The surface potential on the purple membrane measured using a modified bacteriorhodopsin chromophore as the spectroscopic probe

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Abstract
The surface potential of the purple membrane was measured by a novel method by using an artificial bacteriorhodopsin whose chromophore was 13‐CF3 retinal instead of retinal. When attached to the apoprotein by a Schiff base, the intrinsic pK of the 13‐CF3 chromophore is around 7.3. The apparent pK of this pigment depends on the surface potential and thus on the electrolyte concentration. This allowed us to determine the surface charge density using the Gouy‐Chapman equation. The surface charge density was found to be −1.65 ± 0.15 × 10−3 electronic charges per Å2 or about 2 negative charges/bacteriorhodopsin. This large value for the surface potential probably explains both part of the strong apparent association of divalent cations with the membrane and the effect of low salt concentrations on light‐induced proton release from the purple membrane.