The following dinuclear exchange-coupled manganese complexes are investigated:  [dtneMnIIIMnIV(μ-O)2μ-OAc](BPh4)2 (dtne = 1,2-bis(1,4,7-triazacyclonon-1-yl)ethane), [(CH3)4dtneMnIIIMnIV(μ-O)2μ-OAc](BPh4)2 ((CH3)4dtne = 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane), [(CH3)4dtneMnIVMnIV(μ-O)2μ-OAc](ClO4)3, [(tacn)2MnIIIMnIV(μ-O)2μ-OAc](BPh4)2 (tacn = 1,4,7-triazacyclononane), [bpy4MnIIIMnIV(μ-O)2](ClO4)3 (bpy = 2,2‘-bipyridyl), and [phen4MnIIIMnIV(μ-O)2](ClO4)3 (phen = 1,10-phenanthroline). For three of these complexes, X-ray structural data obtained on single crystals are reported here. All complexes are strongly antiferromagnetically coupled, with exchange coupling constants ranging from J = −110 cm-1 (bis-μ-oxo-μ-acetato-bridged) to −150 cm-1 (bis-μ-oxo-bridged). EPR investigations at X- and Q-band frequencies are reported for all five mixed-valence MnIIIMnIV complexes. G tensors and 55Mn hyperfine coupling constants (hfc's) were obtained by simultaneous simulation of the EPR spectra at both frequency bands. By using the vector model of exchange-coupled systems, tensor axes could be related to the molecular structure of the complexes. Hyperfine coupling constants from 55Mn cw-electron−nuclear double-resonance (ENDOR) spectra were in agreement with those obtained from the simulation of the EPR spectra. Ligand hyperfine couplings (1H and 14N) were also measured using cw-ENDOR spectroscopy. Electron spin−echo envelope modulation spectroscopy (ESEEM) spectra yielded information about small 14N hyperfine and quadrupole coupling constants that could not be resolved in the ENDOR spectra. On the basis of specifically deuterated complexes and results from orientation-selection ENDOR spectra, some proton hfc's could be assigned to positions within the complexes. Using an extended point-dipole model and the coordinates provided by the X-ray structure analysis, all dipolar hfc's of the complexes were calculated. Comparison of these hfc's with experimentally obtained values led to a consistent assignment of most hf tensors to molecular positions. The electronic structures of the investigated complexes are compared with each other, and the relevance of the results for metalloenzymes containing at least a dinuclear manganese core is discussed.

A comparative study of X-band EPR and ENDOR of the S2 state of photosystem II membrane fragments and core complexes in the frozen state is presented. The S2 state was generated either by continuous illumination at T=200 K or by a single turn-over light flash at T=273 K yielding entirely the same S2 state EPR signals at 10 K. In membrane fragments and core complex preparations both the multiline and the g=4.1 signals were detected with comparable relative intensity. The absence of the 17 and 23 kDa proteins in the core complex preparation has no effect on the appearance of the EPR signals. (1)H-ENDOR experiments performed at two different field positions of the S2 state multiline signal of core complexes permitted the resolution of four hyperfine (hf) splittings. The hf coupling constants obtained are 4.0, 2.3, 1.1 and 0.6 MHz, in good agreement with results that were previously reported (Tang et al. (1993) J Am Chem Soc 115: 2382-2389). The intensities of all four line pairs belonging to these hf couplings are diminished in D2O. A novel model is presented and on the basis of the two largest hfc's distances between the manganese ions and the exchangeable protons are deduced. The interpretation of the ENDOR data indicates that these hf couplings might arise from water which is directly ligated to the manganese of the water oxidizing complex in redox state S2.

A series of substituted bacteriochlorophyll molecules, all used in reconstitution experiments of reaction centers of Rhodobacter sphaeroides (Struck et al. Biochim. Biophys. Acta 1991,1060,262-270), were characterized by EPR, electron-nuclear double (ENDOR), and electron-nuclear-nuclear triple (TRIPLE) resonance spectroscopy in their monomeric radical cation states. Effects of different substituents at position 3 in the porphyrin macrocycle were considered, especially for two "crosslinks" between plant and bacterial chlorophylls. These are 3-vinylbacteriochlorophyll where the "bacteria" acetyl group at position 3 was substituted by vinyl and 3-acetylchlorophyll where the "plant" vinyl group was substituted by acetyl. In addition, effects of substitutions at position 132 were studied. All major hyperfine coupling constants of proton and nitrogen nuclei were elucidated from the spectra and assigned to molecular positions by comparison with the parent radicals. The data were compared with those calculated by an INDO-type program, showing that INDO essentially models the effect of the different substituents correctly.

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