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 (DELTANS) bias (-150 V), but not at a higher DELTANS bias (> -250 V). In contrast, when the sample was desalted and analyzed by ESI mass spectrometry at a low DELTANS 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(CGCG(n)GCG)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.