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 B

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...