title: Photoexcitation of the O-intermediate in bacteriorhodopsin mutant L93A.
abstract: During the extended lifetime of the O-state in bacteriorhodopsin (bR) mutant L93A, two substates have been distinguished. The first O-intermediate (OI) is in rapid equilibrium with N and apparently still has a 13-cis chromophore. OI undergoes a photoreaction with a small absorbance change, positive charge transport in the pumping direction, and proton release and uptake. None of these effects was detected after photoexcitation of the late O (OII). The most likely interpretation of the effects seen is an accelerated return of the molecule from the OI- to the bR-state. However, with a lifetime approximately 140 ms, the reaction cannot account for the observed high pumping efficiency of the mutant under continuous illumination. We suggest that OII corresponds to the O-intermediate with a twisted all-trans chromophore seen in the photocycle of wild-type bR, where the 13-cis OI-intermediate under the usual conditions does not accumulate in easily detectable amounts and, therefore, has generally been overlooked. Both the OI- and OII-decays are apparently strongly inhibited in the mutant.
authors.affiliation: Institute of Biophysics, Biological Research Centre of the Hungarian Academy of Sciences, H-6701 Szeged, Hungary.
authors.forename: R
authors.initials: R
authors.lastname: Tóth-Boconádi
authors.forename: L
authors.initials: L
authors.lastname: Keszthelyi
authors.forename: W
authors.initials: W
authors.lastname: Stoeckenius
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology / Q000528:radiation effects; D001436:Bacteriorhodopsins / Q000502:physiology* / Q000528:radiation effects*; D002478:Cells, Cultured; D003624:Darkness; D004307:Dose-Response Relationship, Radiation; D019617:Halobacterium salinarum / Q000235:genetics / Q000528:radiation effects*; D007834:Lasers; D008564:Membrane Potentials / Q000502:physiology* / Q000528:radiation effects*; D009154:Mutation; D010507:Periodicity; D010775:Photic Stimulation / Q000379:methods; D018462:Photobiology / Q000379:methods; D011522:Protons; D018717:Purple Membrane / Q000502:physiology* / Q000528:radiation effects*; D012172:Retinaldehyde / Q000502:physiology
chemical_list: D011522:Protons; D001436:Bacteriorhodopsins; D012172:Retinaldehyde