Abrupt onset of large scale nonproton ion release in purple membranes caused by increasing pH or ionic strength.

The abrupt onset of large scale nonproton ion release by photo-excited purple membrane suspensions has been observed near neutral pH using transient conductivity measurements. At pH 7 and low ionic strength, the conductivity transients due to proton and nonproton ions are of comparable magnitude but of opposite sign: fast proton release and ion uptake, followed by slow proton uptake and ion release. By increasing either the pH or the NaCl concentration, the amplitude of the conductivity transient increases sharply and the signal is then dominated by nonproton ion release. These results can be understood in terms of light-induced changes in the population of counterions condensed at the purple membrane surface caused by changes in the surface charge density. The critical charge density required for condensation to occur is evidently achieved near neutral pH by ionizing dissociable groups on the membrane by either titration (increasing the pH) or shifting their pKs (increasing the ionic strength).