Rhombic EPR spectrum

The low-spin Nini06 compound [Ni(bipy02)3]3+ (bipy02 = 2,2 -bipyridyl-1,1 -dioxide) is the product of constant-potential electrolysis of the Ni11 salt.176 Its violet solutions show a rhombic EPR spectrum indicative of a Jahn-Teller-distorted (dz )1 ground state. 

Electrochemistry and spectroscopy of the tt cation radical of meso-tetraalkylchlorin (tetra-methyl) and various porphyrins (tetramethyl, tetraethyl, and tetra-ra-propyl) indicate that these do not convert to Nim at low temperatures.280 Optical evidence reveals, however, that oxidation of the tt cation radical of [Ni(pEt2N)(TPP)] leads to a Ni111 cation radical which can be further oxidized to a Ni111 porphyrin dication. Similar studies have been carried out for various other derivatives of me.so-tetraarylporphyrins such as /V-oxides of TPP and 5,10,15,20-tetramesitylpro-phyrin (TMP). Addition of trifluoroacetic acid (TFA) to the /V-oxide of [NinTMP] at —25 °C in CH2C12 results in [Nim(TMP)]+ with a rhombic EPR spectrum, g = 2.40, 2.12, and 2.04.281... 

The interest in low-valent Ni complexes in S-rich environments has been stimulated by the presence of Ni in [Ni,Fe] hydrogenase and CODH. While thiolate ligation usually favors higher oxidation states, thioethers stabilize Ni1 and Ni°. In most cases, however, Ni1 ions of an NiS4 chromophore are unstable with respect to disproportionation. The cyclic voltam-mogram of square planar (983) with homoleptic thioether coordination exhibits a quasi-reversible wave at —0.42V (vs. NHE), which on the basis of the rhombic EPR spectrum (gi 2.27, g2 2.11, and g3 2.03) of the chemically reduced species (Na/Hg) is assigned to metal-centered reduction. 8... 

Oxyanions also affect the coordination chemistry of the metal center (84). Molybdate and tungstate are tightly bound noncompetitive inhibitors (Ki s of ca. 4 (iM) (85). These anions bind to the reduced form of the enzyme, changing the rhombic EPR spectrum of the native enzyme to axial (Figure 1) and affecting the NMR shifts observed (84,85). Comparisons of the ENDOR spectra of reduced uterofenin and its molybdate complex show that molybdate binding causes the loss of iH features which are also lost when the reduced enzyme is placed in deuterated solvent (86). These observations suggest that molybdate displaces a bound water upon complexation. 

The earliest high-oxidation-state complex of nickel reported was the heteropoly(molybdate) (132, 133) complex [NilvMo90 32]6. which contains nickel(IV) in an octahedral Ni06 coordination environment. There is no evidence for the corresponding nickel(III) species but further work on nickel(IV) complexes of this type has been reported recently (134). Nickel(III) can be prepared in a six-coordinate oxygen donor environment (135) as a tris chelate with 2,2 -bipyridine-l,T-dioxide (bpy02). The complex has a rhombic EPR spectrum and a reduction potential of 1.7 V, from which an estimate of the reduction potential of the ion [Nini(H20)6]3+ of 2.5 V (versus nhe) has been calculated. 

The active sites of [FeNi]-hydrogenases give rise to a number of interesting EPR spectra. There are three EPR active states labeled A, B and C. Each of these states produces a rhombic EPR spectrum. Data for a variety of different [FeNi]-hydrogenases are collected in Table 1. These signals have been assigned to the Ni ion on the basis of Ni-labeling studies [30]. 

Compound 11 has two possible heme binding sites per peptide (based on two histidine residues) and an estimated separation between the Fe atoms of about 20 A. Due to dimerization of the model protein 11, addition of Fe heme leads to the incorporation of four redox centers at the four predetermined binding sites of the protein dimer. CD spectroscopy also confirms in tis case that well-defined binding sites are present prior to heme coordination (0222 = -26000°cm2dmoF ). As expected the heme-protein complex exhibits a rhombic EPR spectrum (g = 2.89, gy = 2.24, gx = 1.54) confirming the coordination of low-spin Fe " through two trans imidazole groups. 

Molybdenum is present in many enzymes and is often EPR detectable (except for Mo-nitrogenase, where it exists in a complex MoFeySg homocitrate cluster). Shown in Figure 9 is the Mo " " (f) X-band spectrum of one form of xanthine oxidase, called the Very Rapid form. Xanthine oxidase is a homodimer containing one Mo center, two Fe-S, centers and one molecule of flavin adenine dinucleotide per subunit. This enzyme catalyzes the oxidation of xanthine at the Mo center with subsequent reduction of O2 at the flavin center. In this enzyme Mo + exhibits a slightly rhombic EPR spectrum (g = 2.0252, gj, = 1.9550, g = 1.9495 gave < 2). 

Calderia and co-workers have characterized enoate reductase with optical and EPR spectroscopy during potentiometric titrations. A flavin radical, with an isotropic EPR signal with g = 2.0 and a linewidth of 1.7 mT, decreasing to 1.3 mT in deuterated buffer, was detected as a redox intermediate. In the reduced form, the [4Fe-4S] moiety was characterized by a single rhombic EPR spectrum, observed in a wide range of temperatures (4.2-60 K). The gfmax value is low... 

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