Nativity of Lignin Carbohydrate Bonds substantiated by Biomimetic Synthesis.

The question of whether lignin is covalently linked to carbohydrates in native wood, forming what is referred to as lignin carbohydrate complexes (LCC), still lacks unequivocal proofs. This is mainly due to the need of isolating lignin from woody materials prior to analysis, at conditions leading to partial chemical modification of the native wood polymers. Thus, the correlation between the structure of the isolated LCC and LCC in situ remains open. As a way to circumvent the problematic isolation, biomimicking lignin polymerization in vivo and in vitro is an interesting option. Herein, we report the detection of lignin carbohydrate bonds in the extracellular lignin formed by tissue-cultured Norway spruce cells, and in modified biomimetic lignin synthesis (dehydrogenation polymers). Semi-quantitative 2D heteronuclear singular quantum coherence (HSQC)-, 31P - and 13C-NMR spectroscopies were applied as analytical tools. Combining results from these systems, four types of lignin carbohydrate bonds were detected; benzyl ether, benzyl ester, gamma-ester and phenyl glycoside linkages providing direct evidence of lignin carbohydrate bond formation in biomimicked lignin polymerization. Based on our findings, we propose a sequence for lignin carbohydrate bond formation in plant cell walls.