THEORETICAL-STUDY ON COMPLEXATION OF PB(I I) AND HG(II) CATIONS WITH D-TALOSE

Molecular modeling of interactions between D-talopyranose and D-talofu ranose conformers and Pb2+ and Hg2+ cations was done in the gaseous st are using the AM1 method, both with ''naked'' species and with some wa ter molecules added. A complete conformational analysis of the free D- talose molecule was carried out using a Monte Carlo method (BATCHMIN p rogram). Lower minima were identified, and the different orientations of the hydroxyl groups given. The calculated equilibrium distribution of anomers in the gaseous phase differs from that obtained experimenta lly in aqueous solution. This could be related to the predominance of intramolecular hydrogen bonding for the isolated molecule and intermol ecular hydrogen bonding in aqueous media. In the presence of a cation, the analysis was carried out using the AM1 method for every conformat ion of the ligand. In the gaseous phase, Angyal's predictions are not respected. The lowest energy is represented by a flexible beta-pyranos ic form and some complexes are tetra or pentacoordinated. For the fura nosic species, the alpha F forms bearing a cis-cis-cis sequence of hyd roxyl groups are the less stable. Amongst the complexes of carbohydrat es with metal cations in aqueous solutions, Pb2+ forms complexes and H g2+ does not. The calculations show that, for the isolated complexes, the reverse should be true. The lesser stability in water of Hg2+ comp lexes with respect to Pb2+ could be interpreted in terms of (1) a more unfavourable entropic hydration effect; (2) a less favourable differe nce of translational entropy. A complete answer requires the dynamical study of the system in water by a Monte Carlo method.