Laccase (EC 1.10.3.2) catalyses the conversion of procyanidin B-2 (epicatechin dimer) to type A-2

We report here the conversion of procyanidin B-2 (epicatechin dimer) to the procyanidin A-2 dimer by laccase (EC 1.10.3.2). The identity of the A-2 dimer was determined by its mass spectrum (m/z = 577), as well as by comparison with a product formed with the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical. This latter system was previously shown to transform procyanidin type-B to type-A. Other quinonoid-type products, including an oxidation product (m/z = 574.6) of A-2, were also observed. We propose that in plants the conversion of natural procyanidins type-B to type-A might occur by enzymatic means rather than via a radical process as was previously suggested.