Reactions of cis-[Pt(NH3)(NH2C6H11)Cl(OH2)](+) with d(Tp(S)T) and d(T(n)p(S)T16-n), n = 1, 4, 8, 12, and 15, were investigated by use of HPLC in an aqueous medium with pH 4.1 +/- 0.1 and sodium and magnesium ion concentrations varying between 1.5 mM and 0.50 M. Platination of the oligonucleotide fragments is favored over platination of d(Tp(S)T) in the whole salt concentration interval studied. The maximum rate enhancement after incorporation of the p(S)-site into the polymeric DNA environment is observed for d(T(8)p(S)T-8), which reacts up to ca. 500 times faster than d(Tp(S)T), after suitable changes of the cation concentrations in the reaction medium. The platination rates of the oligonucleotide fragments d(T(n)p(S)T16-n) decrease with increasing salt concentration. For a given phosphorothioate position, the rate also decreases when the cations in the medium are changed from Na+ or K+ to Mg2+, even at constant ionic strength. The reactions with embedded p(S)-sites in d(T(n)p(S)T16-n), n = 4, 8, and 12, were found to be kinetically favored over reactions with the 5'- and 3'-end positions. In a reaction medium containing monovalent cations there is a strong preference for platination of d(T(8)p(S)T-8), whereas d(T(4)p(S)T-12) and d(T(12)p(S)T-4) show intermediate reactivity compared with fragments with n = 1 and 15. In contrast, no kinetic discrimination is found between the p(S)-sites in d(T(n)p(S)T16-n), n = 4, 8, and 12, in the presence of Mg2+. The results are interpreted in terms of a general mechanism where preaccumulation of the cationic Pt(II) complex on the oligomers is required for product formation. The kinetics are consistent with a reaction model that includes release of cations from the DNA surface during the adduct formation process.