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 winelike medium. When the reaction was performed in darkness at 45 °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 °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.
Keywords: Copper(II); white wine; (+)-catechin; xanthylium cation; model wine; browning; glyoxylic
acid; acetaldehyde; oxidation