Rationalization and In Vitro Modeling of the Chemical Mechanisms of the Enzymatic Oxidation of Phenolic Compounds in Planta: From Flavonols and Stilbenoids to Lignins

Abstract

Enzymatic oxidation of phenolic compounds is a widespread phenomenon in plants. It is responsible for the formation of many oligomers and polymers, which are generally described as the result of a combinatorial coupling of the different radicals formed through oxidation of the phenol group and delocalization of the radical. We focused our interest on several phenolic compounds that are present in plants and known to form, under enzymatic oxidation, oligomers with different type of linkages between monomers. To explain this diversity of inter-monomer linkages and their variation according to the experimental procedure used for the enzymatic oxidation, we report an alternative mechanistic pathway involving dismutation of the radicals, leading to the formation of carbocations which, thereafter, react with nucleophilic species present in the medium. This alternative pathway allows the understanding of peculiar linkages between monomeric units in the oligomer and offers new insights for understanding the formation of phenolic biopolymers in plants.

Chemo-enzymatic Synthesis, Derivatizations, and Polymerizations of Renewable Phenolic Monomers Derived from Ferulic Acid and Biobased Polyols: An Access to Sustainable Copolyesters, Poly(ester-urethane)s, and Poly(ester-alkenamer)s

“…Having developed expertise in lignin related phenol compounds chemistry (42)(43)(44)(45)(46)(47), we recently put a great deal of effort into developing new valorizations of ferulic acid. In this context, we first devised and designed a new class of bio-based polyfunctional molecules incorporating ferulic acid moieties through the development of an enzymatic process involving polystyrene supported Candida antarctica lipase B (aka CAL-B or Novozyme ® 435) (48).…”

Chemical Constituents from the Aerial Parts of Cyrtopodium paniculatum

“…These biphenanthrene derivatives occur mostly in species of the genus Bletilla [33,41,71,[78][79][80], Bulbophyllum [55,[81][82][83] and Cremastra [54,[84][85][86][87][88]. The existence of dimers together their respective monomers in the same species provides support for a proposed biogenetic pathway for biaryl-derived compounds occurring from their corresponding monomers as a result of free radical [56] or an enzymatic oxidative phenolic coupling reaction [82,[89][90][91]. This study reveals for the first time the chemical diversity of phenolic constituents produced by an endemic South-American orchid, Cyrtopodium paniculatum.…”

Optimization of the Laccase-Catalyzed Synthesis of (±)-Syringaresinol and Study of its Thermal and Antiradical Activities

“…We recently showed in the case of flavonols and resveratrol, natural phenolic compounds that are also good substrates of laccases, leading to similar polymerization processes, that C‐C bonds where indeed formed upon direct radical‐radical coupling but C‐O bonds (or C‐C bonds involving nucleophilic aromatic carbons of resorcinol or phloroglucinol type) resulted from a first dismutation of two radicals leading to a reduced phenate ion and a carbocation that is thereafter trapped through nucleophilic addition of a nucleophilic species. Further experiments (work under publication) showed the oxidative oligomerization of monolignols to occur in the same way involving either radicals or cations derived from monolignols, β‐O‐4 linkages mainly resulting from the dismutation/nucleophilic addition pathway (See scheme 2 for the description of both pathways in the case of sinapyl alcohol).…”

“…Further experiments (work under publication) showed the oxidative oligomerization of monolignols to occur in the same way involving either radicals or cations derived from monolignols, β‐O‐4 linkages mainly resulting from the dismutation/nucleophilic addition pathway (See scheme 2 for the description of both pathways in the case of sinapyl alcohol). In in vitro experiments, balance between these two different mechanisms depends on the ratio between radicals and non‐oxidized starting material and therefore on the experimental procedure.…”