The Acidity and Proton Affinity of the Damaged Base 1,N⁶-Ethenoadenine in the Gas Phase versus in Solution: Intrinsic Reactivity and Biological Implications

1,N⁶-Ethenoadenine (ϵA) is a highly mutagenic lesion that is excised from human DNA by the enzyme alkyladenine DNA glycosylase (AAG). In an effort to understand the intrinsic properties of 1,N⁶-ethenoadenine, we examined its gas phase acidity and proton affinity using quantum mechanical calculations and mass spectrometric experimental methods. We measure two acidities for ϵA: a more acidic site (ΔH_acid = 332 kcal mol⁻¹; ΔG_acid = 325 kcal mol⁻¹) and a less acidic site (ΔH_acid = 367 kcal mol⁻¹; ΔG_acid = 360 kcal mol⁻¹). We also find that the proton affinity of the most basic site of 1,N⁶-ethenoadenine is 232−233 kcal mol⁻¹ (GB = 224 kcal mol⁻¹). These measurements, when compared to calculations, establish that, under our experimental conditions, we have only the canonical tautomer of 1,N⁶-ethenoadenine present. We also compare the gas phase acidic properties of ϵA with that of the normal bases adenine and guanine and find that ϵA is the most acidic. This supports the theory that AAG and other related enzymes may cleave damaged bases as anions. Furthermore, comparison of the gas phase and aqueous acidities indicates that the nonpolar environment of the enzyme enhances the acidity differences of ϵA versus adenine and guanine.