Proton transfer in anthocyanins and related flavylium salts.: Determination of ground-state rate constants with nanosecond laser flash photolysis

A new method for the determination of proton-transfer rate constants in the ground state of anthocyanins and related flavylium salts is described. The method is based on the well-known pK(a) shift of phenols on going from ground to the first excited singlet state coupled to the typically very fast excited state proton transfer and very short lifetimes (picosecond range) occurring in these compounds. Under these conditions, a nanosecond light pulse instantaneously shifts the ground-state equilibrium, after which the ground-state transients can be monitored with nanosecond or microsecond resolution. The method is successfully applied to the determination of the deprotonation rate constants, k(d), of two synthetic flavylium salts and a natural anthocyanin (7-hydroxy-4-methylflavylium chloride (HMF), 4',7-dihydroxyflavylium chloride (DHF), and malvidin-3-glucoside chloride (oenin), respectively) and the protonation rate constants, k(p), of their conjugate quinonoidal bases in the ground state. For all three flavylium cations, the protonation of the ground state base form is essentially diffusion-controlled, and the deprotonation occurs in the submicrosecond range. Our directly determined rate constants are 2 orders of magnitude greater than previous values estimated for oenin by T jump. The flash photolysis approach utilized in the present work seems to be the only technique available for measurement of the kinetics of proton transfer in anthocyanins. In addition, our results show clear laser-induced perturbation of the ground-state protonation of oenin. providing the first direct evidence for excited-state proton transfer as a significant energy dissipation process in natural anthocyanins.