RAPID PH CHANGE DUE TO BACTERIORHODOPSIN MEASURED WITH A TIN-OXIDE ELECTRODE

The photocurrent transient generated by bacteriorhodopsin (bR) on a ti n-oxide electrode is due to pH change and not to charge displacement a s previously assumed. Films of either randomly oriented or highly orie nted purple membranes were deposited on transparent electrodes made of tin-oxide-coated glass. The membranes contained either wild-type or D 96N-mutant bR. When excited with yellow light through the glass, the b R pumps protons across the membrane. The result is a rapid local pH ch ange as well as a charge displacement. Experiments with these films sh ow that it is the pH change rather than the displacement that produces the current transient. The calibration for the transient pH measureme nt is given. The sensitivity of a tin-oxide electrode to a transient p H change is very much larger than its sensitivity to a steady-state pH change.