Báo cáo khoa học: Molecular structure and biochemical properties of lignins in relation to possible self-organization of lignin networks

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Báo cáo khoa học: "Molecular structure and biochemical properties of lignins in relation to possible self-organization of lignin networks"Molecular structure and biochemical properties of ligninsin relation to possible self-organization of lignin networksB. Monties Agronomique Paris-Grignon,Laboratoire de Chimie Biologique, INRA (CBAI), Institut NationalCentre de Grignon, 78850 Thiverval-Grignon, France tion, drying of logs and sawings, and hard-Introduction board and paper manufacture, as sug- gested, respectively, for example by Sar-This review briefly recalls chemical data kanen (1971;!, Northcote (1972), Fryrelated to the variations in the molecular (1986), Back(1987), Jouin et al.. (1988),structure of lignin and mainly discusses and Horn and Setterholm (1988).the biochemical heterogeneity and occur- This review focuses thus on self-organi-rence of associations between lignins and zation and recalls only briefly the chemicalother cell wall components. In an attempt and biochemical properties of lignin into relate the formation of such lignin net- relation to other plant cell wall compo-works to possible functions of lignins, a Due to edition constraints, only nents. hypothesis on the self-organizationnew main relevant references are cited.properties of lignin is presented. From a biochemical point of view, ligninsare particularly complex polymers whosechemical structure changes within plant Molecular structure of ligninspecies, organs, tissues, cells and evencell fractions. Furthermore, from a physio- In vitro model studies and in vivo experi-logical point of view, lignin biosynthesis is ments (Freudenberg and Neish, 1968;unusual in that the final polymerization Higuchi, 1985;! have shown that the gen-step is only enzymatically initiated and is eral molecular structure of lignin can berandom chemically directed. Occurrence explained by one-electron oxidation of cin-of such random synthesis raises the cen- namyl alcohols followed by non-enzymatictral question of the origin of the biological polymerization of the correspondingfitness of lignification to the life cycle of mesomeric free radicals.plants. This question is relevant not onlyfor the formation of ’abnormal lignins’ and Fig. 1 shows the phenylpropane’lignin-like compounds’ in reaction woods, (C skeleton of the lignin monomers ) 3 -C 6and wounded and diseased tissues but (M) and the structure of 4 of the mostalso in the case of ’normal’ lignin in wood common linkages found in lignins. Thesexylem. Such random polymerization may structures have been established by inalso be relevant in relation to the evolution vitro peroxidase oxidation of mainly coni-of the quality of the lignocellulosic pro- feryl alcohol ((a Fig. 1followed by iso- =ducts, such as during heartwood forma- lation of dimers (dilignols), oligomers (oli- i-golignols) and dehydropolymers (DHP lignin polymer through copolymerization, but also through heteropolymerization withmodel). Model polymerization studieshave also shown that the relative frequen- other macromolecules, such as polysac-cy of these intermonomeric linkages and, charides (Sarkanen, 1971; Higuchi, 1985).thus, the corresponding macromolecular Fig. 1 shows the addition reaction bet-structure of DHP changes according to ween a compound A-B and a terminal p-polymerization conditions (Sarkanen, methyle ...