ACCELERATED LIGHT-INDUCED AGING OF α-, β-, AND γ-13C-ENRICHED CELL WALL-DEHYDROGENATION POLYMERS STUDIED WITH SOLID STATE 13C NMR SPECTROSCOPY

Abstract

ABSTRACT Light-induced aging of lignocellulosic materials has been studied with a new technique involving selectively α-, β-, and γ-13C-enriched cell wall-dehydrogenation polymers (CW-DHPs) and solid state 13C NMR spectroscopy. The results from cross-polarization magic angle spinning (CP/MAS) 13C NMR experiments of unirradiated and irradiated CW-DHP have revealed mainly a decrease in the amount of end-groups of both coniferaldehyde and coniferyl alcohol type. The results suggest that these end-groups become saturated and that the terminal functionalites, i.e., γ-aldehyde and γ-hydroxymethyl groups, at least to some extent, are retained. The results indicate further that no detectable cleavage of the β-O-4 bonds occurs in the examined lignocellulosic model. In terms of proposed mechanisms of yellowing, there is marginal evidence that up to 2% of the α-labeled sites are converted by irradiation to α-carbonyls (aldehyde or ketones); moreover, we cannot dismiss the possibility that the precursor structures giving rise to these few α-carbonyls are β-O-4 structures. The 13C-enriched CW-DHP was formed directly on spruce (Picea abies) wood tissue (differentiating xylem) by administering selectively 13C-labeled coniferin at pH 6.0 in the presence of glucose oxidase and β-glucosidase, i.e., no phenol-oxidizing enzyme was added and the wood cells’ own enzymes polymerized the precursor.