Evidence that Ala M260 is hydrogen-bonded to the reduced primary acceptor quinone QA−. in reaction centers of Rb. sphaeroides

The binding of the primary quinone anion radical QA−. in reaction centers (Rcs) of Rhodobacter (Rb.) sphaeroides species was investigated with electron spin echo envelope modulation (ESEEM). The ESEEM spectra, at two microwave frequencies, of Zn-substituted RCs of Rb. sphaeroides R26 showed interactions of the unpaired electron of QA−. with two nitrogen nuclei in the protein matrix. From an analysis of the experimental data the nitrogen nuclear quadrupole resonance parameters were determined: e2qQ/h = 1.52 MHz, η = 0.82 and e2qQ/h = 3.04 MHz, η = 0.66, which are assigned to the 14Nδ(1)N group of His M219 and the peptide 14N of Ala M260. The ESEEM spectrum of QA−. in reaction centers of the Rb. sphaeroides mutant W(M252)Y shows that the nitrogen of Trp M252 is not interacting with QA−., that of the mutant H(M266)C shows that manipulating the Fe binding site strongly affects the QA-binding site.