COENZYME F430 FROM METHANOGENIC BACTERIA - DETECTION OF A PARAMAGNETIC METHYLNICKEL(II) DERIVATIVE OF THE PENTAMETHYL ESTER BY H-2-NMR SPECTROSCOPY

A methylnickel(II) derivative of coenzyme F430 (1) was proposed as an intermediate in the enzymic process catalyzed by methyl-CoM reductase. Indirect evidence points to formation of CH3-F430M(II) in the reaction of F430M1 (obtained from F430M(II) (2)) with electrophilic methyl donors. The results presented here show, that such a compound does exist. A paramagnetic CD3-Ni(II) 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 H-2-NMR spectrum. At -40-degrees, the very broad D-signal of the axially coordinated CD3 group is found at -490 ppm. Comparison with the H-2- and H-1-NMR spectra of methyl(tetramethylcyclam)nickel(II) derivatives 4 ([Ni(II)(CH3)(tmc)]CF3SO3 (4a) is the only isolated CH3-Ni derivative of a N4macrocyclic Ni(II) 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 H-2 hyperfine coupling constants of -0.65 (4b) and -0.85 MHz (5b), respectively. The H-1-NMR spectrum indicates that, in contrast to the five-coordinate monochloro complex [Ni(II)Cl(tmc)]+, intermolecular exchange of the axial ligand in [Ni(II)(CH3)(tmc)]+ 4a is either slow at the NMR time scale or does not occur at all.