Lignin-Biosynthetic Study: Reactivity of Quinone Methides in the Diastereopreferential Formation of p-Hydroxyphenyl- and Guaiacyl-Type β-O-4 Structures

Abstract

p-Quinone methides are involved in lignin biosynthesis as transient intermediates, and the aromatization step has a great impact on the chemical structure of the resulting lignin. A series of quinone methides (QMs) were synthesized and allowed to react with water in pH 3–7 buffers at 25 °C to mimic the formation of p-hydroxyphenyl- and guaiacyl-type (H- and G-type, respectively) β-O-4 structures in gymnosperm-plant cell walls. Water addition occurred in 3-methoxy-substituted QMs (G-type QMs) with half-lives of 1.4–15 min. In contrast, nonsubstituted QMs (H-type QMs) were very labile; they were aromatized to β-O-4 products with half-lives of only 10–40 s. The rapid aromatization in H-type QMs may provide an advantage over G-type species for efficiently driving the lignin-polymerization cycle, which possibly contributes to the development of highly lignified compression wood. In the water-addition reaction, the threo isomers of the β-O-4 products were stereopreferentially formed more than the erythro isomers from both G- and H-type QMs (erythro/threo ratios of 24:76 and 50:50, respectively). The proportion of erythro isomers was higher at lower-pH conditions. This pH-dependent trend agrees with findings from a previous study on 3,5-dimethoxy-substituted (syringyl-type, S-type) QMs; thus, this pH-dependent trend is common in H-, G-, and S-type lignin-related QMs. Higher threo-selectivity was obtained by changing the β-etherified aromatic rings from G- to H-type. A similar but weaker effect was also observed by changing the QM moiety from G- to H-type.

Difference in enzymatic dehydrogenative polymerization of dilignols using horseradish peroxidase and crude enzyme obtained from Japanese cypress (Chamaecyparis obtusa)

“…However, this study suggested that the reactivity of erythro form was slightly higher than that of threo form. On the other hand, in the water-addition reaction to quinone methide of I, completely opposite result was shown recently, where threo was preferentially formed more than erythro [17]. The recent study and our experiment were conducted in vitro, and thus may not perfectly be a mimic of lignin biosynthesis.…”

Revisiting the mechanism of β-O-4 bond cleavage during acidolysis of lignin VII: acidolyses of non-phenolic C6-C2-type model compounds using HBr, HCl and H2SO4, and a proposal on the characteristic action of Br− and Cl−

“…The α-bromo-substituting derivative of compound I, 1-bromo-2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethane (VII, Fig. 2), was synthesized from compound I in chloroform-d by adding bromotrimethylsilane, following the method described in our previous report [25], for use as a starting compound in an acidolysis. The obtained chloroform-d solution was analyzed by 1 H-NMR to confirm its formation, disappearance of compound I, and absence of compound II.…”

Stereoselective Formation of β-O-4 Structures Mimicking Softwood Lignin Biosynthesis: Effects of Solvent and the Structures of Quinone Methide Lignin Models

“…Previously, as described above, we conducted water addition experiments similar to those by Brunow’s group, for mimicking the formation of H-type β-O-4 structures in compression wood lignin biosynthesis . The threo -preferential water addition was observed both for H- and G-type QMs examined at pH 3.5–7.…”

“…The structural variation of QMs and their chemical properties are thought to influence the polymerization efficiency and the structure of the resulting lignins. − Kobayashi et al reported in their study of monolignol oxidizability, that coniferyl alcohol (G-type) was consumed faster than p -coumaryl alcohol (H-type) in a peroxidase–H 2 O 2 system. The oxidation potentials determined by cyclic voltammetry indicated that p -coumaryl alcohol can be less oxidized than coniferyl alcohol .…”