Color stabilization of anthocyanins: Effect of SDS micelles on the acid-base and hydration kinetics of malvidin 3-glucoside (Oenin)

The origin of the substantial stabilization of the color of the natural anthocyanin malvidin-3-glucosyl chloride (Oenin) in aqueous micellar SDS solutions has been elucidated by employing laser flash photolysis and pH-jump techniques to determine all four rate constants involved in the acid-base and hydration reactions of Oenin. These rate data show that the pronounced stabilizing effect of SDS micelles on the color of aqueous solutions of oenin, as measured by the shift in the bleaching constant (pK(1/2)) from moderately acid to near-neutral pH, cannot be attributed just to the difference in local pH between the aqueous and micellar phases. Thus, the major contribution to the SDS-induced shift in the acid-base equilibrium constant arises from the large decrease in the deprotonation rate constant in SDS micelles (k(d) = 2 x 10(5) s(-1)) with respect to water (k(d) = 5.0 x 10(6) s(-1)). Although the hydration rate constant also decreases in SDS micelles (k(h) = 4.6 x 10(-3) s(-1) versus k(h) = 8.5 x 10(-2) s(-1) in water), the major contribution to the observed shift in the hydration equilibrium constant comes from the back reaction (k-(h) = 7.1 x 10(3) 1(.)mol(-1.)s(-1) in SDS versus k-(h) = 34 1(.)mol(-1.)s(-1) in water). The present results conclusively demonstrate that color stabilization by anionic SDS micelles involves significant preferential stabilization of the cationic form of oenin with respect to the neutral base, hemiacetal and E-chalcone forms, resulting in profound changes in the energetics of deprotonation and hydration, the two key equilibria that affect color.