Influence of structure on the ionisation constants of anthocyanin and anthocyanin-like wine pigments

The macro ionisation constants of malvidin-3-glucose, vitisin A and the two pairs of diastereoisomers of 8,8-ethyl-linked pigments formed between malvidin-3-glucose and catechin or epicatechin were determined using high voltage paper electrophoresis (HVPE) and compared as a function of their chemical structures. Their UV–vis spectral characteristics were also compared. The low pKa1 value of vitisin A (0.95 ± 0.10) compared to malvidin-3-glucose (1.76 ± 0.07) reflects the extended delocalisation of the former due to the presence of the fourth ring and the electron withdrawing effect of the carboxyl group. The presence of the ethyl bridge at the 8-position of the 8,8-ethyl-linked malvidin-3-glucose–(epi)catechin give rise to slightly higher pKa1 values (2.33–2.44) due to the electron donating effect of the alkyl group into the delocalised electron system. Similarly, the loss of a second proton, reflected by the pKa2 values is influenced by the 8-ethyl substituent. Small differences in pKa1 and pKa2 values between the different diasterioisomers reflects the impact of variation in three-dimensional structures. pKa1 and pKa2 values for these compounds determined by HPVE agree well with those determined by UV–vis spectroscopy indicating that the latter are true ionisation constants rather than a composite of ionisation and hydration constants.