OXIDATIVE REACTIONS OF CAFFEIC ACID IN MODEL SYSTEMS CONTAINING POLYPHENOL OXIDASE

Enzymatic oxidation products of caffeic acid were monitored by HPLC with diode array detection in model solutions containing different amounts of polyphenol oxidase. Caffeic acid o-quinone and a number of secondary products were characterized by their UV-visible spectra. Caffeic acid o-quinone was generated enzymatically within the first minutes of incubation, at a maximum concentration which increased at higher enzyme concentrations. It was highly unstable and rapidly replaced by various condensation products. The first and major products were formed by reaction of caffeic acid and its o-quinone. These products were themselves enzymatically oxidizable. Their maximum level was higher, but it was reached after longer incubation times with lower enzyme concentrations. Following initial enzymatic oxidation steps, the reactions proceeded further essentially via nonoxidative mechanisms, yielding a series of relatively polar compounds.