A possible explanation for the structural inhomogeneity of lignin in LCC networks

Selda Aminzadeh,Liming Zhang,Gunnar Henriksson

doi:10.1007/s00226-017-0941-6

Abstract

Lignin has a very complex structure, and this is partly due to the monomers being connected by many different types of covalent bonds. Furthermore, there are multiple covalent bonds between lignin and polysaccharides in wood, and it is known that the structure of lignin covalently bound to the hemicellulose xylan is different to lignin bound to the hemicellulose glucomannan. Here, synthetic lignin (DHP) is synthesized at different pH and it is shown that lignin made at lower pH has a structure more similar to the lignin bound to xylan, i.e., having higher relative content of β-O-4 ethers. It is hypothesized that xylan due to its carboxylic acids forms a locally lower pH and thus “direct” the lignin structure to have more β-O-4 ethers. The biological significance of these results is discussed.

Highlights

True wood, i.e., secondary xylem produced by plants with secondary growth, such as trees and bushes, is among the most common biological tissues on earth (Sjostrom 1993)
There are multiple covalent bonds between lignin and polysaccharides in wood, and it is known that the structure of lignin covalently bound to the hemicellulose xylan is different to lignin bound to the hemicellulose glucomannan
A method for the quantitative preparation of relatively intact molecules from wood carrying lignin carbohydrate complexes (LCC) was developed, and the results indicated that lignin in wood cross-links different polysaccharides and that most lignin molecules in wood were covalently bound to polysaccharides (Lawoko et al 2006)

Summary

Introduction

I.e., secondary xylem produced by plants with secondary growth, such as trees and bushes, is among the most common biological tissues on earth (Sjostrom 1993). The latter polymer is in many ways an oddity within the group of biomolecules, being racemic, lacking well-defined primary structure, and most likely being branched (Boerjan et al 2003) The explanation for these unusual properties is that the lignin monomers, the monolignols, are polymerized by an uncatalyzed radical–radical coupling reaction from resonance stabilized radicals, directly or indirectly generated by enzymatic oxidation (Fig. 1) (Freudenberg 1959; Westermark 1982). One unexpected observation was that there were differences between the structures of the softwood lignin, dependent on whether it was covalently bound to arabinoxylan or to glucomannan; the glucomannan-bound lignin seems to contain much more of condensed bonds and has a lower content of b–O-4 bonds than the arabinoxylan-bound lignin (Lawoko et al 2005) This is of scientific interest, since it suggests that hemicelluloses direct the lignin structure, and of technical interest, since hardwoods that are rich in xylan are faster to be delignified during chemical pulping; b–O-4 bonds are much easier broken than condensed bonds in chemical pulping (Gierer 1980). Structures of synthetic lignins synthesized at different pH are discussed to investigate whether micro-pH gradients, such as described above, might be an explanation of the differences in lignin structure depending on which hemicellulose it is bound to

Materials and methods

Results and discussion

Conclusion