A SPECTROELECTROCHEMICAL STUDY OF FACTOR F430 NICKEL(II/I) FROM METHANOGENIC BACTERIA IN AQUEOUS-SOLUTION

The nickel(II) ion in factor F430 pentacarboxylic acid, both in the native form and in the 12,13-di-epimer, in aqueous solution can be reversibily reduced in a one-electron step 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 hole in the d(x2 - y2) orbital resulting in an S = 1/2 spin with g(parallel-to) = 2.244, g(perpendicular-to) = 2.06 1, (g(parallel-to) = 2.238, g(perpendicular-to) = 2.057 for the di-epimer), subject to isotropic superhyperfine interaction from the four in-plane nitrogen ligands 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 has an apparent pK of 8.9 (native) or 9.4 (di-epimer). The Ni(II)/Ni(I) couple in F430 has a reduction potential E(m,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 resonance at almost-equal-to 13 MHz. In (H2O)-H-2 solution this line is replaced by a quadrupole-split deuterium resonance around almost-equal-to 2 MHz, indicating that the Ni(I) in both native 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.