Non-enzymatic reduction of quinone methides during oxidative coupling of monolignols: implications for the origin of benzyl structures in lignins

Abstract

Lignin is believed to be synthesized by oxidative coupling of 4-hydroxyphenylpropanoids. In native lignin there are some types of reduced structures that cannot be explained solely by oxidative coupling. In the present work we showed via biomimetic model experiments that nicotinamide adenine dinucleotide (NADH), in an uncatalyzed process, reduced a beta-aryl ether quinone methide to its benzyl derivative. A number of other biologically significant reductants, including the enzyme cellobiose dehydrogenase, failed to produce the reduced structures. Synthetic dehydrogenation polymers of coniferyl alcohol synthesized (under oxidative conditions) in the presence of the reductant NADH produced the same kind of reduced structures as in the model experiment, demonstrating that oxidative and reductive processes can occur in the same environment, and that reduction of the in situ-generated quinone methides was sufficiently competitive with water addition. In situ reduction of beta-beta-quinone methides was not achieved in this study. The origin of racemic benzyl structures in lignins therefore remains unknown, but the potential for simple chemical reduction is demonstrated here.

Epigallocatechin gallate incorporation into lignin enhances the alkaline delignification and enzymatic saccharification of cell walls

“…Spectra were processed with Bruker’s Topspin 3.1 (Mac) software, using the central solvent peaks as internal references [δ H /δ C : acetone, 2.04/29.8; dimethylsulfoxide, 2.49/39.5 ppm]. Adiabatic 2D-HSQC (‘hsqcetgpsisp2.2’) and 2D-HSQC-TOCSY (‘hsqcetgpml’) experiments for DHP samples in the solution-state [58,59], and maize cell wall samples in a gel-state [60,61], were carried out as described previously. Processing used typical matched Gaussian apodization in F2 (LB = −0.3, GB = 0.001), and squared cosine-bell and one level of linear prediction (32 coefficients) in F1.…”

Lignins: A Twenty‐First Century Challenge

“…Interestingly, Holmgren et al (350) also attempted to demonstrate the reductive conversion of pinoresinol (69) into secoisolariciresinol (89) upon incubation of coniferyl alcohol (3) with horseradish peroxidase/H 2 O 2 in presence of NADH, i.e., in the absence of any (PLR) protein. As expected, no such reduction occurred in the absence of functional PLR; indeed, such experiments are generally carried out as controls in our enzyme assays.…”

Biocomposites based on lignin and plasticized poly(L‐lactic acid)

“…1 Because of its poly-disperse polyphenolic structure, with no extended sequences of regularly repeating units and variable degree of polymerization, there may be no two identical lignin macromolecules with the same primary sequence of phenyl units. Lignin is a natural polymer found in wood and in the secondary cell walls of plants and some algae.…”