Mass Spectrometry of Nucleic Acid Constituents. Trimethylsilyl Derivatives of Nucleosides1,2

Article abstract of DOI:10.1021/jo00141a023

Trimethylsilylation of nucleosides provides derivatives which are thermally volatile and whose electron-ionization mass spectra are useful for structural characterization and for determination of chemically or biologically incorporated stable isotopes.The major reaction pathways and mechanisms of fragmentation of silylated nucleosides have been studied, on the basis of the mass spectra of a structural variety of nucleoside analogues and of uridine selectively labeled with deuterium (C-2', C-3', C-5', Si(CD3)3) and oxygen-18 (O², O⁴, O-2', O-3', O-5').Formation of most of the major base-containing ions, which are even-electron species, involving rearrangement of hydrogen from the sugar skeleton to the ionized base.The site selectivity of some of the rearrangement processes indicates that base-H-2' interactions are relatively important and that in those cases significant opening of the ribose ring does not occur prior to hydrogen abstraction by the base.The relative abundance of sugar H ions resulting from transfer of H-2' to the base was found to be greater in derivatives of β-ribofuranosides compared with that for the corresponding α anomers, reflecting differences in steric accessibility of H-2' to the base and providing a means of distinguishing α and β anomers.The determination of the site and extent of isotopic substitution in the sugar is better measured from fragment ions which contain the base plus portions of the sugar than from sugar ions which do not contain the base.This is a consequence of the multiple pathways of formation of most sugar-derived ions.