A Survey of Epr Investigations of Bacterial Photosynthesis

Abstract

Abstract-Investigations in which EPR has been used as a probe of the mechanism of the primary quantum conversion reaction and/or electron transport reactions in bacterial photosynthesis are surveyed. These investigations include studies of whole cell organisms and simpler sub-cellular preparations, chromatophores and bacteriochlorophyll-protein complexes.Electron paramagnetic resonance studies have successfully demonstrated that the primary donor of the photosynthetic, photochemical reaction involves a dimer of bacteriochlorophyll, generally referred to as P870. P870 is photochemically oxidized to a cation radical which exhibits a g = 2.0025 EPR signal. Comparative studies of the time behaviour of this signal in whole cells and in sub-cellular preparations show that electron flow in the whole cells is substantially different than in the cell-free systems.The primary acceptor molecule of the photochemical reaction has not been conclusively identified. When it is photochemically reduced, it exhibits a broad EPR absorbance centered at g = 1.8 observable only at low temperatures. This signal involves an iron atom and a quinone molecule. Two possible identifications of the species responsible for the g = 1.8 signal are an iron-quinone complex and an iron-sulfur protein. The latter identification would require that one primary acceptor function for two primary donors and that the removal of a tightly held quinone alter the integrity of the system so as to inhibit the photochemistry.When the primary acceptor is chemically reduced, a photo-induced, polarized triplet EPR spectrum is detected. Both absorption and emission lines arq observed as if only one substate (m = 0) of the triplet manifold is populated. The zero field parameters of the triplet spectrum suggest that the triplet is formed through a decay of a biradical, not through an optical singlet to triplet transition.Low temperature EPR studies of photosynthetic preparations which had been poised at room temperature by subjecting the preparations to different redox potentials and/or dark adaptation and illumination show the presence of a number of light-influenced, EPR active components. The spectral characteristics of these components are indicative of iron-heme (both low and high-spin forms) and iron-sulfur (both oxidized and reduced) proteins and at least one other organic free radical distinct from P870'. The spectra varied for different strains of bacteria. Also, some of the signals detected in the whole cell organism were not detected in cell free preparations and the kinetics of the light influenced signal amplitudes were different in the whole cells than in chromatophores.

Photoreceptors for biosynthesis, energy storage and vision

“…Purple non-sulphur bacteria have only cyclic photophosphorylation; however, they also have O2 respiration using the same cytoclirome electron-transport chain as is used for photosynthesis (Marrs et al, 1972;Dickerson & Timkovich, 1975). Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopy have provided convincing evidence for the special pair nature of the reaction centre chlorophyll in photosynthetic bacteria (Norris et al, 1974;Feher et al, 1975;Corker, 1976). When the energy from a quantum of liglit arrives in a bacterial photosynthetic reaction centre, an eleetron is removed from the cUorophyU special pair (which is thus also known as the primary donor) and transferred to what is believed to be a nearby molecule of bacteriopheophytin (a bacteriochlorophyll in which the lnagnesiutn has been replaced witli two hydrogens) (Fajer et al, 1975;Dutton, 1976).…”

Picosecond Flash Photolysis in Biology and Biophysics

“…: listed in the text, we have found other review articles (15,26,29,42,83,87) useful in preparing this section.…”