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.