Laccase (EC 1.10.3.2) catalyses the conversion of procyanidin B-2 (epicatechin dimer) to type A-2 A. M. Osman * and K. K. Y. Wong Scion, Private Bag 3020, Rotorua, New Zealand Received 19 October 2006; revised 5 December 2006; accepted 13 December 2006 Available online 8 January 2007 Abstract—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. Ó 2006 Elsevier Ltd. All rights reserved. Procyanidins also known as condensed tannins are widely distributed in the plant kingdom. 1 They are believed to protect plants against predators. Because of their potential beneficial effects to human health, there is a growing interest in these natural prod- ucts. These condensed tannins possess a wide range of biological properties, including antioxidant activity, 2 antibacterial, 3 anticancer 4 and antiallergy effects. 5 For example, procyanidin B-2 was reported to have a hair growing effect, 6 whereas procyanidins of type-A linkage were shown to prevent urinary tract infections. 7 Although the transformation of natural procyanidin type-B to type-A in an alkaline/H 2 O 2 system 8 and in neutral conditions with DPPH (2,2-diphenyl-1-picr- ylhydrazyl) radicals 9 have been previously reported, no direct evidence for enzymatic mediation of this conver- sion has been presented. The only data that suggested a possible involvement of enzymatic activity in this transformation was a report by Tanaka et al., 10 in which phenazine derivatives of quinonoid structures were isolated after epigallocatechin was incubated in banana extracts. However, the isolated reaction products might have been formed via radical chemistry. Recently, we reported that, in the presence and absence of the mediator (2,2 0 -azino-bis(3-ethylbenzothiazoline- 6-sulfonic acid) (ABTS), laccase converts (+)-catechin to oligomeric products, including hydrophilic type-B dimers as well as hydrophobic type-A dimers. 11 We have presented evidence that laccase could convert an enzy- matically generated type-B dimer via an intermediate quinone methide to the type-A dimers. These dimers were shown to be formed by other enzymatic systems, such as peroxidase 12 and grape polyphenol oxidase. 13 However, the natural procyanidin type-B dimers are structurally different from the enzymatically generated type-B dimers. 13 They differ in the position of the in- ter-flavan bond, which in the former mainly occurs be- tween C-4 of a flavan-3-ol unit and C-8 of another flavan-3-ol unit (Fig. 1). The type-A dimer has an addi- tional ether bond between C-2 and O-7. In contrast to natural procyanidin type-B, the coupling of enzymati- cally produced type-B dimers occurs between the A-ring of a flavan-3-ol unit and the B-ring of another unit (head to tail polymerization). Because of this structural differ- ence it is of interest to investigate whether or not laccase is capable of also converting natural procyanidin type-B dimer to type-A. The reaction of procyanidin B-2 and DPPH radicals was performed in methanol and in aqueous ethanol (9:1). A previous report showed that DPPH radicals could trans- form procyanidin type-B to type-A under neutral condi- tions. 9 The reaction between laccase and PB-2 was 0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2006.12.075 Keywords: Laccase; Procyanidin dimers; Transformation; DPPH radical; Quinonoid structures. * Corresponding author. Tel.: +64 73435473; fax: +64 73435507; e-mail: Ahmed.Osman@Scionresearch.com Tetrahedron Letters 48 (2007) 1163–1167