Entropy effect in the evaluation of the proton affinity of N-3-benzoyl-2-deoxycytidines by the kinetic method

The kinetic method has been applied, in its extended form, to the evaluation of the entropy effect in the determination of the proton affinity (PA(N)) of N-3-para substituted benzoyl 2'-deoxycytidines. The latter, mixed with nucleosides of known PA, afforded the appropriate proton-bound heterodimers when subjected to electrospray ionization. Their collisional induced dissociations were studied in a QqTOF hybrid instrument as a function of both the gas pressure and the collision energy of the dimers entering q. The value of DeltaS for each reaction was evaluated and correlated with the effect of the substituent at the para. position of the benzoyl group. For each reaction, a Hammett plot of Delta PA(N) vs. cr values of the substituents on the benzoyl group shows that the carbonyl group participates, in the transition state, to the stabilization/destabilization of the N-3-protonated species. In the cases examined, one can rule out that the formation of intramolecular hydrogen bonds occurs under thermodynamic control. The activation energy for the back reaction can, therefore, be neglected. [See chart on next page.] (C) 2001 Elsevier Science B.V.