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

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Abstract
Using a sensitive differential AC conductance apparatus, transient ion movements in, and the healing of, bacteriorhodopsin suspensions after a light flash were measured. The signal from the heating serves as an internal calibration of the absorbed photons and therefore the method gives the absolute quantum yield (.vphi.) from a single measurement. At pH 4, H+ uptake precedes release, with .vphi. = 0.4. By varying the buffer composition, this signal is shown to be due to protons. At pH 8, the transient conductance increase is virtually independent 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+ (.LAMBDA. = 50 ohm-1 .cntdot. cm2 eq-1), this process has a quantum yield of .gtoreq. 2 at high salt concentrations.