Analysis of platinum adducts with DNA nucleotides and nucleosides by capillary electrophoresis coupled to ESI-MS: Indications of guanosine 5'-monophosphate O6-N7 chelation

DNA is the ultimate target of platinum-based anticancer therapy. Since the N7 of guanine is known to be the major binding site of cisplatin and its analogues, adduct formation with model nucleotides, especially 2'-deoxyguanosine 5'-monophosphate (dGMP), has been studied in detail. During the last few years a coupled capillary eletrophoresis/ electrospray-ionization mass spectrometry (CE/ESI-MS) method has been advantageously used in order to separate and identify platinum adducts with nucleotides in submillimolar concentrations in aqueous solutions. Beside the bis-adduct, [Pt(NH3)2(dNMP)2]2- (NMP = 2'-deoxynucleoside 5'-monophosphate), and the well-known monochloro and monohydroxo adducts, [Pt(NH3)2Cl(dNMP)] - and [Pt(NH3)2(dNMP)OH]-, respectively, a third kind of monoadduct species with a composition of [Pt(NH3)2(dNMP)]- can be separated by CE and detected through the m/z values measured with ESI-MS. Different experimental setups indicate the existence of an O6-N7 chelate, whereas the formation of W-aPO4 macrochelates or dinuclear species is unlikely. Additionally, offline MS experiments with 2'-deoxyguanosine (dG) and stabilization of the controversially discussed O6-N7 chelate by oxidation with hydrogen peroxide support the assumption of the existence of O6-N7 chelation.