Energy-transducing membranes of living organisms couple spontaneous to non-spontaneous processes through the intermediacy of protonmotive force (p.m.f.)--an imbalance in electrochemical potential of protons across the membrane. In most organisms, p.m.f. is generated by redox reactions that are either photochemically driven, such as those in photosynthetic reaction centres, or intrinsically spontaneous, such as those of oxidative phosphorylation in mitochondria. Transmembrane proteins (such as the cytochromes and complexes I, III and IV in the electron-transport chain in the inner mitochondrial membrane) couple the redox reactions to proton translocation, thereby conserving a fraction of the redox chemical potential as p.m.f. Many transducer proteins couple p.m.f. to the performance of biochemical work, such as biochemical synthesis and mechanical and transport processes. Recently, an artificial photosynthetic membrane was reported in which a photocyclic process was used to transport protons across a liposomal membrane, resulting in acidification of the liposome's internal volume. If significant p.m.f. is generated in this system, then incorporating an appropriate transducer into the liposomal bilayer should make it possible to drive a non-spontaneous chemical process. Here we report the incorporation of F0F1-ATP synthase into liposomes containing the components of the proton-pumping photocycle. Irradiation of this artificial membrane with visible light results in the uncoupler- and inhibitor-sensitive synthesis of adenosine triphosphate (ATP) against an ATP chemical potential of approximately 12 kcal mol(-1), with a quantum yield of more than 7%. This system mimics the process by which photosynthetic bacteria convert light energy into ATP chemical potential.

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The photodynamic effect of novel cationic porphyrins, with different pattern of meso-substitution by 4-(3-N,N,N-trimethylammoniumpropoxy)phenyl (A) and 4-(trifluoromethyl)phenyl (B) groups, have been studied in both solution bearing photooxidizable substrates and in vitro on a typical Gram-negative bacterium Escherichia coli. In these sensitizers, the cationic groups are separated from the macrocycle ring by a propoxy spacer. Thus, the charges have a high mobility and a minimal influence on photophysical properties of the porphyrin. These compounds produce singlet molecular oxygen, O2(1Delta(g)), with quantum yields of approximately 0.41-0.53 in N,N-dimethylformamide. In methanol, the l-tryptophan photodecomposition increases with the number of cationic charges in the sensitizer. In vitro investigations show that cationic porphyrins are rapidly bound to E. coli cells in approximately 5 min. A higher binding was found for A3B3+ porphyrin, which is tightly bound to cells still after three washing steps. Photosensitized inactivation of E. coli cellular suspensions follows the order: A3B3+ > A44+>> ABAB2+ > AB3+. Under these conditions, a negligible effect was found for 5,10,15,20-tetra(4-sulfonatophenyl)porphyrin (TPPS4(4-)) that characterizes an anionic sensitizer. Also, the results obtained for these new cationic porphyrins were compared with those of 5,10,15,20-tetra(4-N,N,N-trimethylammonium phenyl)porphyrin (TTAP4+), which is a standard active sensitizer established to eradicate E. coli. The photodynamic activity of TTAP4+ is quite similar to that produced by A4(4+). Studies in an anoxic condition indicate that oxygen is necessary for the mechanism of action of photodynamic inactivation of bacteria. The higher photodynamic activity of A3B3+ was confirmed by growth delay experiments. Photodynamic inactivation capacities of these sensitizers were also evaluated in E. coli cells immobilized on agar surfaces. Under these conditions, A3B3+ porphyrin retains a high activity to inactivate localized bacterial cells. Therefore, tricationic porphyrin A3B3+ is an interesting sensitizer with potential applications in photodynamic inactivation of bacteria in liquid suspensions or on surfaces.

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Carotenoids (Car) act as ''wires'' that discharge unwanted electrons in the reaction center of higher plants. One step in this ''side-path'' electron conduction is thought to be mediated by Car oxidation. We have carried out direct measurements of the conductance of single-Car molecules under potential control in a membrane-mimicking environment, and we found that when Car are oxidized conductance is enhanced and the electronic decay constant (␤) is decreased. However, the neutral molecule may already be conductive enough to account for observed electron transfer rates.carotenoid ͉ molecular electronics ͉ photosynthesis ͉ potential control ͉ single molecule P hotosynthetic systems are natural photoelectronic devices, integrating electronic and photonic elements in a protein scaffold. Understanding the role played by the different components is important both for understanding photosynthetic processes as well as for using them in artificial molecular electronic devices. Carotenoids (Car) in photosynthesis contribute to light harvesting, structural stabilization, and protection from photooxidation. In photosystem II reaction centers, Car participate in the ''side-path'' electron donation reactions for reduction of P 680 ϩ as part of the photoprotective system. The role of the Car is thought to be that of an electron carrier (1, 2). It is believed that ␤-carotene in photosystem II is oxidized under illumination (1, 2). To investigate whether Car oxidation is a prerequisite for electron transport in photosystem II photoprotection, we measured the conductivity and the length-dependence of the tunneling rate for single-Car molecules under potential control. Measuring electron transport under potential control allows us to simulate the redox environment of the Car under natural conditions and to relate our electronic measurements to its function in vivo.Single (neutral) Car polyenes are much more conductive than alkanes of equivalent length because of their conjugated -electron system (3-5). We and others (6-9) have shown that the conductivity of redox-active single molecules can be regulated by changes in the molecule's oxidation state. Here, we have extended our experimental methods to work in a water-and oxygen-free environment that mimics the conditions found in cell membranes. In this way, we hope to understand one of the roles of Car in photosynthesis and to explore their role as gatable electronic components. Finding such components remains a challenge to date (10).Single-molecule electron transport measurements under potential control were carried out by following the method developed by Xu and Tao (6, 11), for which a scanning tunneling microscope (STM) is used with a partially insulated probe to enable measurements in a conductive electrolyte, which is required for maintaining potential control. A scheme of the experimental setup is shown in Fig. 1 (see also the supporting information, which is published on the PNAS web site). Photosystem II is a transmembrane protein complex, so the surrounding environment o...