Analysis of Primary Photochemical Processes in Bacteriorhodopsin

The sequence of primary events following light absorption by light adapted bacteriorhodopsin (bR 570 ) is considered by analyzing recent picosecond absorption and emission data. The analysis is facilitated by theoretical calculations which allow us to characterize the properties of the first excited singlet state. It is concluded that excitation leads to the eventual population of a photochemically important nonfluorescent excited state (I) which decays into a photoproduct (J 625 )‐ In J 625 , 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 reflects a relaxation of the protein environment involving proton transfer.