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 S-1 states, i.e., reactive and nonreactive S-1 states at physiological pH, but their origin remained unclear. In this study, we examined the S-1 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 S-1 state is predominantly formed at pH >9, but its population drastically decreases with decreasing pH while the population of the nonreactive S-1 state(s) increases. The pH dependence of the relative population of the reactive S-1, 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 S-1 states in KR2. The quantitative analysis of the time resolved absorption data led us to conclude that the reactive and nonreactive S-1 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 S-1 states in KR2. So far, the generation of multiple S, 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 S-1 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 S-1 states which have been widely observed for various rhodopsins.