Systematic analysis of desorption chemical ionization (DCI) mass spectra of nucleic acids--7.

The positive ion and negative ion pyrolysis mass spectra of the herring sperm DNA have been studied using desorption chemical ionization. The positive ion desorption chemical ionization spectra have been produced with CH4, i-C4H10, NH3, HCl and Cl2; the negative ones with N2O/CH4, N2O/i-C4H10, Cl2, CCl4, HCl and via electron capture. These spectra have been compared with the electron impact ionization spectra. We have observed an important increase of sensitivity when negative ionization has replaced the positive ionization mode. The series of diagnostic ions resulting from direct chemical ionization belong to the family of base + reagent ion X [BH + X] and base + X - HX ion [B]. Their abundance has increased considerably compared to the electron impact spectra. The application of these new diagnostic ions in nucleic acid studies is interesting especially for the much higher abundance of the usually weak dG fragment ion obtained in the negative ionization mode. The dG-base segment of the DNA is the most nucleophilic centre of the whole nucleic acid and is implicated in numerous important biochemical reactions involving, for example, proteins.