EFFECTIVE LIGHT-INDUCED HYDROXYLAMINE REACTIONS OCCUR WITH C-13=C-14 NONISOMERIZABLE BACTERIORHODOPSIN PIGMENTS

The light-driven proton pump bacteriorhodopsin (bR) undergoes a bleach ing reaction with hydroxylamine in the dark, which is markedly catalyz ed by light. The reaction involves cleavage of the (protonated) Schiff base bond, which links the retinyl chromophore to the protein. The ca talytic light effect is currently attributed to the conformational cha nges associated with the photocycle of all-trans bR, which is responsi ble for its proton pump mechanism and is initiated by the ail-trans -- >13-cis isomerization. This hypothesis is now being tested in a series of experiments, at various temperatures, using three artificial bR mo lecules in which the essential C-13=C-14 bond is locked by a rigid rin g structure into an ail-trans or 13-cis configuration, In all three ca ses we observe an enhancement of the reaction by light despite the fac t that, because of locking of the C-13=C-14 bond, these molecules do n ot exhibit a photocycle, or any proton-pump activity. An analysis of t he rate parameters excludes the possibility that the light-catalyzed r eaction takes place during the similar to 20-ps excited state lifetime s of the locked pigments. It is concluded that the reaction is associa ted with a relatively long-lived (mu s-ms) light-induced conformationa l change that is not reflected by changes in the optical spectrum of t he retinyl chromophore. It is plausible that analogous changes (couple d to those of the photocycle) are also operative in the cases of nativ e bR and visual pigments. These conclusions are discussed in view of t he light-induced conformational changes recently detected in native an d artificial bR with an atomic force sensor.