Charge equilibria and pK(a) of malvidin-3-glucoside by electrophoresis

Paper electrophoresis has been used over the pH range 1.2 to 10.4 to measure apparent pK(a) values for malvidin-3-O-glucoside of pK(a(1)) 1.76+/-0.07, pK(a(2)) 5.36+/-0.04, and pK(a(3)) 8.39+/-0.07. Using solvent partitioning between buffered aqueous solutions and n-octanol, several micro-pK(a) constants for malvidin-3-O-glucoside were also identified, highlighting the complex nature of malvidin-3-glucoside equilibria. As a nonspectrophotometric procedure, the charge-dependent electrophoretic mobility method provided independent information on the net charge and color of anthocyanin species at wine pH (ca. 3.6). At this pH, the color of malvidin-3-glucoside in red wines is consistent only with the uncharged quinonoidal base as a major colored component of the equilibria.