Charge equilibria and pK values of 5-carboxypyranomalvidin-3-glucoside (vitisin A) by electrophoresis and absorption spectroscopy

Abstract Apparent p K values of the wine pigment, 5-carboxypyranomalvidin-3-glucoside (vitisin A), were determined using UV–vis spectroscopy, viz. p K a1 =0.98 (±0.10), p K H1 =4.51 (±0.03) and p K H2 =7.57 (±0.02). An additional ionisation constant at high pH (p K a4 =8.84±0.06) was established by high-voltage paper electrophoresis. These data in conjunction with previously published p K a values determined by high-voltage electrophoresis suggest that in wine (pH 3.2–3.8), 5-carboxypyranomalvidin-3-glucoside exists as a complex mixture of hydrated and non-hydrated, partially ionised species with the predominant species being the quinonoidal base ( λ max 498 nm).