Oxidative conversion of B- to A-type procyanidin trimer: Evidence for quinone methide mechanism

Two main types of procyanidin oligomers (A-type and B-type) are present in plants. Previous studies revealed that B-type procyanidin dimers can be converted into A-type dimers by oxidation. However, the mechanism lacks experimental proof. In this study, the conditions of a B-type procyanidin trimer, procyanidin C-1 (PC-1), conversion to A-type cinnamtannin B-1 (CTB-1), including temperature, pH value and four catalysts were investigated. The catalysts include DPPH, O2-, polyphenol oxidase and xanthine oxidase. Results show that the conversion is significantly affected by temperature and pH. Both free radicals and oxidases accelerate the conversion rate with the same products but show no increase in conversion ratio. Moreover, the isolation and identification of continuous oxidation products (P 1-P 4) from CTB-1 are discussed here. These results demonstrate that the oxidative conversion of B- to A- procyanidins in plants might involve a free radical-driven process or an enzyme-catalyzed free radical reaction via a quinone methide mechanism.