Temporal behavior of DNA thermal stability in the presence of platinum compounds. Role of monofunctional and bifunctional adducts

Penetrating into cell nuclei, antitumor drug cisplatin sequentially forms various intermediate and final adducts destroying local DNA structure. The demonstrated disappearance of the fine structure of melting curve of long DNAs along with a strong decrease in melting enthalpy conforms to the structural impact. However, the negative thermal effect (δTm) caused by cisplatin is relatively small if neutral medium is used in melting experiments. Cisplatin's inactive analogs transplatin and diethylenetriaminechloroplatinum {Pt[(dien)Cl]Cl} also distort DNA structure but their thermal effect is even positive. We have found that the use of alkaline medium in melting experiments strengthens the negative thermal effect for cisplatin. For transplatin and Pt[(dien)Cl]Cl, the thermal effect becomes negative that makes it qualitatively consistent with structural distortions. Those changes are explained by elimination of nonspecific electrostatic stabilization of DNA under platination. Additionally, alkaline medium fixes intermediate states of DNA platination and makes them stable against heating. These results allowed us to monitor δTm under binding of platinum compounds to DNA and their further transformation. The kinetic and thermal characteristics of monofunctional and bifunctional adducts were evaluated. It has been demonstrated that monofunctional adducts of cisplatin, transplatin and Pt[(dien)Cl]Cl produce approximately the same thermal destabilization. Cisplatin intrastrand crosslinks cause a two-fold stronger thermal destabilization than its monofunctional adducts. The value of δTm for cisplatin's final adducts is ten times larger than for transplatin. This difference mainly comes from the much stronger thermal destabilizing power of cisplatin's intrastrand crosslinks, which are responsible for antitumor activity of this compound. An increase in pH to 10.5 for melting experiments strengthens the decrease in DNA thermal stability caused by platinum compounds and makes all products of platination stable against heating. It allowed us to register melting temperature kinetics under platination and to determine the thermal impact of monofunctional and bifunctional adducts. [Display omitted] ► Alkaline medium is used in melting studies of DNA complexes with platinum compounds. ► At pH 10.5, the thermal DNA destabilization under platination is much stronger. ► The mechanism relies on promotion of DNA deprotonation under platination. ► Alkaline medium stabilizes interme