SURFACE AND ELECTROSTATIC CONTRIBUTIONS TO DNA-PROMOTED REACTIONS OF PLATINUM(II) COMPLEXES WITH SHORT OLIGONUCLEOTIDES - A KINETIC-STUDY

The kinetics and mechanism for reactions of cis-[Pt(NH3)(NH2C6H11)Cl-2], 1, and cis-[Pt(NH3)(NH2C6H11)Cl(OH2)](+), 2a, with phosphorothioate- and d(GpG)-containing oligodeoxyribonucleotides were investigated in phosphate-buffered aqueous solution at 25 degrees C. The rate of covalent adduct formation was studied as a function of pH, oligonucleotide length, and ionic strength. The results were consistent with a common mechanism in which 2a serves as the active platinating reagent. The average rate constant for the acid hydrolysis of 1 was determined to be (1.9 +/- 0.7) x 10(-5) s(-1) from experiments with d(Tp(S)T) and d(T(8)p(S)T-8) as trapping reagents. The pK(a) value of 2a was determined to be 6.4 +/- 0.2 from a series of its reactions with d(T(8)p(S)T-8) at [Na+] = 0.064 M and 5.80 less than or equal to pH less than or equal to 8.00. Direct platination of oligonucleotides with 2a at both phosphorothioate sites and d(GpG) sequences exhibited a similar length dependence, with up to a 40-fold rate increase as the DNA lengthened from 2 to 16 nucleotides at [Na+] = 0.064 M. The apparent second-order rate constants for platination with 2a at [Na+] = 0.064 M and 25 degrees C were 0.080 + 0.016 M(-1) s(-1) for d(Tp(S)T), 1.64 +/- O.11 M(-1) s(-1) for d(T(4)p(S)T-4), 3.1 i+/- 0.4 M(-1) s(-1) for d(T(8)p(S)T-8), 0.026 +/- 0.004 M(-1) s(-1) for d(GpG), and 0.92 +/- 0.04 M(-1) s(-1) for d(T(7)GGT(7)). The rate constants of phosphorothiote adduct formation were the same for both single- and double-stranded oligonucleotides. The apparent second-order rate constants for disappearance of unplatinated oligonucleotides were 0.56 +/- 0.09 M(-1) s(-1) for d(T(8)p(S)Ts) and 0.56 +/- 0.08 M(-1) s(-1) for d(T(8)p(S)T-8). d(A(16)), at [Na+] = 0.064 M and 0 degrees C. The influence of sodium ion concentration on the rate of adduct formation, with NaClO4 as supporting electrolyte, was probed by studying the reaction of 2a with d(T(n)p(S)T-n), where n = 1, 4, or 8. The apparent second-order rate constants for platination of the two longer oligonucleotides decreased as a function of increasing ionic strength and exhibited a similar reactivity at [Na+] = 0.772 M. Apparent second-order rate constants for platination with 2a at 25 degrees C were 0.624 +/- 0.007 M(-1) s(-1) for d(T(4)p(S)T-4) and 0.58 +/- 0.06 M(-1) s(-1) for d(T(8)p-(S)T-8). The reactivity of d(Tp(S)T) increased slightly in the same [Na+] interval but remained below that of the longer oligonucleotides even at high ionic strength. The apparent rate constant for platination with 2a was k(2,app) = 0.230 +/- 0.032 M(-1) s(-1) at [Na+] = 0.772 M and 25 degrees C. The variations in reactivity with oligonucleotide length cannot be explained solely on the basis of a mechanism involving nonselective cation condensation which results in local increases of the concentration of 2a on the polymer. A significant pathway for the reaction mechanism is postulated to involve directed diffusion along the DNA, similar to that suggested for protein target location.