The fluorescence yield of Rhodopseudomonas viridis in relation to the redox state of the primary electron donor.

The fluorescence yield of bacteriochlorophyll (BChl) b in membranes of Rhodopseudomonas viridis was measured immediately and at a variable time-interval after a saturating laser flash to bring about charge separation. At 4 K a decrease of the yield by 28% was observed immediately after the flash. This yield recovered mono-exponentially with a time constant of 6.3 +/- 0.4 ms to approximately the original level. The same time constant was observed for the re-reduction of the primary electron donor, indicating that the fluorescence quenching can be ascribed to the oxidation of the primary donor. The extent of quenching decreased with increasing temperature and reversed to a fluorescence increase at temperatures above 50 K. These results may be explained by the presence of long-wavelength absorbing BChls b in the antenna which at low temperature transfer their excitation energy more efficiently to the oxidized than to the reduced primary donor, in support of a similar hypothesis used to explain the quenching of fluorescence by 'oxidized' reaction centers in heliobacterium chlorum (Deinum, G., Kramer, H., Aartsma, T.J. Kleinherenbrink, F.A.M. and Amesz, J. (1991) Biochim. Biophys. Acta 1058, 339-344).