Kinetics and mechanism for platination of thione-containing nucleotides and oligonucleotides: evaluation of the salt dependence

摘要

Reactions of cis-[PtCl(NH_3)(CyNH_2)(OH_2)]~+ (Cy = cyclohexyl) with thione-containing single-stranded oligonucleotides d(T_8XT_8) and d(XT_(16) (X = ~(s6)I or ~(s4)U) and the mononucleotides 4-thiouridine (~(s4)UMP) and 6-mercaptoinosine (~(s6)IMP) have been studied in aqueous solution at pH 4.1. The reaction kinetics was followed using HPLC methodology as a function of ionic strength in the interval 5.0 mM ≤ I ≤ 300 mM. A two-fold kinetic preference for reaction with the ~(56)I moiety over ~(s4)U is observed in both monomeric and oligomeric systems. The rate for adduct formation with the oligonucleotides d(T_8XT_8) and d(XT_(16)) decreases with increasing ionic strength of the medium. The effect is most pronounced for adduct formation with the middle positions, e.g. for d(T_8~(s6)IT_8): k_(2,app) = 370 M~(-1) s~(-1) and 13 M~(-1) s~(-1) at I = 5.0 and 300 mM, respectively, and slightly less pronounced for adduct formation at the end positions, e.g. for d(~(s6)IT_(16): k_(2,app) = 130 M~(-1)s~(-1) and 11 M~(-1) s~(-1) at I = 5.0 and 300 mM, respectively. Analysis of the salt dependence using the Bronsted-Debye-Huckel relationship shows that the reactions with the monomers are well described as an interaction between a monovalent cation and a monovalent anion. In contrast, a similar analysis of the oligonucleotide reactions indicates influence from polyelectrolyte effects. The results support a mechanism in which pre-association on the DNA surface precedes adduct formation, regardless of the exact location of the final binding site.