Temperature and pH-dependent inversion of photoelectric response in bacteriorhodopsin

Abstract

The temperature and pH-related polarity inversions of transient photocurrents developed from randomly deposited bacteriorhodopsin (bR) films (on transparent conductive ITO glass) were investigated. It was observed that the photocurrent reversed its polarity when the bulk pH was changed from alkaline (pH=8) to acid (pH=3), but under extreme acidic conditions (pH=1), in combination with high salt (saturated KCl), the reversed photocurrent regained the same polarity as that obtained in alkaline solution, which supports the notion that at extremely low pH, a high Cl^- concentration can catalyze the rate of the retinal photoisomerization. Moreover, heating the film (from 20 to 60°C) will also trigger a current inversion which differs from the pH-induced reversal. It evolves from a peak splitting process rather than through a cancellation step of the photocurrent. The splitting and the final inversion, which are pH-dependent, occur symmetrically on both the positive and negative stroke of the differential photocurrent. Comparative studies have been carried out and an explanation is discussed in terms of the inversion of the proton pump sequence.