No abstract

The electron spin polarization (ESP) pattern of the donor‐triplet state (PR) 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 PR 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 PR 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.