1-Carboxyethyl-4-cyanopyridinium-mediated photoinduced electron-proton cotransport across phosphatidylcholine vesicle membranes

Asymmetrically organized egg lecithin small unilamellar vesicles containing an electron acceptor (Co(bpy)33+) in the internal aqueous phase with a photosensitizer (ZnTPPS4−) and electron donor (dithiothreitol) in the external medium were prepared. When 1-carboxyethyl-4-cyanopyridinium ions were also present in the medium, illumination with visible light gave rise to net transmembrane oxidation–reduction; however, no transmembrane redox reaction was observed when this ion was absent. Transient spectroscopy revealed that the pyridinium derivative oxidatively quenched the 3ZnTPPS4− photoexcited ion, forming the neutral pyridinium radical, which subsequently diffused across the bilayer to reduce the occluded Co(bpy)33+ ion. Quenching rate constants in 20 mM Tris, pH 3.3, at room temperature were 4.5×109 M−1 s−1 and 2.1×109 M−1 s−1 in the absence and presence of vesicles, respectively; the characteristic time for transmembrane diffusion of the radical was 30 ms. Under continuous illumination, at least seven internal Co(bpy)33+ ions could be reduced to Co(bpy)32+ for each 1-carboxyethyl-4-cyanopyridinium ion present, implying that the redox mediator had cycled on average seven times across the membrane. Transmembrane reduction rates increased when the vesicle interior was made alkaline, consistent with the formally neutral 1-carboxyethyl-4-cyanopyridinium zwitterion being the diffusible form of the oxidized carrier. The data support a transmembrane transport model involving stepwise photosensitized reduction of the carrier, transmembrane transport of the neutral radial, reoxidation accompanied by proton release from the pendant carboxyl group, and diffusion of the oxidized zwitterion to the external medium with subsequent proton uptake to close the cycle.