TEMPERATURE AND PH-DEPENDENT INVERSION OF PHOTOELECTRIC RESPONSE IN BACTERIORHODOPSIN

The temperature and pH-related polarity inversions of transient photoc urrents developed from randomly deposited bacteriorhodopsin (bR) films (on transparent conductive ITO glass) were investigated. It was obser ved that the photocurrent reversed its polarity when the bulk pH was c hanged from alkaline (pH = 8) to acid (pH = 3), but under extreme acid ic 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 pho toisomerization. Moreover. heating the film (from 20 to 60 degrees C) will also trigger a current inversion which differs from the pH-induce d reversal. It evolves from a peak splitting process rather than throu gh a cancellation step of the photocurrent. The splitting and the fina l inversion, which are pH-dependent, occur symmetrically on both the p ositive and negative stroke of the differential photocurrent. Comparat ive studies have been carried out and an explanation is discussed in t erms of the inversion of the proton pump sequence.