On the Nature of the Primary Photochemical Events in Bacteriorhodopsin

Abstract

The sequence of primary events following light absorption by light-adapted (all-trans) bacteriorhodopsin (BR ^t₅₇₀ ) is considered by analyzing recent picosecond absorption and emission data. Theoretical calculations are carried out in order to predict the ground state and the excited state (singlet-singlet) absorption spectra of the molecule. It is concluded that excitation leads to a photochemically important non-fluorescent excited state (I) which decays into a photoproduct (J₆₂₅).in J₆₂₅, which is most probably a ground state molecule, the chromophore has undergone a structural change, presumably trans→13-cis isomerization. It is suggested that the subsequent process $\text{J}_{\text{625}} \underrightarrow {\text{llpsec}}\text{K}_{\text{610}}$, reflects a relaxation of the protein environment involving proton transfer. The conclusions preclude models which, based on the assumption that J₅₂₅ is a primary excited state, attribute the first photochemical event to proton translocation. Based on a variety of common features, the general picture bears also on the nature of the primary processes in vision.