Triplet state EPR of reaction centers from the His(L173)→Leu (L173) mutant of Rhodobacter sphaeroides which contains a heterodimer primary donor.

Electron paramagnetic resonance (EPR) spectroscopy has been used to examine the triplet states in reaction centers of Rhodobacter sphaeroides which have undergone a genetic modification affecting the primary donor. Reaction centers containing the His(L173)→Leu(L173) substitution in the amino acid sequence have a primary donor which consists of a BChl-BPh heterodimer. The triplets formed in this heterodimer reaction center were compared with those formed in the wild-type reaction center which contains the BChl-BChl homodimer. Both reaction centers transfer triplet energy to the carotenoid under illumination at liquid nitrogen temperatures (∼90 K). However, the intensity of the carotenoid triplet signal is significantly decreased in the Leu(L173) mutant compared with the wild-type reaction center. At 12 K, in wild-type reaction centers only the primary donor triplet is observed. The Leu(L173) mutant exhibits a signal similar to that observed by Bylina et al. (1990) in His(M200)→Leu(M200) mutant reaction centers from Rb. capsulatus. The values of the zero-field splitting parameters of this triplet are discussed within the context of various models for the primary donor triplet state. No alteration in the ability of the carotenoid to quench the primary donor triplet state results from mutations at these sites.