Electrospray ionisation mass spectrometry of ruthenium and palladium complexes with oligonucleotides

Electrospray ionisation mass spectrometry (ESI-MS) was used to characterise complexes formed in reactions between ruthenium or palladium compounds and oligonucleotides containing eight bases. ESI mass spectra of reactions involving cis- or trans-[RuCl2(DMSO)(4)] showed in-source fragmentation of the reaction products. In contrast, spectra obtained from reactions involving cis-[Ru(bipy)(2)Cl-2], mer-[Ru(terpy)Cl-3] or [Pd(en)Cl-2] were consistent with the chelating ligands remaining bound to metal-oligonucleotide products. The most abundant complex formed when cis-[Ru(bipy)(2)Cl-2] reacted with 5'-GGCTAGCC-3' comprised one Ru(bipy)(2) unit bound to the oligonucleotide. Smaller amounts of a complex in which two Ru(bipy)(2) units were bound were detected in reactions containing excess ruthenium, Mer-[Ru(terpy)Cl-3] reacted more slowly with oligonucleotides than cis-[Ru(bipy)(2)Cl-2] under otherwise identical conditions. However, the rates of reactions involving mer-[Ru(terpy)Cl-3] were increased by addition of the reducing agent ascorbate, [Pd(en)Cl-2] reacted more extensively with 5'-GGCTAGCC-3' than the ruthenium complexes, since ESI mass spectra showed ions arising from complexes containing two, three and four Pd(en) moieties bound to the oligonucleotide. Mass spectra obtained after purifying the reaction mixture by HPLC indicated that the major complex contained only two Pd(en) units bound to the oligonucleotide, suggesting that ions containing three or four Pd(en) units were formed predominantly by gas-phase chemistry in the ion source.