SPECIFIC AND NONSPECIFIC DIMER FORMATION IN THE ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY OF OLIGONUCLEOTIDES

Specific and nonspecific noncovalent dimer ions of oligonucleotides (O DNs) were observed when mixtures of complementary or noncomplementary strands were analyzed via negative ion electrospray ionization mass sp ectrometry. Dimer formation was concentration dependent and nearly alw ays occurred when the concentration of ODN exceeded 100 mu M. Dimers w ere observed even for short-length ODNs for which the melting temperat ure (T-m) was well below the experimental temperature and which, there fore, would not be expected to form stable solution duplexes. The abun dance of the heterodimer ions seems to correlate with the number of ex pected hydrogen bonds from Watson-Crick base pairing. As the energy of the incoming ion beam (orifice potential) was increased, the absolute and relative abundance of the dimer ions unexpectedly increased.