Biomimetic Initiation of Lignol Dehydropolymerization with Metal Salts

Abstract

Dehydropolymerization of p-hydroxycinnamyl alcohols with metal salt oxidants can lead to dehydropolymerisates (DHPs) that more closely. resemble native lignin than those prepared by conventional techniques. Intermediate dimers, trimers, and tetramers obtained by this technique have provided the building blocks that are necessary for precise characterization of the DHPs by 13 C NMR spectroscopy. Individual dilignols have been used for specific preparation of higher oligolignols with readily definable (i.e. interpretable) features. Guaiacyl DHPs prepared with metal salts exhibit, in certain cases, distributions of interunit linkages more representative of native lignin than those prepared by conventional enzymatic techniques.

13C NMR Characterization of Guaiacyl, Guaiacyl/Syringyl and Syringyl Dehydrogenation Polymers

“…Also, many of the ß-O-4 entities in G-DHPs are crosslinked by a-O-4 bonds. These linkages have been previously found in DHPs by NMR methods (Nimz and Lüdemann 1976;Gagnaire and Robert 1977;Landucci and Ralph 1996). However, the occurrence of non-cyclic benzyl-aryl ether bonds in lignin have been somewhat controversial and estimates ranging from about 7% to zero have been reported ( Adler 1977;Nimz 1981;Leary and Sawtell 1984).…”

“…The broad and relatively featureless appearance of ] H NMR spectra of DHPs are similar to corresponding spectra of MWLs and are indicative of their polymeric nature (Brunow and Lundquist 1980). In contrast, a 1 H NMR spectrum that contains relatively sharp lines is typical of a mixture of low molecular weight oligolignols such as dimers to pentamers (Landucci and Ralph 1996). The signals in the carbon NMR spectra can be divided into three main types: those from carboxyls of acetyl groups (168-171 ppm); those from aromatic carbons, which can be divided further into quaternary (125-160ppm) and methine carbons (110-125ppm); and those from side-chain carbons (50-90ppm).…”