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) to 5′-GMP, 5′-ATP and 5′-d(CGCGCG)2 and to their sugar constituents (D-ribose and 2-deoxy-D-ribose) was investigated in aqueous solution by means of potentiometric titration and 1H and 31P NMR spectroscopic methods. The results showed that in acidic media the phosphate groups can provide suitable sites for metal ion co-ordination, while the hydroxy groups of the sugars or the sugar moieties of the two nucleotides play a role in this process at higher pH. The base moieties of 5′-GMP and 5′-ATP were not co-ordinated to dimethyltin(IV). 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 co-ordination. The observed chemical shift changes of the 31P 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) between pH 4.5 and 7. The 1- and 2-D 1H NMR spectra indicated that the base and sugar moieties do not participate in the co-ordination process under these conditions.