Solvation of Nucleobases in 1,3-Dialkylimidazolium Acetate Ionic Liquids: NMR Spectroscopy Insights into the Dissolution Mechanism

NMR studies of uracil, thymine, and adenine dissolved in 1-ethyl-3-methyl-imidazolium acetate ([C(2)mim]-[CH3COO]) and 1-butyl-3-methyl-imidazolium acetate ( [C(4)mim]-[CH3COO]) show that hydrogen bonds (HB) dictate the dissolution mechanism and that both cations and anions participate in the solvation process. For that, the 1,3-dialkylimidazolium acetate ionic liquids (ILs) were considered to be bifunctional solvation ionic liquids. In the solvation of uracil and thymine, the [CH3COO](-) anion favors the formation of hydrogen bonds with the hydrogen atoms of the N1-H and N3-H groups of the nucleobases, while the aromatic protons in the bulky cations ([C(2)mim](+) and [C(4)mim](+)), especially the most acidic H2, interact with the oxygen atoms of the carbonyl groups. In the adenine solvation, while the [CH3COO](-) anion favors the formation of hydrogen bonds with the hydrogen atoms of the amino and N9-H groups of adenine, the aromatic protons in the bulky cations ([C(2)mim](+) and [C(4)mim](+)), especially the most acidic H2, prefer to interact with the unprotonated nitrogen atoms (N1, N3, and N7) of adenine. It is clearly demonstrated that hydrogen bonding is the major driving force in the dissolution of nucleobases in 1,3-dialkylimidazolium acetate ILs. Our results show that the ionic liquid must be a good hydrogen bond acceptor and a moderate hydrogen bond donor to dissolve nucleic acid bases. To strengthen the evidence of the proposed mechanism, NMR studies in the absence of deuterated cosolvents have been used, because the use of deuterated solvents could seriously hinder the dissolving capability of the IL for nucleobases.