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Photoyellowing of lignocellulosic materials has been studied with a new technique based on solid state  13 C-NMR analysis of  13 C-enriched DHP in cell wall tissue. The selectively  13 C-enriched cell wall-dehydrogenation polymer (CW-DHP) was prepared directly on differentiating xylem from spruce ( Picea abies ) at pH 6.0 by administering β- 13 C-enriched coniferin in an enzymatic system consisting of glucose oxidase, β-glucosidase, and the naturally occurring water-insoluble enzymes remaining in the cell wall. The bonding pattern of the formed CW-DHP was found to be: 42% β-β, β-5, and β-1 substructures; 36% β-O-4 derived substructures; and 22% coniferyl alcohol and coniferaldehyde end-groups. The  13 C-NMR analysis of unirradiated and irradiated tissue revealed a decrease in the relative amount of coniferaldehyde and/or coniferyl alcohol end-groups during irradiation. Prolonged irradiation also introduced new signals centered at 37, 70, and 102 ppm. The results indicate that the present technique, with the formation of DHP in a naturally lignifying carbohydrate environment, has the potential of being a valuable tool for the study of structural changes of lignin during light-induced yellowing.

Solid State NMR Analysis of β-13C-Enriched Lignocellulosic Material During Light-Induced Yellowing