The pH-dependence of the O and P intermediates in the photocycle of bacteriorhodopsin (bR) on the intensity and duration of the exciting flash was investigated for bR glycerol suspensions and bR gelatin films. Green and red laser flashes (532 and 670 nm) were utilized to generate a photoequilibrium state of bR and O at ambient temperature, and UV-vis spectroscopy was used to determine the photoconversion for the bR suspensions and films. The maximal concentration of the O intermediate was observed to be pH-dependent and the dependency was most pronounced at a slightly alkaline pH values. The photochemical conversion from the O to P intermediate was investieated for both bR suspensions and films. The P intermediate was only found in bR gelatin film. These results indicate that bR gelatin film may be an attractive candidate for the information storage based on P intermediate. It is possible, with red light, to create photoproducts which are thermally stable at ambient temperature and that can be photochemically erased. (c) 2006 Elsevier Inc. All rights reserved.