Distonic Isomers and Tautomers of the Adenine Cation Radical in the Gas Phase and Aqueous Solution

Abstract

The gas-phase protonation of adenine is predicted to occur preferentially at N1, N3, and N7, which have the highest proton affinities PA = 939, 932, and 904 kJ mol-1, respectively, as calculated by coupled-cluster ab initio theory and combined density functional and Møller−Plesset theory, B3-MP2, with the 6-311++G(3df,2p) basis set. The collisionally activated dissociation of protonated adenine tautomers at kiloelectronvolt energies in the gas phase proceeds by the loss of a hydrogen atom, which is accompanied by substantial hydrogen migration in the dissociating ion. The product is the adenine cation-radical (1+•), which is the most stable C5H5N5+• isomer. The adiabatic and vertical ionization energies of adenine have been calculated to be 8.28 and 8.43 eV, respectively, in excellent agreement with previous photoionization and photoelectron spectroscopy measurements. Structures and relative energies of several isomers of 1+• (e.g., three imine tautomers and six distonic ions) have been obtained computationally. The imine tautomers are 16−79 kJ mol-1 less stable than 1+• at 0 K in the gas phase. The distonic ions, in which a hydrogen atom has been shifted from C2 or C8 to N1, N3, or N7, are 51−142 kJ mol-1 less stable than 1+•. Dissociations of protonated adenine tautomers by the loss of H require threshold energies in the range of 381−415 kJ mol-1 and proceed at or close to the thermochemical thresholds. The energy differences between 1+• and its isomers are diminished to ΔG298(w) = 14−121 kJ mol-1 in the polar dielectric corresponding to aqueous solution. However, 1+• remains the most stable isomer in water. H9 and the protons of the amino group are calculated to be the most acidic protons in 1+• (pKa = 3.2−3.3). The deprotonation of the amino group in the ionized adenine moiety in DNA is expected to produce a highly reactive radical that is able to abstract a hydrogen atom from thymine and amino acid residues exothermically.