The light-driven proton pump bacteriorhodopsin (bR) undergoes a bleaching reaction with hydroxylamine in the dark, which is markedly catalyzed by light. The reaction involves cleavage of the (protonated) Schiff base bond, which links the retinyl chromophore to the protein. The catalytic light effect is currently attributed to the conformational changes associated with the photocycle of all-trans bR, which is responsible for its proton pump mechanism and is initiated by the all-trans → 13-cis isomerization. This hypothesis is now being tested in a series of experiments, at various temperatures, using three artificial bR molecules in which the essential C13=C14 bond is locked by a rigid ring structure into an all-trans or 13- cis configuration. In all three cases we observe an enhancement of the reaction by light despite the fact that, because of locking of the C13=C14 bond, these molecules do not exhibit a photocycle, or any proton- pump activity. An analysis of the rate parameters excludes the possibility that the light-catalyzed reaction takes place during the ~20-ps excited state lifetimes of the locked pigments. It is concluded that the reaction is associated with a relatively long-lived (μs-ms) light-induced conformational change that is not reflected by changes in the optical spectrum of the retinyl chromophore. It is plausible that analogous changes (coupled to those of the photocycle) are also operative in the cases of native bR and visual pigments. These conclusions are discussed in view of the light-induced conformational changes recently detected in native and artificial bR with an atomic force sensor.
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