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

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Molecular structure and biochemical properties of lignins in relation to possible self-organization of lignin networks B. Monties Agronomique Paris-Grignon, Laboratoire de Chimie Biologique, INRA (CBAI), Institut National Centre 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), Fry related 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 chemical other cell wall components. In an attempt and biochemical properties of lignin in to 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-organization new main relevant references are cited. properties of lignin is presented. From a biochemical point of view, lignins are particularly complex polymers whose chemical structure changes within plant Molecular structure of lignin species, organs, tissues, cells and even cell 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 be random chemically directed. Occurrence explained by one-electron oxidation of cin- of such random synthesis raises the cen- namyl alcohols followed by non-enzymatic tral question of the origin of the biological polymerization of the corresponding fitness of lignification to the life cycle of mesomeric free radicals. plants. This question is relevant not only for 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 6 and wounded and diseased tissues but (M) and the structure of 4 of the most also in the case of ’normal’ lignin in wood common linkages found in lignins. These xylem. Such random polymerization may structures have been established by in also 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 with model). Model polymerization studies have 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, methylene quinone unit (a’: Fig. 1Addi- 1971),such as, the concentrations and tion of A-B led to the formation of the the rate of addition of the reagents, the corresponding A,B-(a) substituted hex- polarity of the medium or solvents, the alignol (a-f: Fig. 1). Depending upon the electronic and steric effects of the substi- structure of A-B and when A is hydrogen, tuents in the aromatic cycle, according to the aromatic character of the a-monomeric the various substitution patterns of the unit is recovered with reformation of a lignin monomeric units: H, G, S (Table I). phenolic group. This phenolic unit may Formation of para- and ortho-quinone further polymerize, leading to a trisub- methide has also been suggested during stituted monomeric unit or ’branch point’ of the dimerization of mesomeric oligolignols the lignin network (Pla and Yan, 1984). or monomeric units and during chemical Such a reticulation process with reforma- oxidation of simple phenolic model com- tion of a phenolic group could be a signifi- pounds (Harkin, 1966). Intermediate oligo- cant self-organization property of lignin (see below). Depending upon the A-B methides are implicated lignol-p-quinone in the formation of lignin networks. Ac- structure, the addition reaction shown in cording to in vitro experiments, such struc- Fig. 1 may also be important and thus tures are involved in the growth of the explain certain macromolecular regulari-
ties in in the 3 lignin structure. As early as in 1968, appearance of certain regularities Freudenberg and Neish stressed that &dquo;the dimensional network. sequence of the individual (monomeric) units in lignin is fortuitous, for they are not moulded like proteins on a template. This Biochemical properties does not exclude the occurrence of a cer- tain regularity in the distribution of weak Biochemical heterogeneity or inhomoge- and strong bonds between the units. As a neity (Monties, 1985) is the second main rough estimate, 7 to 9 weak bonds are feature of lignin. Characteristic variations randomly distributed among 100 units, in lignin structure and monomeric com- ’gluing’ together more resistant clusters, of position have indeed been found and an average, 14 units.&dquo; Such ’clusters’ or confirmed between plant species (Logan ’primary chains’ of about 18 strongly link- and Thomas, 1985), between plant organs ed monomeric units have been reported and tissues grown either in vitro or in vivo after delignification experiments by Bolker and also betvveen cell wall fractions (Hoff- and Brener (1971) and by Yan et aL man et al., 1985; Sorvari et aL, 1986; (1984). According to these authors, the Saka etal., 1 !)88; Eriksson et al., 1988). In weak-bonds suggested by Freudenberg agreement with these data, which cannot and Neish are mainly a-aryl ether link- be discussed here in detail, heterogeneity intermolecular respectively, ages, in lignin formation and molecular structure, (Ca !B bond in a-unit: Fig. 1) and intra- has been demonstrated in the case of molecular (C bond in b-unit: Fig. 1 -04 a ). gymnosperms (Terashima and Fukushi- Confirming the importance of addition ma, 1988) and in the case of angiosperms reactions with p-methylene quinone, such (Higuchi, 19135; Monties, 1985; Lapierre, a weak a-aryl ether bond may correspond 1986; Tollier et al., 1988; Terashima and to a Ca !B linkage (a Fig. 1 ) where B = Fukushima, 1988). From a biochemical is a phenoxy substituent corresponding to of view, lignin thus appears to be point the addition of a BA phenolic terminal non-random heterogeneous copolymers monomeric unit. Summarizing the most enriched by either non-methoxylated (p- characteristic chemical properties, lignin hydroxyphemyl H), monomethoxylated does not appear to be a defined chemical = (guaiacyl G) and dimethoxylated (syrin- compound but a group of high molecular = gyl S) monomeric units (Fig. 1These weight polymers whose random structure, = copolymers are unequally distributed which is related to their chemically driven amongst cells and subcellular layers, in polymerization, does not exclude the
these authors: in Fagus, the lignin moiety tissues according to patterns changing of LCC would consist of a small number of with species. The biosynthesis of the pre- extremely large molecular fractions, while cursors and the regulation of lignification pine would have relatively smaller and most likely occurs within individual cells more numerous fractions, confirming the and variations are observed according to hypothesis of biochemical heterogeneity of the type and the age of cells (Wardrop, 1976), as in the case of secondary me- lignins. tabolism (Terashima and Fukushima, known to be bound Phenolic acids are 1988). lignin, especially in the cases of mono- to cotyledons (grasses and bamboos) and Salicaceae (poplars). Ester bonds of phe- nolic acids to C and Cy-hydroxyls of a Molecular associations and cell wall monomeric propane chains (Fig. 1C5- lignification carbon-carbon bonds and ether bonds at C oxygen of aromatic cycles -phenolic 4 (Fig. 1 ) have been reported in the cases of Formation of molecular associations with model DHP (Higuchi, 1980) and gra- other cell wall components is the third mineae lignins from wheat (Scalbert et main feature of lignins. Indirect evidence al., 1985) and reed, Arundo sp. (Tai ef aL, of the occurrence of such heteropolymers, 1987). Ether linkages of phenolic acids mainly based on extractability or liquid have been tentatively implicated in the chromatographic experiments, has been characteristic alkali solubility of grass reported in the case of polysaccharides, lignins; however, free phenolic hydroxyl phenolic acids and proteins, tannins and groups would also participate in this some other simple compounds. The types solubility (Lapierre et aL, 1989). of chemical bonds involved in these asso- Lignin-protein complexes in the cell wall ciations have been established only for of pine (Pinus sp.) callus culture have polysaccharides, phenolic acids and pro- been reported: covalent bonds, formed teins, mainly based on model experiments preferentially with hydroxyproline, have of addition to p-methylenequinone dis- been suggested on the basis of selective cussed previously. extraction experiments and of the reactivi- The most frequently suggested types of ty of model DHPs containing hydroxypro- lignin-carbohydrate complex (LCC) link- stable to acid line, which were more ages are a benzyl ester bond with the C - 6 hydrolysis than carbohydrate-DHP com- carboxyl group of uronic acids, a benzyl plexes (Whitmore, 1982). Chemical bonds ether bond with the hydroxyl of the primary between lignin and protein have also been alcohol of hexose or pentose, a glycosidic recently indicated during the differentiation bond with either the C hydroxyl -phenolic 4 of xylem in birch wood, Betula sp. (Eom or the Cy-primary alcohol of phenylpro- et al., 1987). A gradual decrease in phe- pane units (M Fig. 1The synthesis of = nolic hydroxyl group content and changes LCC model compounds, their reactivity in molecular weight distribution during the and their chemical or enzymatic stability lignification have also been shown by have been compared to those of wood authors. These variations were these LCC (Higuchi, 1983; Minor, 1982; Enoki in terms of changes in lignin explained et al., 1983). Recently, using a selective structure in relation to variations in depolymerization procedure, Takahashi concentrations of available monomers and and Koshijima (1988) have concluded that effects of the conditions of polymerization xylose participates in lignin-carbohydrate as discussed above . linkages through benzyl ether bonds in LCC from angiosperm (Fagus sp.) and Possible associations with other pheno- as condensed and hydrolyzable gymnosperm (Pinus sp.) woods. Macro- lics, such molecular differences were reported by tannins have also been suggested in rela-
process (Back, 1987; Horn ’press-drying’ tion to the difficulties in completely remov- and Setterholm, 1988). ing tannins, after solvent and mild chemi- cal extractions of woods and, also in rela- In order to try to understand the general tion to coprecipitation, such as sulfuric phenolic networks by non- formation of acid-insoluble lignin fractions. Mecha- enzymatic pol’ymerization processes, self- nisms of random, i.e., chemically-driven organizing properties of lignin can be sug- polymerization of tannins with cell wall gested. The self-organization concept components, have been discussed recent- comes from the general theory of systems. ly (Haslam and Lilley, 1985; Jouin et a/., Self-organization accounts for the manner 1987). However, no evidence of chemical in which complex systems adapt to and bonds between tannins and lignins was increase their organization under the sti- given. mulation of random environmental factors. This theory has been applied extensively to the growth of organisms and transmis- sion of information (Atlan, 1972). Self- Network formation and self-organiza- organization also seems relevant in the tion properties case of lignin, since lignin is a non-enzy- matic polymerized macromolecule, its structure changes as a function of random Formation of molecular associations be- external environmental factors, it rear- lignins and cell wall components tween ranges during maturation, ageing or tech- sheds light on the importance of the phe- nological transformations and, finally, nolic group’s reactivity, such as the addi- these changes provide a better fitness of tion to methylene quinone with phenolic cell wall functions, such as resistance group reformation (Fig. 1in the reticula- against biotic and abiotic factors. tion of the plant cell wall. Such reactivity is According to Atlan (1974), a self-orga- not unique, since phenol dimerization, by nizing system is a complex system in formation of diphenyl and of diarylether which changes in organization occur with bonds, has also been reported for tyrosine increasing efficiency in spite of the fact during cell wall cross-linking processes that they are induced by random environ- (Fry, 1986). Recently, similar reactions mental factors; changes are not directed have also been suggested for tyramine in template. Self-organization capacity by a the phenolic fraction associated with su- expressed as a function of 2 main be can berin (Borg-Olivier and Monties, 1989). As parameters: redundancy and reliability. very clearly stressed by Northcote as early When the organization is defined as ’varie- 1972, with reference to synthetic as ty and inhomogeneity’ of the system, fibrous composite, the formation of such redundancy is viewed as ’regularity or cross-linked phenolic polymers may be order as repetitive order’ and reliability significant in regard to the structure and expresses the system’s ’inertia opposed to functions of plant cell walls. Reticulation random perturbation’. According to these may be of importance in durability and definitions, the information content, i.e., mechanical properties, as recently dis- the organization of a system, can be cussed in the case of cell wall proteins by function of redundancy expressed as a Cassab and Warner (1988). Furthermore, (see Annex). Evolution of the and of time in the case of lignins, this cross-linking function of time can thus organization as a phenomenon may be of much more gen- different types of be calculated showing eral interest. For example, the formation of organization. chemical bonds in the residual lignin net- Thus, a self-organizing system is char- work of thermomechanical pulps has been acterized by a defined maximum organiza- implicated in the autocross-linking of these tion resulting from an initial increase in cellulosic fibers during the production of inhomogeneity associated with a contin- paper and hardboard in the so called
lignin macromolecules, explaining their decrease in redundancy under the uous effect of random environmental factors. At functional fitness and the biological signifi- cance of the ’random process’ of lignifica- the other extreme, a non-self-organizing tion. However, until now, this theory suf- system shows a continuous decrease of fers from 2 main drawbacks: a lack of organization, mainly due to a low initial quantitative evaluation and a definite redundancy. Furthermore, intermediate cases have also been described by Atlan account of the phylogenic and ontogenic significance of the substitution pattern of (1972, 1974) corresponding to relatively high or very low reliability and lead- the lignin monomeric units. very ing, respectively, to a very long or a very short duration of the initial phase of in- crease in organization. According to Atlan Acknowledgments (1974), crystals can be viewed as a non- self-organizing system because of low in- itial reliability in spite of their large redun- Thanks are due to Drs. Catherine Lapierre, C. dancy. At the other extreme, less repetitive Costes and E. Odier for critical assessment of and more flexible structures, such as the manuscript and to Kate Herve du Penhoat for linguistic revisions. macromolecular systems, can be self- organizing. In agreement with this model, it is sug- gested that lignin networks be considered References self-organizing systems, thus ex- as plaining the formation of molecular com- Atlan H. (1972) In: L’organisation biologique et plexes by auto- and heteropolymerization la theorie de I’information. Hermann, Paris, pp. in plant cell walls with an increase of lignin 229 functional properties. Atlan H. (1974) On a formal definition of organi- The high frequency of relatively labile zation. J. Theor. Biol. 45, 295-304 intermonomeric linkages, such as /3- and Back E.I. (1987) The bonding mechanism in mainly a-ether bonds, and also of easily hardboard manufacture. Holzforschung 41, activated groups, such as free phenolic 247-258 terminal units (Fig. 1), may allow rear- Bolker H.I. & Brener H.S. (1971) Polymeric rangement reactions and, thus, easy evo- structure of spruce lignin. Science 170, 173-176 lution of the system as a function of ran- Borg-Olivier O. & Monties B. (1989) Characteri- dom environmental factors. Occurrence of zation of lignins, phenolic acids and tyramine in chemical and biochemical regularities, the suberized tissues of natural and wound- induced potatoe periderm. C.R. Acad. Sci. Ser. previously discussed, may, in addition, 111308, 141-147 provide enough initial redundancy. Finally, a high reliability, i.e., inertia to perturba- Cassab G.I.& Varner J.E. (1988) Cell wall pro- teins. Annu. Rev. Plant PhysioL 39, 321-353 tion, may result from the ability to reform phenolic groups after, for example, an Enoki A., Yaku F. & Koshijima T. (1983) Synthe- sis of LCC model compounds and their chemi- addition reaction as shown in Fig. 1, but cal and enzymatic stabilities. Holzforschung 37, also from the release of reactive phenolic 135-141 and/or benzylic groups after /3- and mainly Eom T.J., Meshitsuka G., Ishizu A. & Nakano T. a-ether cleavage. (1987) Chemical characteristics of lignin in dif- In conclusion, when lignin forma- even ferentiating xylem of a hardwood III. Mokuzai tion appears as enzyme-initiated and an Gakkaishi 33, 716-723 chemically driven process, structural stu- Eriksson I., Lindbrandt O. & Westermark U. dies have provided evidence of regulari- (1988) Lignin distribution in birch (Betula veru- ties in chemical and biochemical proper- cosa) as determined by mercurization with ties in lignin networks. Such regularities SEM- and TEM-EDXA. Wood Sci. Technol. 22, may allow self-organizing properties of 251-257
Freudenberg K. & Neish A.C. (1968) In: Saka S., Hosoya S. & Goring D.A.I. (1988) A comparison of bromination of syringyl and Constitution and Biosynthesis of Lignin. Sprin- ger-Verlag, Berlin, pp. 129 guaiacyl type lignins. Holzforschung 42, 79-83 Sarkanen K.V. ( 1971 ) Precursors and their poly- Fry S.C. (1986) Cross-linking of matrix poly- merization. In: I.ignins: Occurrence, Formation, mers in the growing cell walls of angiosperms. Structure and Reactions. (Sarkanen K.V. & Lud- Annu. Rev. Plant Physiol. 37, 165-186 wig C.H., eds.), Wiley Interscience, New York, Harkin J.M. (1966) O-Quinonemethide as tenta- pp. 138-156 tive structural elements in lignin. Adv. Chem. Scalbert A., Monties B., Lalemand J.Y., Guittet Ser. 59, 65-75 E. & Rolando C. (1985) Ether linkage between Haslam E. & Lilley T.H. (1985) New polyphenols phenolic acids and lignin fractions from wheat from old tannins. In: The Biochemistry of Plant straw. Phytochemistry 24, 1359-1362 Phenolics. Annu. Proc. Phytochem. Soc. Eur. Sorvari J., Sjostrom E., Klemola A. & Laine J.E. (van Sumere C.F. & Lea P.J., eds.), 25, 237-256 (1986) Chemical characterization of wood T. (1983) Biochemistry of Higuchi lignification. constituents especially lignin in fractions sepa- Wood Res. 66, 1-16 6 rated from midd’le lamella and secondary wall of Norway spruce (Picea abies). Wood Sci. Tech- Higuchi T. (1985) Biosynthesis of lignin. In: nol. 20, 35-51 Biosynthesis and Biodegradation of Wood Components. (Higuchi T, ed), Academic Press, Tai D., Cho W. & Ji W. (1987) Studies on Arun- Orlando, pp. 141-160 do donax lignins. Proc. Fourth Int. Symp. Wood Pulping Cnem. 2, C.T.P., Grenoble, pp. Hoffman A. Sr., Miller R.A. & Pengelly W.L. 13-17 7 (1985) Characterizations of polyphenols in cell walls of cultured Populus trichocarpa tissues. Takahashi & Koshijima (1988) Molecular prop- Phytochemistry 24, 2685-2687 erties of lignin--carbohydrate complexes from beech (Fagus c:renata) and pine (Pinus densi- Horn R.A. & Setterholm V. (1988) Press drying: flora) woods. Waod Sci. Technol. 22, 177-189 a way to use hardwood CTMP for high-strength paperboard. TAPPI 71, 143-146 Tanahashi M., Takeuchi H. & Higuchi T. (1976) Dehydrogenative polymerization of 3,5-disubsti- Jouin D., Tollier M.T. & Monties B. (1988) Ligni- tuted p!oumaryl alcohols. Wood Res. 61, 44- fication of oak wood: lignin determinations in 53 sapwood and heartwood. Cell. Chem. Technol. Terashima N. & Fukushima K. (1988) Heteroge- 22, 231-243 neity in formation of lignin: autoradiographic Lapierre C. (1986) H6t6rog6n6it6 des lignines study of formation of guaiacyl and syringyl lignin de peuplier: mise en evidence syst6matique. in Mangnolia Icobus D.C. Holzforschung 40 Ph.D. Thesis, Universit6 d’Orsay, France suppl., 101-105 Lapierre C., Jouin D. & Monties B. (1989) On Tollier M.T., Monties B. & Lapierre C. (1988) the molecular origin of the alkali solubility of Heterogeneity in angiosperm lignins. Holzfor- gramineae lignins. Phytochemistry 28, 1401- schung, 40 suppl., 75-79 1403 Wardrop A.B. (1976) Lignification in plant cell Logan K.J. & Thomas B.A. (1985) Distribution wall. Appl. Pofyin. Symp. 28, 1041-1063 of lignin derivatives in plants. New Phytol. 99, Whitmore F.A. (1982) Lignin-protein complex in 571-585 cell walls of Pinus elliottii: amino acid consti- Minor J.L. (1982) Chemical linkage of pine poly- 8 tuents. Phytochl!mistry 21, 315-318 saccharide to lignin. J. Wood Chem. TechnoL 2, Yan J.F., Pla F., Kondo R., Dolk M. & McCarthy 1-16 6 J.L. (1984) Lignin: 21: depolymerization by Monties B. (1985) Recent advances in lignin bond d reactions and degelation. eavagfi inhomogeneity. In: The Biochemistry of Plant Macrofno/ecu/es 17, 2137-2142 Phenolics. Annu. Proc. Phytochem. Soc. Eur. (van Sumere C.F. & Lea P.J., eds.), 25, 161-181 Annex Northcote D.H. (1972) Chemistry of plant cell wall. Annu. Rev. Plant Physiol. 23, 113-132 to Atlan’s proposal, organiza- According Pla F. & Yan Y.F. (1984) Branching and func- tion should correspond to an optimum tionality of lignin molecules. J. Wood Chem. compromise between maximum informa- Technol. 4, 285-299
tion content (Hm!) and redundancy (R) tion effects due to random perturbations. both considered as a function of time. The second term, however, is positive Starting from Shannon’s definition: explaining a possible increase in organiza- tion and thus self-organization under the H t (1-R ) max &dquo; = effect of random perturbations. A self- differentiating H versus time, with the and organizing system appears, thus, to be assumption that time means accumulated redundant enough to sustain a continuous random perturbation from the environ- process of disorganization, first term, ment, one gets: constantly associated with reorganization dMt)f(1 -R)(dNm!ldt) + H (-dH/dQ (1 ) ax m and increased efficiency of the system due As perturbations decrease both H and to its reliability, i.e., its inertia opposed to ax m R, the first term on the right side of eqn. 1 random perturbations, the second term of is negative and thus shows disorganiza- eqn. 1.