Potentiometric and spectroscopic evidence for co-ordination of dimethyltin(IV) to phosphate groups of DNA fragments and related ligands

The co-ordination of dimethyltin(IV)(2+) to nucleic acid fragments (5'-GMP, 5'-ATP and 5'-d(CGCGCG)(2)) and their sugar constituents (D-ribose and 2-deoxy-D-ribose) as ligands was investigated in aqueous solution by means of potentiometric titration and H-1 and P-31 NMR spectroscopic methods. The results showed that in acidic medium the phosphate groups can provide suitable sites for metal ion co-ordination, while the hydroxyl groups of the studied sugars or the sugar moieties of the two nucleotides play a role in this process in the higher pH range. The base moieties of 5'-GMP and 5'-ATP were not co-ordinated to dimethyltin(IV)(2+). The stability constants of the complexes formed in the above systems were determined by pH-metric titration. The data revealed a stronger co-ordination ability of the triphosphate as compared with that of the monophosphate. The comparison of the stability constants of the D-ribose and 2-deoxy-D-ribose complexes showed that more stable species were formed when neighbouring alcoholic hydroxy groups were available for the co-ordination of metal ions. The observed chemical shift changes of the P-31 NMR resonances, as compared with those measured for the metal-free systems, demonstrated that the phosphate groups of the DNA fragment [5'-d(CGCGCG)(2)] chains act as binding sites for dimethyltin(IV)(2+) between pH 4.5 and 7. The 1D and 2D H-1 NMR spectra indicated that the base and sugar moieties do not participate in the coordination process under these conditions.