The experimental and theoretical gas phase acidities of adenine, guanine, cytosine, uracil, thymine and halouracils

The gas phase acidities GPA (ΔH (298) for deprotonation) of the most stable tautomers of adenine, guanine, cytosine, uracil and thymine are evaluated. New GPA are obtained from electron impact spectra and acid dissociation constants measured in dimethylsulfoxide for A, U and 5-FU. The average experimental GPA are: [N1H] C 340(2); T 333(2); U 333(2); 5-FU 329(4); [N9H] A 333(1); G 332(4); all in kcal/mol. Only cytosine is a weaker acid than HCl in the gas phase. The most acidic hydrogens in the nucleotides are replaced by the sugar in DNA and RNA. The experimental N3H GPA are G 334(4); U 347(2), T 347(4), while the predicted N3H 5-FU GPA is 343 kcal/mol. The NHH GPA are: C 346(4); A 352(2); G 336(4) (all in kcal/mol). These are supported by semi-empirical multiconfiguration configuration interaction calculations. The predicted C8H acidities of G and A and the C6H of T are about the same, 360(2) kcal/mol. The remaining CH acidities are 370–380 kcal/mol. The 5-halouracils are predicted to be more acidic than HCl.