Speciation 98: Abstracts

The stability of metal-ligand complexes is usually quantitated in terms of macroconstants. This classical parameter is related unambiguously to the binding site in the case of monodentate ligands only. Most bioligands are, however, multidentate, with the capacity of alternative binding modes. For example, the small biomolecule and prodrug dihydroxyphenylalanine (DOPA, Levodopa) forms metal complexes in "catechol-type" and "amino acid-type" binding fashion, of comparable stability. Large biomolecules can form a large number of various complexes with isomeric binding mode that are of comparable stability and always coexist in solution.

Site-specific metal- or proton-binding strength combined with the defined complexation/protonation state of the side-chains can only be correctly characterised in terms of microconstants. In a broader sense, not only complexation, but several other reactions of multifunctional ligands can only be correctly characterised in terms of microscopic parameters. Such reactions of biochemical importance are ester hydrolysis, thiol-disulfide conversions, partition, etc.

An even more subtle approach is when not only the binding site and complexation/protonation states of the side chains are definite, but also the rotational/conformational status of the molecule. The related rota-micro species are often doubly isomeric ones, e. g. they are of the same stoichiometry, but differ both in the sense of binding sites and conformational state. They provide concentration-weighted, composite spectroscopic and other signals, but act individually on biological targets. The related submicro constants can be estimated by the combination of various equilibrium and spectroscopic methods and corresponding calculations.

Micro- and rota-micro speciation will be illustrated by several examples of biomedicinal significance.

Back to the list of abstracts