title: A nonbleachable rhodopsin analogue with a slow photocycle.
abstract: Archaeal rhodopsins, e.g. bacteriorhodopsin, all have cyclic photoreactions. Such cycles are achieved by a light-induced isomerization step of their retinal chromophores, which thermally re-isomerize in the dark. Visual pigment rhodopsins, which contain in the dark state an 11-cis retinal Schiff base, do not share such a mechanism, and following light absorption, they experience a bleaching process and a subsequent release of the photo-isomerized all-trans chromophore from the binding pocket. The pigment is eventually regenerated by the rebinding of a new 11-cis retinal. In the artificial visual pigment, Rh(6.10), in which the retinal chromophore is locked in an 11-cis geometry by the introduction of a six-member ring structure, an activated receptor may be formed by light-induced isomerization around other double bonds. We have examined this activation of Rh(6.10) by UV-visible and FTIR spectroscopy and have revealed that Rh(6.10) is a nonbleachable pigment. We could further show that the activated receptor consists of two different subspecies corresponding to 9-trans and 9-cis isomers of the chromophore. Both subspecies relax in the dark via separate pathways back to their respective inactive states by thermal isomerization presumably around the C(13)=C(14) double bond. This nonbleachable pigment can be repeatedly photolyzed to undergo identical activation-relaxation cycles. The rate constants of these photocycles are pH-dependent, and the half-times vary between several hours at acidic pH and about 1.5 min at neutral to alkaline pH, which is several orders of magnitude longer than for bacteriorhodopsin.
authors.affiliation: Biophysics Group, Institut für Molekulare Medizin und Zellforschung, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Strasse 9, D-79104 Freiburg, Germany. Reiner.Vogel@biophysik.uni-freiburg.de
authors.forename: Reiner
authors.initials: R
authors.lastname: Vogel
authors.forename: Gui-Bao
authors.initials: GB
authors.lastname: Fan
authors.forename: Steffen
authors.initials: S
authors.lastname: Ludeke
authors.forename: Friedrich
authors.initials: F
authors.lastname: Siebert
authors.forename: Mordechai
authors.initials: M
authors.lastname: Sheves
mesh_terms: D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010777:Photochemistry; D012243:Rhodopsin / Q000737:chemistry / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D012243:Rhodopsin