Proton and hydrogen atom adducts to cytosine. An experimental and computational study

Cytosine cations were generated by chemical ionization and fast-atom bombardment of cytosine, and their dissociations in the gas phase were studied by tandem mass spectrometry and quantum chemistry calculations at levels of theory up to CCSD(T)/aug-cc-pVTZ. Metastable cytosine cations undergo loss of NH3, H2O, and [CHNO] molecules as major dissociations. Mechanisms for these dissociations were established by a combination of MS3, deuterium labeling, and calculations. Several tautomers of protonated cytosine were identified to exist as local energy minima. The tautomers are predicted to undergo facile prototropic isomerizations at internal energies below the lowest ion-dissociation thresholds. The lowest-energy elimination of ammonia proceeds from open-ring structures and is accompanied by exchange of the N-3 and N-7 positions in cytosine, so that either nitrogen atom can be lost in the ammonia molecule. Cytosine radicals corresponding to hydrogen atom adducts are stable when formed by femtosecond electron transfer to the cations. A fraction of the radicals dissociate by loss of hydrogen atom to form cytosine tautomers. Ring-cleavage dissociations leading to the loss of CO and HNCO are less abundant, and are predicted to proceed from excited electronic states accessed by vertical electron transfer.