1 and 3a. Indeed, for both monoesters, a second acylation was observed at C-3 position. However, for D-xylose monoester, the acylation on C-3 position could be in competition with acylation at C-2 maybe due to the cis-relationship(s) between OH in C-3 position and the acylated C-5 position which could generate steric hindrance.The surface-active properties of the pentose-based esters produced a 50 °C can be compared to other sugar-based laurate esters described in literature. The CAC values were rather similar to those for xylofuranose laurate monoester for which CMC value of 41 µM or 18 µM have been determined[START_REF] Fernandez Bolanos | Synthesis of Special Surfactants[END_REF][START_REF] Garofalakis | Surface Activity and Critical Aggregation Concentration of Pure Sugar Esters with Different Sugar Headgroups[END_REF]. Other laurate esters with different polar heads have shown variable CMC or CAC values depending on the polar head, such as maltopyranose and glucopyranose laurate ester with respective CMC values of 120 µM and 180 µM[START_REF] Garofalakis | Surface Activity and Critical Aggregation Concentration of Pure Sugar Esters with Different Sugar Headgroups[END_REF]. A CMC value of 3 mM was obtained for galactopyranose laurate esters[START_REF] An | Enzymatic synthesis and characterization of galactosyl monoesters[END_REF]. Measurement of CMC/CAC is highly dependent on the pH and the temperature conditions. The CAC values obtained for our pentosebased laurate esters are inferior to 100 µM compared to hexose-based laurate esters and these low values could be related to the lower hydrophobicity of the smaller polar heads (pentoses vs. hexoses)[START_REF] Ducret | Characterization of enzymatically prepared biosurfactants[END_REF].The low CAC values of the D-xylofuranose and L-arabinofuranose esters produced in the present study indicate that these molecules present interesting surfactant properties as their CAC are lower than those of commercially available sugar esters such as Tween 20 (80 µM). Furthermore, the mixtures of esters also exhibit low CAC values which are of interest in a context of an industrial production with limited purification steps.

  biomass-derived pentoses, D-xylose and L-arabinose, reports the full characterization of the esters produced and a better understanding of the regioselectivity of the lipasecatalysed transesterification with both pentoses. In this work, the enzymatic synthesis of D-xylose and L-arabinose laurate esters using C. antarctica lipase B (N435) as a biocatalyst was reported. Both L-arabinose and D-xylose gave rise to the synthesis of pentose laurate esters as furanose isomers indicating a regioselective first acylation onto the primary hydroxyl group. A second acylation could then occur either selectively on O-3 for L-arabinose or as a mixture of diacylated products on O-2 and O-3 positions for D-xylose. The nature of the pentose influenced kinetics of production and obtained yields. Kinetics of synthesis of D-xylose based esters were slower but yields were higher than those obtained for L-arabinose based esters. Investigation of surfactant properties of pentoses laurate esters indicated that Dxylose and L-arabinose mono-and diesters exhibited good surfactant properties, such as low CAC, whether purified or in mixtures.

  Figure 1. (A) Transesterification reaction of L-arabinose and vinyllaurate performed by the lipase Novozym435. (B) Kinetics of synthesis of arabinose laurate esters quantified by HPLC. Reactions were performed at 50°C during 48h with 50 mM arabinose, 150 mM vinyllaurate, 1% (w/v) lipase. 1: arabinose laurate monoester, 2: arabinose laurate diester.

Figure 2 .

 2 Figure 2. (A) Transesterification reaction of D-xylose and vinyllaurate performed by the lipase Novozym435. (B) Kinetics of synthesis of xylose laurate esters quantified by HPLC. Reactions were performed at 50°C during 48h with 50 mM xylose, 150 mM vinyllaurate, 1% (w/v) lipase. 3a-b: xylose laurate monoesters, 4 and 5: xylose laurate diesters.

Figure 3 .Figure 4 .

 34 Figure 3. Surface activity of the xylose laurate monoester 3a. Kinetics of monoester adsorption at the air-water interface, time zero corresponds to the injection into the subphase

Acknowledgements

The authors are grateful to the French Region Grand Est, Grand Reims and Feder for their financial support (chaire AFERE) as well as to the European Regional Development Fund (ERDF) and Region Grand Est for the financial support of the Interreg Valbran project. PhD thesis (T. Méline) was supported by the Region Grand Est. M.D. thanks the F.R.S.-F.N.R.S. (National Funds for Scientific Research, Belgium) for her position as Senior Research Associate.