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Abstract
Reduction of the primary acceptor quinone (QA) in the photosynthetic reaction center Rhodobacter sphaeroides generates a semiquinone anion radical. This radical species has been studied using electron spin echo envelope modulation (ESEEM). Evidence has supported hydrogen bonding between the carbonyl groups of QA and nitrogen from His M219 and peptide nitrogen from Ala M260. In this study 3-pulse ESEEM and 2D-HYSCORE measurements on native and 15N labeled reaction centers were used to directly measure the hyperfine interactions (hfi) between the semiquinone and surrounding nitrogen nuclei. 3-pulse ESEEM spectra of native reaction centers looked similar to previously reported results (Spoyalov et. al. 1996). Nuclear quadrupole coupling (nqc) produced a very complicated HYSCORE spectrum, but 15N labeling eliminated the nqc and allowed for hfi to be measured. HYSCORE measurements showed the semiquinone coupled to 2 unique nitrogen nuclei. One set of cross peaks appeared far more intense in the spectrum. The difference in intensity suggests contributions to nitrogen coupling in addition to geometry.
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