Direct observation of a DNA quadruplex by electrospray ionization mass spectrometry

In 10 mM sodium phosphate, pH 7.6, containing 0.1 mM ethylenediaminetetraacetic acid, ions corresponding to the non‐covalent, four‐stranded oligonucleotide, d (CGCG 4 GCG) 4 , were detected by negative ion electrospray ionization (ESI) mass spectrometry at a low nozzle‐skimmer (ΔNS) bias (−150 V), but not at a higher ΔNS bias (> −250 V). In contrast, when the sample was desalted and analyzed by ESI mass spectrometry at a low ΔNS bias only ions for the single‐stranded d (CGCG 4 GCG) species were observed. These data agree with spectroscopic evidence which showed that oligonucleotides with the sequence motif 5 ′ d (CGCG n GCG) 3 ′, where n = 2–5, formed stable four‐stranded complexes in the presence of monatomic cations, like K + , Ca 2+ , Na + and Li + , but not in their absence.