Estimation of gas-phase acidities of deoxyribonucleosides: an experimental and theoretical study.

We determined the gas-phase acidities (DeltaH(acid)) of four deoxyribonucleosides, i.e., 2'-deoxyadenosine (dA), 2'-deoxyguanosine (dG), 2'-deoxycytidine (dC), and 2'-deoxythymidine (dT) by applying the extended kinetic method. The negatively charged proton-bound hetero-dimeric anions, [A - H - B](-) of the deoxyribonucleosides (A) and reference compounds (B) were generated under electrospray ionization conditions. Collision-induced dissociation spectra of [A - H - B](-) were recorded at four different collision energies using a triple quadrupole mass spectrometer. The abundance ratios of the individual monomeric product ions were used to determine the DeltaH(acid) of the deoxyribonucleosides. The obtained DeltaH(acid) value follows the order dA > dC > dT > dG. The DeltaG(acid) (298 K) values were determined by using DeltaG(acid) = DeltaH(acid) - TDeltaS(acid) where the DeltaH(acid) and DeltaS(acid) values were determined directly from the kinetic method plots. The DeltaH(acid) values were also predicted for the deoxyribonucleosides at the B3LYP/6-311+G**//B3LYP/6-311G** level of theory. The acidity trend obtained from the computational investigation shows good agreement with that obtained experimentally by the extended kinetic method. Theoretical calculations provided the most preferred deprotonation site as C5'-OH from sugar moiety in case of dA, and as -NH(2) (dC and dG) or -NH- (dT) from nitrogenous base moiety in the case of other deoxyribonucleosides.