Structural analysis of drug-DNA adducts by tandem mass spectrometry

The utility of electrospray ionisation (ESI) tandem mass spectrometry (MS/M S) for the characterisation of ligand-oligonucleotide adducts is demonstrat ed with adducts formed between the oligonucleotide 5'-CACGTG-3' and both a platinating agent, cis-diamminedichloroplatinum(II) (cisplatin), and an alk ylating ligand, n-bromohexylphenanthridinium bromide (phenC(6)Br). We have demonstrated previously that negative ion MS/MS spectra of alkylated oligon ucleotides show a highly specific fragmentation pathway that enables the si te of binding of the ligand to be readily identified. In comparison, the po sitive ion ESI-MS/MS spectra reported here also show a single major fragmen tation pathway, but the dominant ion is the protonated ligand-base adduct. MS/MS of this ion confirms the site on binding of the ligand to the guanine base. MS/MS spectra of cisplatin adducts show much less specific fragmenta tion than alkylated adducts, particularly in the negative ion mode. This su ggests that the ESI-MS/MS spectra of ligand DNA adducts are strongly influe nced by the extent to which the ligand weakens the glycosidic bond in the r esidue to which it is bound. For platinating agents, which do not labilise the glycosidic bond, additional experiments involving MS/MS of source-gener ated product ions were required to enable isomeric adducts to be distinguis hed.