Transport of calcium ions across membranes and against a thermodynamic gradient is essential to many biological processes, including muscle contraction, the citric acid cycle, glycogen metabolism, release of neurotransmitters, vision, biological signal transduction and immune response. Synthetic systems that transport metal ions across lipid or liquid membranes are well known, and in some cases light has been used to facilitate transport. Typically, a carrier molecule located in a symmetric membrane binds the ion from aqueous solution on one side and releases it on the other. The thermodynamic driving force is provided by an ion concentration difference between the two aqueous solutions, coupling to such a gradient in an auxiliary species, or photomodulation of the carrier by an asymmetric photon flux. Here we report a different approach, in which active transport is driven not by concentration gradients, but by light-induced electron transfer in a photoactive molecule that is asymmetrically disposed across a lipid bilayer. The system comprises a synthetic, light-driven transmembrane Ca2+ pump based on a redox-sensitive, lipophilic Ca2+-binding shuttle molecule whose function is powered by an intramembrane artificial photosynthetic reaction centre. The resulting structure transports calcium ions across the bilayer of a liposome to develop both a calcium ion concentration gradient and a membrane potential, expanding Mitchell's concept of a redox loop mechanism for protons to include divalent cations. Although the quantum yield is relatively low (approximately 1 per cent), the Ca2+ electrochemical potential developed is significant.

Liposomes have been used for biomimetic containers and to study phenomena ranging from photosynthetic systems to membrane fusion and dynamics. An important aspect of many preparations and in biological processes is the presence of a pH gradient across the membrane. Here, experiments were conducted using capillary electrophoresis to investigate the effects of this gradient on liposomes composed of phosphatidic acid, phosphatidylcholine, and cholesterol. pH gradients for the liposomes were created by titration of the exterior buffer; then the electrophoretic properties were analyzed by capillary electrophoresis and the size was measured by laser light scattering. Our results show that the presence of a pH gradient has a significant effect on the electrophoretic migration of liposome samples, induced principally by a change in effective charge. The differences in charge for the liposome samples are evaluated with regard to acid-base equilibria, which is shown to be inadequate to describe the dynamics of the system. A more complex capacitive theory incorporating elements of the Overbeek-Booth theory and the relaxation effect appears to more effectively describe the results and could aid in predicting liposome behavior under various pH gradient conditions.