Origin of the Reactive and Nonreactive Excited States in the Primary Reaction of Rhodopsins: pH Dependence of Femtosecond Absorption of Light-Driven Sodium Ion Pump Rhodopsin KR2.

KR2 is the first light-driven Na+-pumping rhodopsin discovered. It was reported that the photoexcitation of KR2 generates multiple S1 states, i.e., "reactive" and "nonreactive" S1 states at physiological pH, but their origin remained unclear. In this study, we examined the S1 state dynamics of KR2 using femtosecond time-resolved absorption spectroscopy at different pH's in the range from 4 to 11. It was found that the reactive S1 state is predominantly formed at pH >9, but its population drastically decreases with decreasing pH while the population of the nonreactive S1 state(s) increases. The pH dependence of the relative population of the reactive S1 state correlates very well with the pH titration curve of Asp116, which is the counterion of the protonated retinal Schiff base (PRSB) in KR2. This strongly indicates that the deprotonation/protonation of Asp116 is directly related to the generation of the multiple S1 states in KR2. The quantitative analysis of the time-resolved absorption data led us to conclude that the reactive and nonreactive S1 states of KR2 originate from KR2 proteins having a hydrogen bond between Asp116 and PRSB or not, respectively. In other words, it is the ground-state inhomogeneity that is the origin of the coexistence of the reactive and nonreactive S1 states in KR2. So far, the generation of multiple S1 states having a different photoreactivity of rhodopsins has been mainly explained with the branching of the relaxation pathway in the Franck-Condon region in the S1 state. The present study shows that the structural inhomogeneity in the ground state, in particular that of the hydrogen-bond network, is the more plausible origin of the reactive and nonreactive S1 states which have been widely observed for various rhodopsins.