Photocurrent from oriented membrane films containing acid-blue and acid-purple bacteriorhodopsin and its mutants

This paper investigates the fast photocurrent components, B1 and B2, from oriented bacteriorhodopsin (BR) membrane films at low pH, under pulsed laser excitation. Adding chloride ion changes the acid-blue BR to its acid-purple form. In the presence of chloride, the acid-purple BR shows a positive B2 component in the same direction as that of BR at neutral pH, indicating a rapid intramolecular charge transfer. In the absence of chloride, the acid-blue BR shows only a negative B1 with multi-components, indicating a rapid charge separation process associated with retinal photoisomerization. The multi-components in B1 are possibly formed due to the heterogeneity of the acid-blue BR. In addition, BR mutants, D85N and D115N, at low pH and in the presence of chloride, generate the B2 component as well. The observation of chloride-dependent B2 component in various cases at low pH, is in favor of a possible transient chloride ion transfer, although the nature of the charge being transferred cannot be identified so far.