Probing of the tautomeric amino-oxo vs imino-oxo equilibrium of substituted deoxycytidines by fast atom bombardment mass spectrometry

The position of the amino-oxo-imino-oxo tautomeric equilibrium of gaseous deoxycytidines is affected by the acylation of the exocyclic nitrogen atom of the nucleobase. AM1 quantum-mechanical calculations show that the imino-oxo tautomers are thermodynamically more stable and possess lower-lying lowest unoccupied molecular orbitals than the related amino-oxo isomers. Accordingly, ionization of the acylated derivatives 2-7 by fast atom bombardment produces stable M(-.) radical anions. The lowest critical-energy unimolecular dissociation of the (M-H)(-) species which corresponds to the elimination of isocyanate units, gave further insights into the existence of the gaseous deoxycytidines 2-7 in their imino-oxo form.