Lignin is the second natural biopolymer after cellulose, formed in planta by polymerization of different p-hydroxycinnamic alcohols, which gives it a complex and still incompletely described structure. Associated with cellulose and hemicelluloses, it constitutes the majority of the secondary wall of plants.  

Lignins are phenolic polymers involved in the vertical growth of plants. They play the role of waterproofing agents, causing the exclusion of water from the secondary walls and facilitating its flow through the plant's vascular tissue. This phenolic polymer plays a structural and protective role in the development and life of plants. Lignins are a brake on the transformation of plant biomass, whether in the paper industry or biorefineries, due to their strong interactions with the other wall constituents, high chemical reactivity, and low degradability. These last two factors are closely linked to its molecular structure; therefore, It is necessary to have the most complete understanding of the enzymatic and chemical mechanisms that govern its establishment within plant walls. Only in this way will it be possible to understand the specificities of lignocellulosic biomass and to estimate its potential in an approach to valorization and use of lignins, or wood residues, for the functionalization of materials or the production of bio-sourced molecules for the chemical industry. 

Lignins are polymers made up of three different types of monomers: p-coumaryl, coniferyl, and sinapyl monolignols. These give rise respectively to p-hydroxyphenyl, guaiacyl and syringyl type units. These monomers differ by the degree of methoxylation in positions 3 and 5 of the aromatic cycle. The fraction of each unit varies significantly depending on the plant line, species, organ, and tissue from which they are derived. Lignins from different natural or industrial sources vary widely in structure and chemical properties (average molecular weight, polydispersity, degree of condensation, number of groups present, type of monomers, type of inter-unit links, quantity of each unit present). The polymerization mechanisms responsible for the variation in the composition of lignins use the peroxidase/H2O2 or laccase/O2 catalytic systems. The polymerization mode involves a radical mechanism between two dehydrogenated complexes. To date, there are two hypotheses: the coupling can be between the oxidized monomer and the phenol radical of the growing polymer or between two phenol radicals from two oligomers. Due to the complexity of lignins in biosynthesis, structure, and interaction, studies of lignification on normal or mutated plants are insufficient.

This is why it is interesting to develop model systems such as dehydrogenated polymers (or DHP) to have more information about the structure of lignins and the frequency of the different types of interaction. To find the mechanisms closest to those involved in the formation of native lignins in planta, it is necessary to test various operating modes and different biosynthetic pathways and optimize the formation conditions. Then, from an analytical point of view, the structure of the model lignins formed must be characterized by the analysis of the fragments present. The structural elucidation of the structure formed is crucial for understanding these systems. The analysis methods of lignin involve GC/MS mass spectroscopy, LC/MS, size exclusion chromatography, NMR, and degradative analysis techniques such as thioacidolysis.  

In a second step, the work on industrial lignins resulting from the fractionation of lignocellulosic resources aims to evaluate their potential uses in relation to their aromatic macromolecular structure and the associated process. Due to the inhomogeneity in their formation, lignins and their derivatives have had only limited uses until now. These natural substrates are more challenging to valorize than natural (poly)saccharides, although they are more promising regarding chemical diversity and potential field of application. The free phenols of lignins, highly reactive functional groups, can be used as primers for polymerization or to confer antioxidant potential to products containing them. Today, lignins are commonly used for energy production in the paper or biofuel industries, partly due to their low solubility and free phenol content. However, they can be modified and depolymerized to increase their reactivity, solubility, and free phenol content to produce higher-value chemicals. A lignin conversion process that allows the recovery of high-value molecules would improve the economics of biorefineries. These conversion processes are not intended to replace existing valorization pathways completely but to divert part of the existing lignin stream to generate new value streams alongside the combustion or purification of lignins into polymers. One limitation of most 2G biorefineries is that the quality of the feedstocks strongly influences the processes. Given the high variability and heterogeneity of biorefinery waste and by-products, there is now a crucial need for robust and flexible valorization processes. Lignin research involves a combination of disciplines: biochemistry, chemistry, molecular biology, and genetics. My projects are at the chemistry-biology interface, as in most of my previous research experiences. My scientific project at INRAE ​​starts from the understanding of the structure of lignins in relation to their formation mechanisms in planta and from the study of the different parameters involved in lignification, by in vitro modeling of the initial stages of the polymerization of hydroxycinnamic alcohols up to the valorization of native or industrial lignins, either directly as biomaterials (functionalization, transformation) or as a source of synthons for organic chemistry and the synthesis of new bio-sourced polymers, by combining chemical and biocatalytic treatment for better compliance with environmental constraints.

I have developed several lines of research:

1. the study of lignification and oxidation mechanisms of plant phenols: kinetic monitoring of in vitro polymerization, the study of in vivo biosynthesis pathways and electrochemical studies have made it possible to clarify the role of different oxidases and different substrates in the establishment of lignins and to propose a new specific reaction mechanism for the formation of b-O-4 type bonds.
2. the synthesis of model phenolic compounds: a set of methods has been developed allowing the custom synthesis of monomers, dimers and oligomers of hydroxycinnamic alcohols and acids.
3. the valorization of lignins in innovative media. The aim is to explore ways of depolymerizing lignins in unconventional environments, with or without (bio)catalysis, and providing access to synthons or oligomers, allowing their valorization as additives or macromonomers in the formulation of bio-sourced polymers.
4. structural analyses of lignins.