RIBOFLAVIN-PHOTOSENSITIZED OXIDATIVE DEGRADATION OF A VARIETY OF LIGNIN MODEL COMPOUNDS

Oxidation of several lignin model compounds with alkylated paraphenolic groups by photosensitizing riboflavin (RF). rose bengal (RB) and methylene blue (MB) was examined. Photosensitizing RF cleaved l–(3'-4'-diethoxyphenyl)-1.3 dihydroxy-2-(4-methoxyphenyl)propane (I). 4-ethoxy-3-methoxyphenylglycerol-(3-guaiacyl ether (II) and l-(4'-ethoxy-3'-methoxyphenyI)-1,3 dihydroxypropane (IV) at their respective C_α-C_β bonds. Riboflavin also oxidized 3.4-diethoxy-benzaldehyde (VI) to the corresponding acid, and hydroxylated the conjugated olefin l-(4'-ethoxy-3'-methoxyphenyl)1.2 propene (III) to yield the initial product IV. In contrast, MB and RB hydroxylated III but had no effect on I, II, IV or VI under identical conditions. This suggested RF effected transformations via a hydrogen radical abstraction (Type I) rather than a ¹O₂ mediated reaction.
To confirm this, the effects of deuterium oxide (D_:0) and oxygen pressure on the photosensitizing dye reactions were examined. The effect of D₂0 on tryptophan (Trp) oxidation and hydroxylation of III by MB and RB was significant, indicating involvement of ¹O₂. D₂O had no significant effect on cleavage of the diarylpropane (I) and the olefin (III) by RF, indicating these reactions did not proceed by a Type II mechanism. While O₂ pressure effect on Trp oxidation by MB was insignificant, it had a large negative effect on cleavage of I by RF. These results, coupled with the relatively slow oxidation rate of Trp by RF. indicate that photosensitizing RF produces ¹O₂ inefficiently and is not effecting oxidation of these lignin model compounds via a Type II mechanism.