Fast-atom bombardment mass spectrometry of some nucleoside analogs

A new series of thymine derivative acyclic nucleosides, with mono- or polyfluorosubstitution on the acyclic portion of the molecules have been studied by fast-atom bombardment mass spectrometry, with the aid of metastable ion studies and deuterium labelling experiments. The protonated molecules show four main metastable-supported fragmentation processes, consisting of loss of CX; (where X,=H$ or HFz or F3 respectively), loss of H,O, formation of protonated thymine, and formation of N-methyl thymine. Clear differences in the yields of these reactions have been found among the compounds examined and these yields have been rationalized in terms of the different electron-withdrawing powers of the different CX, groups. For one of the compounds examined, the presence of a metastable peak related to the formation of methyl thymine indicates the occurrence of two different mechanisms which have been investigated by means of deuterium labelling experiments.