Quantum efficiency of light-driven proton extrusion in Halobacterium halobium. pH dependence.

The quantum yield for light-induced proton extrusion in Halobacterium halobium cells pretreated with an ATPase inhibitor was measured between pH 5 and 9 using two separate spectrophotometric techniques. The transmittance of the cell suspension was measured with a spectrometer equipped with "end-on" photomultipliers, whereas the reflectance was measured using a light-integrating sphere. The potentialities of the two techniques are critically compared. These measurements are used to evaluate the intensities of light absorbed by the cells. Since the initial rates of proton release into the extracellular medium were simultaneously measured, the quantum yield values [QY(H+)] could be determined. The results obtained with the two techniques are in reasonable agreement. QY(H+) is 0.64 at pH 5.9 and decreases gradually to 0.28 at alkaline pH values.