Electron nuclear double resonance (ENDOR) and special triple (ST) resonance spectroscopies have been used to study the cation radicals of the primary donor, P680, and two secondary donor chlorophylls (Chl) in photosystem 2 (PS2). Two different preparations were employed, Tris-washed PS2 membranes and PS2 reaction centers (D1-D2-I-Cytb559 complex). One secondary donor Chl a cation radical, Chl1.+, was generated in the Tris-washed preparation, while the P680.+ radical cation and a further Chl a cation radical, Chl2.+, were produced in the reaction center preparation. The ENDOR spectrum of the primary donor radical cation of photosystem 1 (P700.+) is also presented for comparison. Hyperfine coupling constants for methyl groups have been measured for all three PS2 radical species and assigned by comparison with previously published spectra of Chl a radicals in vitro. Electron spin densities were calculated from these hyperfine couplings. Comparison of ENDOR spectral features with those of Chla.+ in vitro indicates similar values for Chl1.+ and Chl2.+ radicals but an apparent reduction in unpaired electron spin density for P680.+. It has been proposed from the more detailed studies of purple bacterial reaction centers that such a reduction in spin density can be interpreted as a delocalization over two Chl a molecules. Our calculations therefore suggest that P680.+ is a weakly coupled chlorophyll pair with 82% of the unpaired electron spin located on one chlorophyll of the pair at 15 K. Environmental or geometrical changes to the chlorin ring structure to give a novel monomeric primary donor are also possible.(ABSTRACT TRUNCATED AT 250 WORDS)

The dark stable neutral tyrosine radical YD. of photosystem 2 (PS2) has been studied using electron nuclear double-resonance (ENDOR) and electron paramagnetic resonance (EPR) spectroscopies. The proton hyperfine coupling constants of all four ring protons and both beta-methylene protons have been determined for YD. in three species covering the range of oxygenic organisms; a higher plant (spinach), an alga (Chlamydomonas reinhardtii), and a cyanobacterium (Phormidium laminosum). It has generally been assumed that the properties of Yd. are the same in all oxygenic organisms, while in fact there are small but significant differences. The beta-proton coupling constants are shown to be species dependent while the ring proton coupling constants are not. Estimation of the electron spin density distribution of Yd. from all three organisms has been done. This shows that changes in beta-proton coupling constants in each organism arise from the slightly different orientation of the tyrosine ring, relative to the beta-protons. The electron spin density distribution within the tyrosine ring is organism independent. The variations in the beta-proton coupling constants are reflected in the corresponding EPR spectra, where small variations in line width have been detected. These data delineate the range of natural variation in the spectroscopic properties of YD., and by assigning the features of the ENDOR spectrum, provide a basis for both the unification of studies of YD. in different organisms and the study of YZ.. The results are discussed in relation to data in the recent study (Hoganson & Babcock, 1992) using YD. in the cyanobacterium, Synechocystis PCC 6803.

Hybrid density functional calculations (B3LYP) are performed on the p-benzosemiquinone anion radical in its free and hydrogen-bonded forms. Geometries and hyperfine couplings are reported. A variety of basis sets ranging from split valence to full triple-ζ are employed. Converged results for hyperfine couplings are observed at the double-ζ level. Hydrogen bonding principally leads to increased spin density on the carbonyl carbon leading to an increase in the 13C isotropic and anisotropic hyperfine coupling of this atom. Comparison with experimentally determined isotropic and anisotropic hyperfine couplings shows good quantitative agreement between theoretical calculation and experiment.

A novel mechanism for the final stages of Nature's photosynthetic water oxidation to molecular oxygen is proposed. This is based on a comparison of experimental and broken symmetry density functional theory (BS-DFT) calculated geometries and magnetic resonance properties of water oxidising complex models in the final metastable oxidation state, S 3. We show that peroxo models of the S 3 state are in vastly superior agreement with the current experimental structural determinations compared with oxohydroxo models. Comparison of experimental and BS-DFT calculated 55 Mn hyperfine couplings for the EPR visible form shows better agreement for the oxo-hydroxo model. An equilibrium between oxo-hydroxo and peroxo models is proposed for the S 3 state and the major implications for the final steps in the water oxidation mechanism are analysed and discussed.

The identity of a key intermediate in the S2 to S3 transition of nature’s water-oxidizing complex (WOC) in Photosystem 2 is presented. Broken-symmetry density functional theory (BS-DFT) calculations and Heisenberg–Dirac–van Vleck (HDvV) spin ladder calculations show that an S2 state open cubane model of the WOC containing a μ-hydroxo O4 changes from an S = 5/2 form to an S = 7/2, form upon deprotonation of W1. Combined with X-band electron paramagnetic resonance (EPR) spectral analysis, this indicates that the g = 4.1 EPR signal corresponds to an S = 5/2 form of the WOC with W1 present as a water ligand to Mn4, while the g = 4.8/4.9 form observed at high pH values corresponds to an S = 7/2 form, with W1 as a hydroxo ligand. The latter is also likely to represent the form needed to progress to S3 in the functioning enzyme.