1D- and 2D-ESEEM Study of the Semiquinone Radical Q_A^- of Photosystem II

Abstract

The semiquinone radical QA - has been studied by Electron Spin−Echo Envelope Modulation (ESEEM) spectroscopy in Photosystem II membranes at various pH values. The observed nuclear modulations have been assigned by the use of two-dimensional Hyperfine Sublevel Correlation Spectroscopy (HYSCORE) and numerical simulations. Two protein 14N nuclei (NI and NII) were found to be magnetically coupled with the QA - spin, and on the basis of 14N-NQR and 14N-ESEEM data from the literature, NI is assigned to an amide nitrogen from the protein backbone while NII is assigned to the amino nitrogen, Nδ, of an imidazole. A physical explanation for such couplings is suggested where the coupling occurs through H-bonds from the protein to the carbonyls of the semiquinone. In PSII membranes treated with CN-, only the NI coupling is present above pH 8.5 while both NI and NII couplings are present at lower pH values. In samples treated at high pH to remove the iron, both NI and NII couplings are present throughout the pH range studied but at pH <6 these couplings strengthen. These results are interpreted in terms of a model based on the structure of the bacterial reaction center and involving two determining factors. (1) The nonheme iron, when present, is liganded to the imidazole that H-bonds to one of the QA - carbonyls. This physical attachment of the imidazole to the iron limits the strength of the H-bond to QA -. (2) A pH-dependent group on the protein controls the strength of the H-bonds to QA -. The pK a of this group is influenced by the biochemical treatment used to uncouple the iron, being around pH 7.5 in CN--treated PSII but around pH 6 in high pH-treated PSII. It is proposed that such a pH effect on the H-bond strength exists in untreated PSII and that earlier observations of pH-induced changes in the EPR signal from the semiquinone iron may reflect this change.

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.…”

Intensity of Cross-Peaks in Hyscore Spectra of S = 1/2, I = 1/2 Spin Systems

“…[ 16] In contrast to the (Ϯ, Ϯ) cross-peaks considered above, now the b 1 Ͼ 0 orientations dominate experimental spectra, and b 1 Ͻ 0 cross-peaks are suppressed (Figs. 5C and 5D).…”

Coordination and binding geometry of methyl-coenzyme M in the red1m state of methyl-coenzyme M reductase

“…Simulation of the data (HYSCORE spectra recorded at middle-and low-field positions in the EPR spectrum are given in Fig. S3 [57][58][59], whereas the NH nitrogens of histidine and proline have |e 2 qQ/h| & 1.4-1.7 MHz, g = 0.6-1.0 [60], and the NH nitrogen of guanine has |e 2 qQ/" h| = 2.63 MHz, g = 0.60. Our parameters most closely resemble those of a glutamine [61].…”