Interaction of cis-diamminediaquoplatinum(II) with adenosylcobalamin and alkylcobalamins

The interaction of cis-diamminediaquoplatinum(II) nitrate with adenosylcobalamin and a series of alkylcobalamins was studied by carbon-13 nuclear magnetic resonance spectroscopy and by electronic spectroscopy. With these cobalamins cis-[Pt(NH3)2(OH2)2]2+ forms adducts in which N(3) of the 5,6-dimethylbenzimidazole moiety is co-ordinated to platinum(II) rather than to cobalt(III) of the corrin. The chemical shifts of the 5′-methylene carbon of adenosylcobalamin–platinum adduct and of the Co–methyl carbon of the methylcobalamin–platinum adduct are characteristic of these cobalamins in the ‘base-off’ form. Furthermore, these cobalamin–platinum complexes have visible spectra identical to those of the cobalamins in acidic solution. The 13C n.m.r. spectrum of the adenosylcobalamin–platinum complex suggests the presence of a second adduct in which platinum(II) complexes are co-ordinated to both the N(3) of the 5,6-dimethylbenzimidazole nucleotide and to N(7) of the 5′-deoxyadenosyl moiety. The rate of the alkylcobalamin–platinum(II) adduct formation is determined by the strength of the cobalt(III)–5,6-dimethylbenzimidazole co-ordinate bond. Thus, adenosylcobalamin reacts much faster than adenosylepicobalamin to form the cobalamin–platinum(II) adduct.