Relaxation process of photoexcited berberine via aggregation and dissociation state-dependent intramolecular electron transfer.

The fluorescence quantum yield of berberine in aqueous solution is significantly smaller than those of organic solution. The time profile of fluorescence intensity of berberine was analyzed by a bi-exponential function, showing that two kinds of states of berberine exist in the solutions. The observed fluorescence lifetime of shorter lifetime species of berberine in water (0.08 ns) was markedly smaller than those of organic solvents and the relative amplitude of the shorter lifetime was dominated in the aqueous solution. Thus, this shorter lifetime can be explained by the deactivation via intramolecular electron transfer. These two states of berberine were independent of pH. The enthalpy and entropy changes between these two states were - 23.2 kJ mol^-1 and - 90 J K^-1 mol^-1, supporting the aggregation of berberine. In the aggregation state, an electrostatic interaction between cationic berberine and chloride ion decreases the electron accepting ability of the isoquinoline moiety of berberine, resulting in the suppression of intramolecular electron transfer. Furthermore, in the presence of clay, the interaction between berberine and clay increased the fluorescence intensity of berberine and its lifetime, showing that the negative charge of clay suppresses the intramolecular electron transfer. Since the electron transfer quenching of the photo-excited berberine is advantageous for suppressing the phototoxic effect of berberine, the inhibition of berberine aggregation is an important process for the phototoxicity prevention.