Bisaurones - Enzymatic Production and Biological Evaluation
The Trametes versicolor laccase catalyzed oxidation of chalcone butein afforded four dimers of aurone sulfuretin (i.e two regioisomeric pairs of diasteromers, 1 - 4) as the major products. The formation of the dimers was explained by a two step process involving initial cyclization of butein into aurone sulfuretin, followed by combining of two molecules of sulfuretin. The coupling process occurred between 2,10-double bond of one molecule of sulfuretin and (3′,4′) catechol group of the another to yield dimeric structure. This was confirmed by the experiment involving laccase catalyzed oxidation of sulfuretin yielding the same dimeric bisaurones. Compounds 1, 3 and 4, were isolated using semipreparatve HPLC and characterized by detailed analysis of NMR, MS, IR, and UV-Vis data. The structure of compound 2, isolated as a mixture containing ca. 25% of 1 was proposed by comparison of 1H NMR data to 1 and by LC-ESIMS analyses. The starting chalcone butein and products of the biocatalytic transformation, aurone sulfuretin and sulfuretin dimers 1, 3 and 4 were evaluated for cytotoxic and antioxidative properties in vitro using healthy human fibroblasts (MRC5) cell line. Biotransformation products showed lower cytotoxicity but higher antioxidative properties. C. coggygria bark methanol extract rich in butein and sulfuretin was also biotransformed by laccase. Transformed extract exhibited significantly improved antioxidative activities.