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Mutants of Rhodobacter sphaeroides, in which hydrogen bonds were added or removed in the reaction center (RC), produced an increase or decrease in the oxidation midpoint potential. The RC is a membrane protein, which brings about electron transfer in photosynthesis and has three protein subunits that bind the cofactors: four bacteriochlorophyll, a (BChl a) and other units. Electro nuclear double resonance and triple resonance reactions for mutants with various combinations of the four carbonyl oxygens of the BChl dimer were studied.

A comparative study of the reaction centers (RCs) of the four species of purple bacteria, Rhodobacter sphaeroides, Rhodobacter capsulatus, Rhodospirillum rubrum and Rhodospirillum centenum, using electron paramagnetic resonance, electron nuclear double resonance and electron nuclear triple resonance spectroscopies, reveals differences in the spin density distribution of the dimer across the various species. The asymmetries are due to different orbital energies of the two bacteriochlorophylls halves of the dimer. The reaction center environment influences the electronic structure of the cofactors and the electron transfer rates.

Electron paramagnetic resonance (EPR) spectroscopy at 94 GHz is used to study the dark-stable tyrosine radical [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in single crystals of photosystem II core complexes (cc) isolated from the thermophilic cyanobacterium Synechococcus elongatus. These complexes contain at least 17 subunits, including the water-oxidizing complex (WOC), and 32 chlorophyll a molecules/PS II; they are active in light-induced electron transfer and water oxidation. The crystals belong to the orthorhombic space group [P2.sub.1][2.sub.1][2.sub.1], with four PS II dimers per unit cell. High-frequency EPR is used for enhancing the sensitivity of experiments performed on small single crystals as well as for increasing the spectral resolution of the g tensor components and of the different crystal sites. Magnitude and orientation of the g tensor of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and related information on several proton hyperfine tensors are deduced from analysis of angular-dependent EPR spectra. The precise orientation of tyrosine [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in PS II is obtained as a first step in the EPR characterization of paramagnetic species in these single crystals.

Research was conducted to examine the replacement of the Fe2+ by Zn2+ in the bacterial reaction center (RC) of Rhodopseudomonas viridis by a chemical method. The procedure for the iron substitution is described. The obtained ZnRCs were characterized and the primary quinone radical anion Q(sub A)-* and the radical pair P960+*Q(sub A)-* were investigated through ENDOR, ESEEM, electron paramagnetic resonance and transient/pulsed EPR methods.