Ground- and excited-state proton transfer in anthocyanins: From weak acids to superphotoacids

Malvidin-3,5-diglucoside (malvin), cyanidin-3,5-diglucoside (cyanin), and pelargonidin-3,5-diglucoside (pelargonin) are among the most representative anthocyanins because of their abundance in the most common red flowers and fruits. Anthocyanin color is directly affected by the pH-dependent chemistry of the red (acid) form of these compounds, while anthocyanin photostability is a function of the photophysics of the first excited singlet state. In the present work, we employ laser flash photolysis and picosecond time-correlated single-photon counting to determine the dynamics of the proton-transfer reactions of these three anthocyanins in the ground [deprotonation rate constants, k(d) = 1.3 x 10(6) s(-1) (pelargonin), 1.8 x 10(6) s(-1) (cyanin), and 3.8 x 10(6) s(-1) (malvin)] and first excited singlet state [deprotonation rate constants, k(d) = 4.3 x 10(10) s(-1) (pelargonin), 4.0 x 10(10) s(-1) (cyanin), and 1.6 x 10(11) s(-1) (malvin)], respectively. The ground- and excited-state proton-transfer rate constants for anthocyanins and for photoacids of the naphthol type are found to correlate with an empirical parameter related to the ionic character of the dissociable OH bond. The present results show that the typically weak fluorescence of the flavylium cation form of anthocyanins is due primarily to competitive ultrafast, adiabatic proton transfer to water. This process is highly efficient as an energy-wasting mechanism, as would be required by an in vivo role such as protection of plant tissues from potentially deleterious excess radiant energy.