The pH-sensitive dye 5-iodoacetamidofluorescein was covalently bound to a single cysteine residue introduced by site-directed mutagenesis in position 101 on the cytoplasmic surface or in position 130 on the extracellular surface of the proton pump bacteriorhodopsin. Using time-resolved absorption spectroscopy at 495 nm a transient increase was observed in the apparent pK of the dye attached at residue 101. At pH 7.3 the rise and decay times of this pK-change (∼2 ms and ∼60 ms) correlate well with decay times observed for the M and O intermediates and with the proton uptake time. Interpreting the pK-increase of +0.18 pH-unit in terms of a transiently more negative surface charge density, we calculate a change of −0.80 elementary charge per bacteriorhodopsin at the cytoplasmic surface. It is likely that this charge change is due to the transient deprotonation of aspartate-96. With the label in position 130 on the extracellular surface no transient pK-shift was detected.
