Spectrally silent intermediates during the photochemical reactions of Salinibacter sensory rhodopsin i

Salinibacter sensory rhodopsin I (SrSRI) is a microbial rhodopsin discovered from the eubacterium Salinibacter ruber. It is thought to be a photoreceptor engaging the signal transductions for both positive and negative phototaxis. To elucidate the photoreactions of SrSRI in the presence and absence of chloride ions, we measured the refractive index change after the photoexcitation by the transient grating method. As a result, two spectrally silent processes were identified after the formation of M intermediate, and we named the spectrally identical intermediates M₁, M₂, and M₃. The enthalpy changes (ΔH) were estimated as ΔH = 136, 99, and 63 kJ/mol for K, M₁, and M₂ intermediates, respectively. The ΔH values were significantly decreased (36-55 kJ/mol) by the removal of chloride ions, suggesting their importance for structural changes of SrSRI. Volume expansions of SrSRI were observed on the spectrally silent steps (44 and 11 mL/mol). They may be related to the signaling process because blue-shifted intermediates of sensory rhodopsins are thought to be active state(s) for phototaxis.