The differentiation of methyl guanosine isomers by laser ionization Fourier transform mass spectrometry.

Laser ionization of guanosines containing methyl substitutions in the 1-, N2-, 3'-O-, O6- and 7-positions generated two characteristic negative ions: loss of hydrogen to generate [M - H]- and elimination of the sugar ring to form the nucleic base ion. The ions generated by elimination of the sugar ring provided the information necessary to determine whether the methyl group was on the nucleic base or sugar ring. Fourier transform mass spectrometry was used to isolate and collisionally dissociate selected negative ions from these nucleosides. The collisional dissociation spectra indicated daughter ions which were sufficient to differentiate all the isomers with methyl substitution of the nucleic bases. In addition, accurate mass measurement and sequential collisional dissociation experiments were employed to investigate fragmentation mechanisms.