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Purple membrane (PM) fragments were adsorbed on a dioleoylphosphatidylcholine (DOPC) monolayer and on a mixed alkanethiol/DOPC bilayer supported by mercury to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). The light-on and light-off capacitive currents on an alkanethiol/DOPC bilayer at pH 6.4 were interpreted on the basis of a simple equivalent circuit. The pH dependence of the biphasic decay kinetics of the light-on currents was analyzed to estimate the pK(a) values for the transitions releasing protons to, and taking up protons from, the solution. The linear dependence of the stationary light-on current of bR on a DOPC monolayer self-assembled on mercury upon the applied potential was interpreted on the basis of an equivalent circuit.

Purple membrane (PM) fragments were adsorbed on a dioleoylphosphatidylcholine (DOPC) monolayer supported by mercury to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). PM fragments were also adsorbed on a mercury-supported triethyleneoxythiol (TET) monolayer. On both monolayers, the light-on current exhibits a finite, potential dependent stationary component that decreases linearly with a positive shift in the applied potential. The light-on and light-off capacitive photocurrents were interpreted on the basis of a simple equivalent circuit, which accounts for the potential dependence of the stationary light-on current. The potential of zero stationary current is about equal to +0.010 V vs. saturated calomel electrode (SCE) on DOPC-coated mercury. The absolute potential difference across the PM fragments adsorbed at this applied potential was estimated on the basis of extrathermodynamic considerations and amounts to about +260 mV; it compares favorably with the value, +250 mV, of the transmembrane potential of zero stationary current across an oocyte plasma membrane incorporating bR [Biophys. J. 74 (1998) 403.]. The effect of the proton pumping activity of photoexcited PM fragments on the electroreduction kinetics of ubiquinone-10 incorporated in the DOPC monolayer underlying the PM fragments was investigated. [Copyright &y& Elsevier]

The homogeneous, defect-free surface of a hanging mercury drop electrode (HMDE) is used to self-assemble films apt to the investigation of problems of biological interest. The electrochemical behavior in the dark of a film of chlorophyll (Chl) molecules self-assembled on mercury and the peculiar features of the photocurrents recorded upon illuminating this film at different applied potentials are discussed. The limiting flux of hydrated Tl+ ions through the channel-forming polypeptide gramicidin (GR), incorporated in a dioleoylphosphatidylcholine (DOPC) monolayer self-assembled on mercury, is shown to be controlled both by diffusion and by a dehydration step, while the limiting flux of dehydrated Tl+ ions stemming from Tl amalgam electrooxidation is exclusively controlled by the diffusion of thallium atoms within the amalgam. The photocurrents of purple membrane (PM) fragments adsorbed on a DOPC monolayer and on a mixed alkanethiol/DOPC bilayer self-assembled on mercury are examined to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). The photocurrents on the mixed bilayer are interpreted on the basis of a simple equivalent circuit and their pH dependence is analyzed to estimate the pKa values of the transitions releasing protons to, and taking up protons from the solution. [Copyright &y& Elsevier]