Native Lignin. II. Native Aspen Lignin

The isolation of lignin with native-like structure

Searching for renewable alternatives for fossil carbon resources to produce chemicals, fuels and materials is essential for the development of a sustainable society. Lignin, a major component of lignocellulosic biomass, is an abundant renewable source of aromatics and is currently underutilized as it is often burned as an undesired side stream in the production of paper and bioethanol. This lignin harbors great potential as source of high value aromatic chemicals and materials. Biorefinery schemes focused on lignin are currently under development with aim of acquiring added value from lignin. However, the performance of these novel lignin-focused biorefineries is closely linked with the quality of extracted lignin in terms of the level of degradation and modification. Thus, the reactivity including the degradation pathways of the native lignin contained in the plant material needs to be understood in detail to potentially achieve higher value from lignin. Undegraded native-like lignin with an as close as possible structure to native lignin contained in the lignocellulosic plant material serves as a promising model lignin to support detailed studies on the structure and reactivity of native lignin, yielding key understanding for the development of lignin-focused biorefineries. The aim of this review is to highlight the different methods to attain "native-like" lignins that can be valuable for such studies. This is done by giving a basic introduction on what is known about the native lignin structure and the techniques and methods used to analyze it followed by an overview of the fractionation and isolation methods to isolate native-like lignin. Finally, a perspective on the isolation and use of native-like lignin is provided, showing the great potential that this type of lignin brings for understanding the effect of different biomass treatments on the native lignin structure.

Structure and action mechanism of ligninolytic enzymes

Lignin is the most abundant renewable source of aromatic polymer in nature, and its decomposition is indispensable for carbon recycling. It is chemically recalcitrant to breakdown by most organisms because of the complex, heterogeneous structure. The white-rot fungi produce an array of extracellular oxidative enzymes that synergistically and efficiently degrade lignin. The major groups of ligninolytic enzymes include lignin peroxidases, manganese peroxidases, versatile peroxidases, and laccases. The peroxidases are heme-containing enzymes with catalytic cycles that involve the activation by H2O2 and substrate reduction of compound I and compound II intermediates. Lignin peroxidases have the unique ability to catalyze oxidative cleavage of C-C bonds and ether (C-O-C) bonds in non-phenolic aromatic substrates of high redox potential. Manganese peroxidases oxidize Mn(II) to Mn(III), which facilitates the degradation of phenolic compounds or, in turn, oxidizes a second mediator for the breakdown of non-phenolic compounds. Versatile peroxidases are hybrids of lignin peroxidase and manganese peroxidase with a bifunctional characteristic. Laccases are multi-copper-containing proteins that catalyze the oxidation of phenolic substrates with concomitant reduction of molecular oxygen to water. This review covers the chemical nature of lignin substrates and focuses on the biochemical properties, molecular structures, reaction mechanisms, and related structures/functions of these enzymes.

Polychlorinated acetamides from the cyanobacterium Microcoleus lyngbyaceus

Several new compounds were isolated from the organic extract of the cyanobacterium Microcoleus lyngbyaceus, and their structures were determined by spectroscopic means. Polychlorinated acetamidoalkynes and alkanes were the major metabolites. 6-Acetamido-1,1,1-trichloroundecane, a positional isomer of the naturally occurring 5-acetamido-1,1,1-trichloroundecane, was synthesized in six steps from delta-decanolactone.

THE EFFECT OF INITIAL ACID CONCENTRATION ON THE LIGNIN ISOLATED BY THE ACIDOLYSIS OF ASPEN WOOD

A further study has been made of the isolation, by acidolysis, of lignin from aspen wood meal (Populustremuloides Michx.) using a dioxane–water (9:1) solvent medium containing hydrogen chloride and a low extraction temperature of 87 ± 2° for a short extraction time of 0.5 hour. Lignins so isolated included those: (a) under different initial acid concentrations of 0.0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, and 1.7 equivalents of HCl/liter of solvent medium; (b) from successive extractions under increasing initial acid concentrations of 0.2, 0.4, 0.8, and 1.6 equivalents/liter; and (c) from successive extractions under constant initial acid concentrations of 0.2 equivalent/liter.All the lignin fractions so isolated were studied with respect to their yield, methoxyl content, infrared absorption spectra, and the yields of vanillin and syringaldehyde obtained from them by oxidation using alkaline nitrobenzene.Evidence is presented in support of the non-homogeneity of the whole protolignin. This may be the result of the presence of possibly two basic types. One fraction, comprising around 70–85%, is most readily extracted, has a consistently high methoxyl content (ca. 21%), and is capable of being oxidized to give high yields of syringaldehyde and vanillin in a ratio close to 2.5:1. The second fraction is characterized by a greater difficulty of extraction, lower methoxyl content, and significantly lower yields of the aldehydes in a ratio closer to 1:1.

THE ISOLATION AND PROPERTIES OF LIGNINS OBTAINED BY THE ACIDOLYSIS OF SPRUCE AND ASPEN WOODS IN DIOXANE–WATER MEDIUM

A study has been made of the isolation of a lignin fraction from spruce and aspen wood meals using a low temperature (90–95 °C) acidolysis involving a dioxane–water (9:1) solvent medium containing the equivalent of 0.2 N hydrochloric acid. The effect of extraction time on the yield of precipitated lignin and ether-soluble material, and on the methoxyl content of these products, was investigated. For both species prolonged extraction times led to greater yields of lignin with lower methoxyl content.Further acidolysis of isolated spruce lignin led to no significant loss of methoxyl indicating the stability of this group to these reaction conditions. With increasing times of acidolysis of the aspen wood, a slow rise in the carbon–methyl/methoxyl ratio was observed for the isolated lignins. This is in agreement with earlier findings for the spruce lignin. The yields of vanillin and syringaldehyde from aspen lignin on alkaline nitrobenzene oxidation were much less than those from wood meal, and the relative yield of syringaldehyde was higher.It is suggested that this type of lignin may be only mildly modified during extraction and hence suitable for subsequent chemical investigation. The significance of the findings is discussed.