Coenzyme F430 from Methanogenic Bacteria: Detection of a Paramagnetic Methylnickel(II) Derivative of the Pentamethyl Ester by 2H‐NMR Spectroscopy

Abstract

AbstractA methylnickel(II) derivative of coenzyme F430 (1) was proposed as an intermediate in the enzymic process catalyzed by methyl‐CoM reductasc. Indirect evidence points to formation of CH3–F430MII in the reaction of F30M1 (obtained from F430MII (2)) with eleclrophilic methyl donors. The results presented here show, that such a compound does exist. A paramagnetic CD3–NiII derivative 5b of the pentamethyl ester 2 (F430M) of coenzyme F430 was prepared by in situ methylation with (CD3)2Mg and characterized by its isotropically shifted 2H‐NMR spectrum. At −40°, the very broad D‐signal of the axially coordinated CD3 group is found at −490 ppm. Comparison with the 2H‐ and 1H‐NMR spectra of mcthyl(tetramethylcyclam)nickel(II) derivatives 4 ([NiII(CH3))(tmc)]CF3SO3 (4a) is the only isolated CH3–Ni derivative of a N4macrocyclic NiII complex' shows that the large isotropic shift to high field is characteristic for a Me group axially bound to the Ni center. The temperature dependence of the isotropic shift of the CD3–Ni group in both 4b and 5b follows Curie's law and yields 2H hyperfine coupling constants of −0.65 (4b) and −0.85 MHz (5b), respectively. The 1H‐NMR spectrum indicates that, in contrast to the five‐coordinate monochloro complex [NiIICl(tmc)]+, intermolecular exchange of the axial ligand in [NiII(CH3)(tmc)]+ 4a is either slow at the NMR time scale or does not occur at all.