Speciation of dimethyltin(IV) and trimethyltin(IV) cations with some biologically important ligands in aqueous medium: A potentiometric investigation

The coordination of Me 2 Sn(IV) 2+ and Me 3 Sn(IV) + to uracil, thymine, 5-fluorouracil, thymidine and L-ascorbic acid in aqueous solution has been investigated by means of potentiometric titration. The concentration distribution of the various complex species in solution has been evaluated as a function of pH. It has been found that the ligands (L) with potential oxygen donor groups such as thymidine and L-ascorbic acid interact more strongly with Me 2 Sn(IV) 2+ and Me 3 Sn(IV) + as compared to those with potential nitrogen donor groups, viz. uraci], thymine and 5-fluorouracil at pH 7. The complexes are water soluble in the pH range 2-11. Thymidine forms -28% of Me 2 SnL and ∼60% of Me 3 SnLH, whereas, L-ascorbic acid forms -60% Me 2 SnL(OH) and ∼60% Me 3 SnL with Me 2 Sn(IV) 2+ and Me 3 sn(IV) + , respectively, at physiological pH. On the other hand, the nucleic acid bases, viz., uracil, thymine and 5-fluorouracil have been found to form only small amount of Me 2 SnL(OH) (-10-20%) and Me 3 SnL (∼10-15%) under similar conditions. Significant amounts of hydroxo species, viz. Me 3 SnOH and Me 2 Sn(OH) 2 , are also formed at pH7.