Order of proton uptake and release by bacteriorhodopsin at low pH

The order of proton uptake and release in an aqueous suspension of purple membrane in response to a light flash has been investigated at lowered pH. pH indicator dyes and a flash spectrophotometer were used for the study. At pH 6.6 it was found that the release of protons from the purple membrane precedes uptake, as reported by other investigators. At pH 5.9, 4.9, and 4.1 it was also found that release precedes uptake. These results are not in agreement with those of previous investigators.