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 gase ous deoxycytidines is affected by the acylation of the exocyclic nitro gen atom of the nucleobase. AM1 quantum-mechanical calculations show t hat the imino-oxo tautomers are thermodynamically more stable and poss ess 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. Th e lowest critical-energy unimolecular dissociation of the (M-H)(-) spe cies which corresponds to the elimination of isocyanate units, gave fu rther insights into the existence of the gaseous deoxycytidines 2-7 in their imino-oxo form.