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(CGCG4GCG)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(CGCG4GCG) species were observed. These data agree with spectroscopic evidence which showed that oligonucleotides with the sequence motif 5′d(CGCGnGCG)3′, where n = 2–5, formed stable four‐stranded complexes in the presence of monatomic cations, like K+, Ca2+, Na+ and Li+, but not in their absence.