STUDIES OF THE DISSOCIATION MECHANISMS OF DEPROTONATED MONONUCLEOTIDES BY ENERGY-RESOLVED COLLISION-INDUCED DISSOCIATION

The energy thresholds for the formation of the major product ions from the collision-induced dissociation of 10 deprotonated mononucleotides (M - H)(-) were determined with a modified triple quadrupole mass spe ctrometer. The experimentally determined threshold energies for the pr oduct ions (M - H - BH)(-) where B is the nucleobase, when corrected f or the kinetic shift, were found to agree with activation energies eva luated with semiempirical PM3 calculations of the transition state ene rgies corresponding to the E2-type mechanism proposed by Beauchamp et al. The (M - H - BH)(-) ions formed from the deoxynucleoside 3'-monoph osphates had activation energies that were lower than those required f or the formation of these ions from the deoxynucleoside 5'-monophospha tes. Experimental energy thresholds were obtained also for the PO3- an d H2PO4- product ions. The mechanisms for the formation of these ions were verified by comparison of the experimental thresholds with theore tically (PM3) evaluated activation energies for given, proposed reacti on mechanisms. It is shown that the present findings provide also rele vant information concerning the fragmentation energetics and mechanism s of deprotonated dinucleotides and oligonucleotides. (C) 1997 Elsevie r Science B.V.