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

We determined the gas-phase acidities (ΔH_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 ΔH_acid of the deoxyribonucleosides. The obtained ΔH_acid value follows the order dA7>dC7>dT7>dG. The ΔG_acid (298 K) values were determined by using ΔG_acid=ΔH_acid-TΔS_acid where the ΔH_acid and ΔS_acid values were determined directly from the kinetic method plots. The ΔH_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 −NH₂ (dC and dG) or -NH- (dT) from nitrogenous base moiety in the case of other deoxyribonucleosides.