THE ELECTRON SPIN POLARIZATION OF THE DONOR-TRIPLET STATE IN NATIVE and MODIFIED PHOTOSYNTHETIC REACTION CENTERS FROM

The electron spin polarization (ESP) pattern of the donor-triplet state (P^R) of reaction centers (RC's) of the purple bacterium Rhodobacter (formerly Rhodopseudomonas) sphaeroides R-26 was investigated. δm =±1 triplet EPR spectra were recorded of unmodified RC's as well as of RC's from which Fe²⁺ or ubiquinone was removed, or ubiquinone was substituted by menaquinone.
The relative amplitude of the Y peaks in the triplet EPR powder spectrum of P^R decreases when the temperature is increased from 8 K to 100 K in RC's with an intact quinone-iron complex. This decrease is more pronounced when the primary ubiquinone is substituted by menaquinone. These observations provide further support for the hypothesis that the observed lineshape of the P^R triplet state EPR spectrum reflects the presence of a third electron spin, magnetically coupled to I^- in the P⁺I^- radical pair, as suggested by Van Wijk et al. (1986) (Photobiochem. Photobiophys . 11, 95–100). Our observations suggest that this phenomenon may be general in purple bacteria.