Rafail F. Khairutdinov

Cyclic transmembrane charge transport by pyrylium ions in a vesicle-based photocatalytic system

Solar energy is being used for power generation, but also attracts increasing interest as a renewable energy source for the photocatalytic production of useful chemicals. Simple systems based on vesicles with transmembrane redox mediators have been used to transform photon energy into long-lived, membrane-separated photoredox products. However, these systems are not suitable for high-throughput applications because the transmembrane electron carriers are oxidized inside the vesicle into charged species that are no longer able to readily traverse the membrane bilayer. This leads to continuous trapping of these carriers during photolysis and, ultimately, to the termination of the redox reaction due to accumulation of the available carriers within the vesicle interior. Living cells circumvent this problem by using quinones to simultaneously transport electrons and protons, thus allowing the carrier to remain neutral in its reduced and oxidized states and so retain the ability to undergo transmembrane diffusion throughout the redox cycle. But the incorporation of quinones into artificial systems is not practical because of their susceptibility to oxidative degradation and slow transmembrane diffusion. Here we describe an alternative mechanism for rapid electroneutral charge transport across vesicle membranes: we use pyrylium cations as the electron carrier, which undergo reversible ring-opening hydrolysis to form neutral diketones after deposition of the electron inside the vesicle. As the pyrylium cations are also the primary acceptors for the photoproduced electrons, our approach greatly simplifies the design of vesicle-based photocatalytic devices.

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

Abstract 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.