Temperature dependence of photovoltages generated by bacteriorhodopsin.

The temperature dependence of the photovoltage developed by a model membrane containing bacteriorhodopsin (BR) is studied. The model membrane is formed by first coating a thin Teflon sheet with lipid and then fusing BR vesicles to it. The time course of the photoresponse is resolved down to 1 microsecond. The photoresponse is taken to be a sum of exponentials. Exponential time constants and amplitudes are determined by an analysis of the photoresponse with a photovoltage vs. log time plot, correlation filter, and nonlinear least-squares routine. The photovoltage is taken to be the sum of three exponentials but only two of the three time constants are resolved. Both are temperature dependent and indicate a thermally activated transport process. The corresponding activation energies are 55 kJ/mol and 62 kJ/mol. Since the photovoltage is proportional to charge times displacement the corresponding charge displacements are 11 and 34 A assuming a total displacement of 45 A. The remaining exponential term corresponds to a small negative transient in the photovoltage that has a rise time less than 1 microsecond even at -20 degrees C. The calculated charge displacement is estimated to be less than 2 A.