A complete kinetic study of GG versus AG platination suggests that the doubly aquated derivatives of cisplatin are the actual DNA binding species

The hairpin-stabilized double-stranded oligonucleotides d(TATGGTATT(4)ATACC ATA) (I) and d(TATAGTATT(4)ATACTATA) (II) were allowed to react with the th ree aquated forms of the antitumor drug cisplatin (cis[PtCl2(NH3)(2)], (1) which are likely candidates for DNA binding, that is, cis[PtCl(NH3)(2)(H2O) ](+) (2), cis-[Pt(NH3)(2)(H2O)(2)](2)(+) (3), and its conjugate base cis[Pt (OH)(NH3)(2)(H2O)](+) (4). The reaction between I and [Pt(NH3)(2)(H2O)](2+) (5) was also studied for comparison. All reactions were monitored by HPLC. The platination reactions of I and II were carried out in NaClO4 (0.1M) at 293 K and at a constant pH of 4.5 +/- 0.1 for 2, 3, and 5. The data relati ve to the platination by 4 were obtained from measurements in unbuffered Na ClO4 solutions (0.1M) at a starting pH close to neutrality, where 3 and 4 a re present in equilibrium. In this case, a fit function describing the pH-t ime curve allowed the determination of the actual concentrations of 3, 4, a nd the dihydroxo complex. The platination rate constants characterizing the bimolecular reactions between either I or II and 2, 3, and 4 were individu ally determined along with the rate constants for hydrolysis of the chloro- monoadducts and for the chelation reactions of the aqua-monoadducts. The re activity of compounds 2-5, which have the general formula cis-[Pt(NH3)(2)(H 2O)(Y)](2+/+), decreases in the order 3>4>5>>2, that is, Y = H2O>OH->NH3>>C l-, which is the order of decreasing hydrogen-bond donating ability of Y. D eprotonation of 3 to 4 reduces the reactivity of the platinum complex only by a factor of approximate to 2, and both complexes discriminate between th e different: purines of I and II in the same manner. Whereas 3 and 4 react approximately three times faster with the GG sequence of I than with the AG sequence of II, 2 shows a similar reactivity towards both sequences. In vi ew of the well-established preferential binding of cisplatin to GG sequence s of DNA in vivo and in vitro, this result suggests that the actual DNA pla tination species are derived from double hydrolysis of cisplatin.