AN IDENTIFICATION OF RADICAL GIVING RISE TO LIGHT-INDUCED ELECTRON SPIN RESONANCE SIGNAL IN PHOTOSYNTHETIC BACTERIA

Abstract— A reaction‐center fraction isolated from Rhodopseudomonas spheroides chromatophores exhibits light‐induced changes in its optical and electron spin‐resonance (ESR) spectra. In particular, a bleaching at 870nm (P870) has been found to be closely correlated with the appearance of an ESR signal with a g factor of 2.0025 and a peak‐to‐peak line width of 10 G. The ESR signal is indistinguishable from light‐induced signals found in chromatophores or whole cells. A careful measurement of the spin concentration showed that the ratio of the light‐induced spins to bleached P870 molecules is 1.1 ± 0.1. In addition the formation and decay kinetics are identical within experimental error under a variety of experimental conditions. Previous work has shown that P870 is a bacteriochlorophyll molecule in a specialized environment and that the bleaching signifies oxidation. The present work provides strong evidence that the photo‐bleaching of P870 produces the radical cation of bacteriochlorophyll, P870+, and that this radical is the source of the ESR signal in whole cells. The quantum yield for the bleaching of P870 in reaction centers has been measured, using actinic light of wavelengths 880, 800, 760 and 680 nm. For light absorbed at 880 or 800 nm the efficiency is close to 100 per cent. In a coupled reaction, the oxidation of mammalian cytochrome c by P870+ proceeds with nearly the same efficiency. The above results place definite limits on the possibilities for the identity of the primary acceptor. These possibilities are discussed. Copyright © 1969, Wiley Blackwell. All rights reserved