Report reviews attempts to duplicate the process of photosynthesis using a physical system wherein the two quantum acts are separated in space. Included is a brief description of the natural photosynthetic systems with special emphasis on the lamellar or membrane system. The chemistry of artificial photosynthesis is discussed in terms of a photosynthetic electron-transfer system involving a phase boundary, a sensitizer, a sink, another phase boundary, sensitizer, and a source. Criteria for the choice of a sensitizer (dyestuff) and the method of construction of a semiconductor with the appropriate band gap are outlined. The author concludes with a discussion of photoelectron transfer across a phase boundary, and states that this essential feature of the natural photosynthetic membrane must be duplicated in any artificial system. 43 references, 8 figures.

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Simulating photosynthetic quantum conversion

Concentration quenching of chlorophyll fluorescence in bilayer lipid vesicles and liposomes

Electron transfer across vesicle bilayers

THE process of electron transfer from water to NAD in living photosynthetic systems involves a crystalline or liquid crystalline two-dimensional lattice of chlorophyll and other pigments. We describe here an artificial model system, with one of the characteristics of pigment system II, in which oxidation of water under the influence of red light (680 nm) is mediated by chlorophyll incorporated into bilayer membranes.

Photo-oxidation of water in phospholipid bilayer membranes containing chlorophyll a

—

The fate of excitation energy and electron transfer to quinones within Chl-a-containing phosphatidyl choline liposomes has been investigated The bilayer membrane of the liposome stabilizes the Chl triplet state, as evidenced by a three-fold increase in the lifetime over that observed in ethanol solution The relative triplet yield follows the relative fluorescence yield, indicative of quenching at the singlet level Triplet state lifetimes are markedly shortened as the Chl concentration is increased, demonstrating that quenching occurs at the triplet level as well This process is shown to be due to a collisional de-excitation In the presence of quinones, the Chl triplet reduces the quinone resulting in production of long-lived electron transfer products The percent conversion of Chl triplet to cation radical when benzoquinone is employed as acceptor is approximately 60 ± 10%, which is slightly less than in ethanol solution (70 ± 10%) The lifetime of the radical, however, can be as much as 1900 times longer With respect to potentially useful photochemical energy conversion, the magnitude of this increased lifetime is far more significant than is the decreased radical yield

Chlorophyll photochemistry in condensed media—ii. triplet state quenching and electron transfer to quinone in liposomes*

— Chlorophyll (chl) a in sonicated egg yolk lecithin vesicles, even with local concentrations of 0.1 M, exists in its monomeric form as is evident by the absorption, circular dichroic and EPR spectra of such suspensions. With the water soluble oxidants, K3Fe(CN)6 or SmCl3, it is possible to photoproduce the chl cation radical. With K3Fe(CN)6 as acceptor the formation and decay kinetics can be explained by a second order process influenced by diffusion of the water soluble components in this heterogenous system. With the exception of an iron-sulfur protein isolated from Rhodospirillum ruhrum, we have not detected a photochemical reaction of chl with various iron-sulfur proteins or ubiquinone.

Chlorophyll sensitized charge separation in phospholipid vesicles.

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Chlorophyll sensitized charge separation in phospholipid vesicles.