The nickel(II) ion in factor F430 pentacarboxylic acid, both in the native form and in the 12,13-di-epimer, in solution can be reversibily in a one-electron to the monovalent state by the action of titanium-(III) citrate. The 3d9 Ni(I) is quantitatively detected by EPR spectroscopy as a tetragonally distorted octahedral system with the in the d(x2-y2) orbital resulting in an S = 1/2 spin with g∥ = 2.244, g⊥ = 2.061, (g∥ = 2.238, g⊥ = 2.057 for the di-epimer), subject to isotropic superhyperfine interaction from the four in-plane nitrogen ligand with A = 1.0 mT. The UV-visible spectrum of the reduced native F430 has peaks (extinction coefficients in mM-1 cm-1) at 268 (24.0), 378 (34.7), and 715 nm (2.3), In the reduced di-epimer these values are 265 (23.7), 376 (27.6), and 710 nm (1.6). Complete reduction is achieved only at alkaline pH; the process an apparent pK of 8.9 (native) or 9.4 (di-epimer). The Ni(II)/Ni(I) in F430 has a reduction Em,10.4 = -0.65 (native) or -0.62 V(diepimer). Three-pulse stimulated ESEEM spectroscopy on the reduced complexes reveals a single, near isotropic proton at ≈13 MHz. In 2H2O solution this line is replaced by a quadrupole-split deuterium resonance around ≈2 MHz, indicating that the Ni(I) in and 12,13-di-epimeric F430 is weakly coordinated by water. The absence of a strong axial bond with water would leave the ability of the d(z2) pair for nucleophilic attack on substrate carbon largely intact.
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Holliger et al. (1993) studied this question.