Polarized Fourier transform infrared spectroscopy and far ultraviolet circular dichroism of oriented multilamellar films of photoreceptor membranes indicate rhodopsin alpha-helices are predominantly oriented perpendicular to the bilayer plane.
Entropy and unavoidable irreversibility place a limit on the efficiency of photochemical solar energy conversion which is substantially lower than that placed by the first law of thermodynamics alone. Shockley and Queisser’s ’’detailed balance limit’’ on the efficiency of p-n-junction photovoltaic devices is a special case of this general thermodynamic limit on the efficiency of all quantum-utilizing solar energy converters. For a single photochemical system operating at 20 °C in sunlight not attenuated by the atmosphere, this efficiency cannot exceed 29%. Under the same conditions, the efficiency of a solar converter composed of two photochemical systems can reach 41%.
Abstract— Photosynthetic quantum conversion and early electron transport is modeled as a stochastic process on a digital computer to determine what free‐energy losses are a necessary consequence of specific assumptions about the reaction structure, kinetics, and thermodynamics of the participating molecules. Maximal free‐energy yield requires that all dark reactions be near equilibrium, so the potentials of all half‐cells on each side of the light act are nearly the same. This near equilibrium requires that all forward rate constants be at least 102 times the rate of light absorption, and that all reverse rate constants be at least the rate of light absorption. The behavior of model systems with one primary donor and one primary acceptor is comparatively independent whether there is one or an infinite number of secondary electron donors and acceptors. A system having no metastable (e.g. triplet) state of the light‐activated donor can convert light energy with nearly ideal efficiency, provided that the standard electrode potentials of the primary donor and primary acceptor half‐cells are precisely located with respect to one another and to the potentials of the ultimate donor and acceptor. While not necessary for near maximal free‐energy yield, a metastable intermediate allows a flexibility in the choice of half‐cell potentials which is not possible in the absence of such an intermediate.
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