title: pH-dependent photoisomerization of retinal in proteorhodopsin.
abstract: The early steps in the photocycle of the bacterial proton pump proteorhodopsin (PR) were analyzed by ultrafast pump/probe spectroscopy to compare the rate of retinal isomerization at alkaline and acidic pH values. At pH 9, the functionally important primary proton acceptor (Asp97, pK(a) = 7.7) is negatively charged; consequently, a reaction cycle analogous to the archaeal bacteriorhodopsin (BR) is observed. The excited electronic state of PR displays a pronounced biphasic decay with time constants of 400 fs and 8 ps. At pH 6 where Asp97 is protonated a similar biphasic decay is observed, although it is significantly slower (700 fs and 15 ps). The results indicate, in agreement to similar findings in other retinal proteins, that also in PR the charge distribution within the chromophore binding pocket is a major determinant for the rate and the efficiency of the primary reaction.
authors.affiliation: Institut für Physikalische und Theoretische Chemie, Marie-Curie-Strasse 11, Johann Wolfgang Goethe-Universität Frankfurt, 60439 Frankfurt am Main, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Huber
authors.forename: Thomas
authors.initials: T
authors.lastname: Köhler
authors.forename: Martin O
authors.initials: MO
authors.lastname: Lenz
authors.forename: Ernst
authors.initials: E
authors.lastname: Bamberg
authors.forename: Rolf
authors.initials: R
authors.lastname: Kalmbach
authors.forename: Martin
authors.initials: M
authors.lastname: Engelhard
authors.forename: Josef
authors.initials: J
authors.lastname: Wachtveitl
mesh_terms: D019843:Archaeal Proteins / Q000737:chemistry / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007700:Kinetics; D010777:Photochemistry; D017494:Proton Pumps / Q000737:chemistry / Q000378:metabolism; D012172:Retinaldehyde / Q000737:chemistry* / Q000378:metabolism*; D012243:Rhodopsin / Q000737:chemistry* / Q000378:metabolism*; D025604:Rhodopsins, Microbial; D013056:Spectrophotometry, Ultraviolet; D013057:Spectrum Analysis; D055672:Static Electricity
chemical_list: D019843:Archaeal Proteins; D017494:Proton Pumps; D025604:Rhodopsins, Microbial; C413548:proteorhodopsin; D012243:Rhodopsin; D012172:Retinaldehyde