An electron spin-echo envelope modulation (ESEEM) study of the QA binding pocket of PS II reaction centers from spinach and Synechocystis

We have used electron spin-echo envelope modulation spectroscopy (ESEEM) to characterize the protein-cofactor interactions present in the QA- binding pocket of PS II centers isolated from spinach and Synechocystis. We conclude that the ESEEM spectrum of QA- is the result of interactions of the S = 1/2 electron spin of QA- with the I = 1 nuclear spins of the peptide nitrogens of two different amino acids. One peptide nitrogen has ESEEM peaks near 0.7, 2.0, 2.85, and 5.0 MHz with isotropic and dipolar hyperfine couplings of Aiso = 2.0 MHz and Adip = 0.25 MHz, respectively. On the basis of these hyperfine couplings we predict the existence of a strong hydrogen bond between QA- and the peptide nitrogen with a hydrogen bond distance of about 2 A. We have not identified the amino acid origin of this peptide nitrogen. By using amino acid specific isotopic labeling in conjunction with site-directed mutagenesis, we demonstrate that the second peptide nitrogen is that of D2-Ala260, with ESEEM peaks near 0.6 and 1.5 MHz and an isotropic hyperfine coupling, Aiso, less than 0.2 MHz. This small isotropic coupling suggests that the D2-Ala260 peptide nitrogen at best forms a weak hydrogen bond with QA-.