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Speciation 98: Abstracts
Eugenio Garribba,1 Giovanni Micera,1 Angelo Panzanelli,1 Daniele Sanna,2 and Elzbieta Lodyga-Chruscinska3
1 Department of Chemistry, University of Sassari, Via
Vienna 2, 07100 Sassari, Italy;
2 C. N. R. - I. A. T. C. A. P. A., Via Vienna 2, 07100
Sassari, Italy;
3 Institute of General Food Chemistry, Technical University
of Lodz, Stefanowski St. 4/10, 90-924 Lodz, Poland
Mimosine, a-amino-b-[N-(3-hydroxy-4-pyridone)]-propionic acid, is a relatively rare a-amino acid which acts as a pyridoxal antagonist and inhibits growth and protein synthesis in microorganism. Mimosine is a quite interesting ligand because it contains two different residues, one of the maltol and another of the amino acid type. Therefore, two potential coordination modes are available to the ligand, both of them involving a five-membered chelated ring. A study of the interaction of mimosine with a metal ion could give information about the relative strength of two chelating groups of biochemical and pharmacological relevance.
In this paper the behaviour of the Cu(II)-mimosine system in aqueous solution, as studied by EPR spectroscopy with varying pH and ligand to metal molar ratio, is described. In order to identify the complex species, a comparative examination of the Cu(II)-maltol and Cu(II)-glycine systems has been carried out.
The study of the Cu(II)-mimosine system indicates that the maltolate (CO, O-) site can compete effectively with the aminocarboxylate (NH2, COO-) site by forming complexes of higher stability. In fact, the EPR parameters of the monometallic species are coincident with those observed in the binary Cu(II)-maltol system. However, as substantiated by EPR spectroscopy, a dimeric species is favoured in equimolar solution over a wide pH range (from 4 to 10.5). In the dimetallic arrangement, two mimosine ligands adopt a bridging mode by using the two different donor sets and each copper ion is coordinated by the maltolate-like site of a molecule and the aminocarboxylate site of another. Although different isomeric structures can be proposed for the complex, a distance in the range 5.0-5.5 Å between the copper ions is expected, consistent with the EPR data. A potentiometric study has been undertaken in order to confirm the spectroscopic results.
