Transient electron paramagnetic resonance spectroscopy on green-sulfur bacteria and heliobacteria at two microwave frequencies

Spin polarized transient EPR spectra taken at X-band (9 GHz) and K-band (24 GHz) of membrane fragments of Chlorobium tepidum and Heliobacillus mobilis are presented along with the spectra of two fractions obtained in the puri fication of reaction centers (RC) from C. tepidum. The lifetime of P+. is d etermined by measuring the decay of the EPR signals following relaxation of the initial spin polarization. All samples except one of the RC fractions show evidence of light induced charge separation and formation of chlorophy ll triplet states. The lifetime of P+. is found to be biexponential with co mponents of 1.5 ms and 30 ms for C. tepidum and 1.0 and 4.5 ms for He. mobi lis at 100 K. In both cases, the rates are assigned to recombination from F -x(-). The spin polarized radical pair spectra for both species are similar and those from He. mobilis at room temperature and 100 K are identical. In all cases, an emission/absorption polarization pattern with a net absorpti on is observed. A slight narrowing of the spectra and a larger absorptive n et polarization is found at K-band. No out-of-phase echo modulation is obse rved. Taken together, the recombination kinetics, the frequency dependence of the spin polarization and the absence of an out-of-phase echo signal lea d to the assignment of the spectra to the contribution from P+. to the stat e P+.Fx-. The origin of the net polarization and its frequency dependence a re discussed in terms of singlet-triplet mixing in the precursor. It is sho wn that the field-dependent polarization expected to develop during the 600 -700 ps lifetime of P(+.)A(0)(-.) is in qualitative agreement with the obse rved spectra. The identity that the acceptor preceding F-x and the conflict ing evidence from EPR, optical methods and chemical analyses of the samples are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.