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

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.

ACKNOWLEDGMENTS

Financial support from the research foundation Stiftelsen Nils och Dorthi Troëdssons Forskningsfond is gratefully acknowledged. The authors thank Professor Noritsugu Terashima, Nagoya University, Japan, for valuable discussion. Thanks are also due to Ms. Brita Ohlsson, Department of Forest Products and Chemical Engineering, Chalmers University of Technology, for her skilful technical assistance and to Dr. Jón Hauksson, Department of Chemistry, Organic Chemistry, Umeå University, for preliminary solid state NMR spectroscopic analyses.

Notes

¹Certain commercial companies are named in order to specify adequately the experimental procedure. This in no way implies endorsement or recommendation by the authors or their agencies.