A nonbleachable rhodopsin analogue with a slow photocycle. 
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.