A COBALT(II) COMPLEX CONTAINING A NOVEL CATIONIC AMINO ACID LIGAND

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Abstract
For a large number of years inorganic chemists have investigated the type of complex formed between metal ions and amino acids—the study of such simple systems being a first step to an understanding of the much more complicated interaction of metal ions with proteins. We have been especially interested in transition metal derivatives of the sulphur-containing amino acids methionine', S-methylcysteine2, penillamine, and cysteine4. Methionine biosynthesis from methylcobalamin and homocysteine is well documentedsi6, and because transmethylation and methionine biosynthesisG is so important biologically, we decided to examine the donor properties of a sulfur-containing amino acid with a cationic sulfur center. We choose to study the reactions of DL-methylsulfoniummethionine chloride (LHCI) with transition metal salts. Although this species can react with metal ions eliminating HCl to form complexes of the neutral zwitterion, L,7 we have also discovered that it can coordinate to cobalt(I1) as a cationic species, LH', which contains two positive centers, (CH3), S+- and -+NH3, and a negative center, -COO-. Such a ligand with formal charge separation of such a degree has hitherto not been encountered in the literature. The cationic nature of t h s ligand represents the first type of overall cationic carboxylate donor, other cationic ligands having N, P, As, or S donor atoms8