Influence of dT20 and [d(AT)10]2 on Cisplatin Hydrolysis Studied by Two-Dimensional [1H,15N] HMQC NMR Spectroscopy

The influence of the presence of DNA on the kinetics of cisplatin (cis‐[PtCl2(NH3)2]) aquation (replacement of Cl− by H2O) and anation (replacement of H2O by Cl−) involved in the hydrolysis of cisplatin have been determined by two‐dimensional [1H,15N] HMQC NMR spectroscopy. Single‐stranded dT20 and double‐stranded [d(AT)10]2 oligonucleotides were used as DNA models, avoiding guanines which are known to react rapidly with aquated cisplatin forms. Reactions starting from cis‐[PtCl2(15NH3)2], or from a stoichiometric mixture of cis‐[Pt(15NH3)2(H2O)2]2+ and Cl− (all 0.5 mM PtII; in ionic strength, adjusted to 0.095 M or 0.011 M with NaClO4, pH between 3.0 and 4.0) were followed in an NMR tube in both the absence and presence of 0.7 mM dT20 or [d(AT)10]2. In the presence of dT20, we observed a slight and ionic‐strength‐independent decrease (15–20 %) of the first aquation rate constant, and a more significant decrease of the second anation rate constant. The latter was more important at low ionic strength, and can be explained by efficient condensation of cis‐[Pt(15NH3)2(H2O)2]2+ on the surface of single‐stranded DNA, in a region depleted of chloride anions. At low ionic strength, we observed an additional set of [1H,15N] HMQC spectral signals indicative of an asymmetric species of PtN2O2 coordination, and we assigned them to phosphate‐bound monoadducts of cis‐[Pt(15NH3)2(H2O)2]2+. Double‐stranded [d(AT)10]2 slowed down the first aquation step also by approximately 15 %; however, we could not determine the influence on the second hydrolysis step because of a significant background reaction with cis‐[Pt(NH3)2(H2O)2]2+. DNA has a significant influence on the kinetics of cisplatin hydrolysis: The greatest effect is observed for the replacement of H2O by Cl− in the diaqua form, cis‐[Pt(H2O)2(NH3)2]2+ (see graphic). This dicationic species efficiently condenses on the chloride‐depleted DNA surface where no chloride anation takes place. At low ionic strength, cis‐[Pt(H2O)2(NH3)2]2+ reversibly binds to an oxygen ligand of DNA, identified tentatively as the phosphodiester group.