Gas-phase quasi-degeneracy of zwitterionic and canonical tautomers of glycine and proline induced by the presence of the MAlF4 (M = Li,Na, K) salts

The ionic MAlF₄ salts (M = Li, Na, K) are postulated to act as effective stabilising agents with respect to glycine and proline zwitterionic tautomers. On the basis of ab-initio MP2/6-311++G(d,p) theoretical calculations, it is demonstrated that strong electrostatic interactions between the polar MAlF₄ moiety and either Gly and Pro zwitterion should allow to suppress the intra-molecular proton transfer in the amino acid and thus render the zwitterion tautomer stable in the gas phase. Among the systems proposed, the K⁺AlF₄^– turned out to be the most efficient stabilising compound as its presence is expected to lead to the energetic quasi-degeneracy of the canonical and zwitterionic tautomers of glycine and proline.