Absorption and photochemistry of sensory rhodopsin--I: pH effects.

Pyranine (8-hydroxyl-1,3,6-pyrene-trisulfonate) was used as a pH-probe to test whether there is a light-induced proton release to the bulk phase during the photochemical reaction cycle of sensory rhodopsin-I (SR-I). We conclude that the retinylidene Schiff-base proton is retained by SR-I-containing envelope vesicles during the SR-I photocycle under the conditions described here. Bacteriorhodopsin containing vesicles were used as a control to show that light-induced proton release can be observed under identical data acquisition parameters as those used for SR-I-containing vesicles. In addition, the effects of extravesicular pH on the absorption maximum (lambda max) and the SR-I photocycle were studied. SR-I properties are insensitive to pH in the range approximately 3 to approximately 8 with lambda max remaining at 587 nm. The lambda max shifts to 565 nm below pH 3.0 and to 552 nm at pH 10.8 with an apparent pKa of 8.5. Flash-induced absorbance changes of SR-I are described under neutral, alkaline and acidic conditions. The neutral, alkaline and acid SR-I forms each undergo similar photoreactions producing long-lived (> 500 ms decay half-time) blue-shifted intermediates. The UV/near-UV absorption of the photoproducts from neutral and alkaline SR-I indicate a deprotonated Schiff base, whereas acid SR-I produces a species with lambda max > 460 nm indicative of a protonated Schiff base.