Mass spectrometry of some modified nucleosides of the pyrimidine and purine series

Abstract

The mass spectral fragmentation patterns of some pyrimidine and adenine nucleosides modified in a carbohydrate moiety have been investigated. The position of the chlorine atom in a carbohydrate moiety is characterized in the mass spectrum by the following peaks. In the case of pyrimidine nucleosides the location of the chlorine atom at C-3′ leads as a general fragmentation pathway to a loss of a Cl radical to yield ion a, which further decomposes into ions h, n and j having the composition base plus 30,68 and 56 mass units, respectively. The location of the chlorine atom at C-2′ involves initial elimination of HCI to form the ion(s) p which are transformed into the characteristic O2,2′-cyclo ion r. The latter is one of the most abundant ions in the mass spectra of O2,2′-cyclonuleosides. In the case of adenine chlorodeoxynucleosides the principal fragmentation routes from the molecular ion involve (i) a loss of a Cl radical and, further, the elements of H2O and formaldehyde 5′-CH2O result in the formation of the characteristic Ion n (b+68mass units); (ii) a reverse sequence of transformations of the molecular ion leading to ion n. An intense ion d containing a C-1′–C-2′ fragment of the carbohydrate ring with a substituent at C-2′is of considerable interest in elucidating the isomeric structure of the adenine chlorodeoxynucleosides. The same ion d is informative in establishing the isomeric structure of adenine 2′,3′- and 3′,5′-anhydronucleosides.