Characterization of caffeic acid enzymatic oxidation by-products by liquid chromatography coupled to electrospray ionization tandem mass spectrometry.

The products of tyrosinase-catalyzed caffeic acid oxidation at pH 6.5 were investigated using high-performance liquid chromatography (HPLC) coupled to electrospray ionization mass spectrometry (ESI-MS). Aliquots of reaction mixtures were withdrawn at different times, varying from 0 to 24 h, and directly analysed by HPLC-ESI-MS and, in the case of 1 and 5 h, by HPLC-ESI-MS/MS to obtain structural information on caffeic acid derivatives. In particular, two different classes of caffeic acid dimers were identified: caffeicins-like structures and dimers originated by CC coupling between the benzene rings. Evidences for the presence of trimeric derivatives of caffeic acid were also obtained from MS data.