The Role of Copper(II) in the Bridging Reactions of (+)-Catechin by Glyoxylic Acid in a Model White Wine

The influence of copper(II) on the bridging reactions between (+)-catechin and glyoxylic acid was studied in a white wine-like medium. When the reaction was performed in darkness at 45 degrees C, copper(II) increased the maximum levels of carboxymethine-linked (+)-catechin dimer and xanthylium cation pigment as monitored by high-performance liquid chromatography/photodiode array detection (HPLC/DAD) and liquid chromatography/mass spectrometry (LC/MS). At 10 degrees C, similar results were observed except that the xanthene intermediate was monitored and found to also increase in concentration at higher copper(II) concentrations. The kinetics for the formation of these species suggested that copper(II) accelerated the bridging of two (+)-catechin units by glyoxylic acid. The acid group of glyoxylic acid allowed copper(II) to influence this reaction, as no copper(II) enhancement was observed when acetaldehyde was used in place of glyoxylic acid.