Absolute quantum yields and proof of proton and nonproton transient release and uptake in photoexcited

Using a sensitive differential ac conductance ap- paratus, we have measured transient ion movements in and the heating of bacteriorhodopsin suspensions after a light flash. The signal from the heating serves as an internal calibration of the absorbed photons and therefore the method gives the absolute quantum yield (4) from a single measurement. At pH 4, H+ up- take precedes release, with b = 0.4. By varying the buffer com- position, we can prove that this signal is due to protons. At pH 8, however, the transient conductance increase is virtually indepen- dent of the buffer composition, showing that ions other than H+ are first released and then taken up by the purple membrane. If these ions are typical monovalent cations such as Na+ (A = 50 ohm-l cm2.equiv 1), this process has a quantum yield of 2 or more