An electron spin echo envelope modulation study of the primary acceptor quinone in Zn-substituted plant photosystem II

Abstract

A Fe2+ ion on the acceptor side of plant photosystem II has been substituted by Zn2+ and an anion radical of the primary acceptor quinone, Q−A, has been studied by electron spin echo method. The electron spin echo modulation shows the interaction of the unpaired electron of Q−A with nitrogen nuclei of the histidine and, probably, alanine residues situated nearby. The comparison of the modulation spectra of Q−A with those of the anion radical of plastoquinone-9 stabilized in protonated and deuterated isopropanol matrices allows one to distinguish between the spectrum lines due to the quinone protons and due to the protons of other molecules that form hydrogen bonds with the oxygen atoms of the quinones.

Direct Evidence for Nitrogen Ligation to the High Stability Semiquinone Intermediate in Escherichia coli Nitrate Reductase A

“…As the latter has been shown to be much more influenced by the geometry of the N-H bond than the parameter, the relatively small value determined in this work may reflect a slightly different H-bonding pattern compared with that occurring for Q A . in bacterial reaction centers (30,35,36) or in photosystem II (25,28). Interestingly, the quadrupole characteristics determined here for the 14 N nucleus interacting with MSQ D in NarGHI are very close to the values calculated by Fritscher for an imidazole making a strong in-plane hydrogen bond with a water molecule (r O-N ϳ2.7 Å) (46).…”

“…Thus, the presence of an H-bond to a histidine residue appears to be a common feature of both PCP and MSQ D binding. This contrasts with the situation observed for the menaquinol oxidation site of the polysulfide reductase from Thermus thermophilus that has been recently characterized by cocrystallization experiments using the natural substrate menaquinone or the inhibitor PCP (27), N ⑀ Arg at the ubiquinone site Q H in cytochrome bo 3 (34,45), N ␦ His and backbone nitrogen at the Q A and Q B sites of bacterial reaction centers (30,35,36) and photosystem II (25,28,29). On the basis of our spectroscopic data and the structure of NarGHI co-crystallized with PCP, we propose that the N ␦ of the His-66 imidazole H-bonds to the semiquinone O 1 while the N ⑀ coordinates the heme b D iron.…”

Radicals and Radical Pairs in Photosynthesis

“…Two distinct matrix nitrogens were observed in an ESEEM study of Q i by Astashkin et al (266) and ascribed to His-215 and Ala-261 of the D2-protein. In this model both nitrogens are hydrogen bound to the plastoquinone-9 oxygens.…”

Matrix Line in Pulsed Electron-Nuclear Double Resonance Spectra

“…In this case, if is shorter than the spectrometer dead time, t d , one may introduce a fourth (180°) mw pulse in time tЈ Ͼ t d after the stimulated ESE signal in order to refocus it and make it observable. To our knowledge, this technique was first used in (14). Later it was described in detail in (15), where it was called refocused Mims (ReMims) ENDOR.…”