Stabilization of amino acid zwitterions with varieties of anionic species: the intrinsic mechanism.

With high-level ab initio theoretical methods, varieties of novel anions (two monoanions and dianions with two binding sites) were explored to stabilize the glycine zwitterions. Unlike the malonic and oxalic dianions (J. Am. Chem. Soc. 2005, 127, 13098.), the presently found anions are self-stable and widely available, ensuring the direct and convenient applications to stabilize the zwitterions. Some of the complexes formed with the anions and glycine zwitterions have very large vertical dissociation energies (>500.0 kJ mol (-1)), implying that the contained anions can be used to stabilize much more unstable zwitterions than the glycine zwitterions. Further studies revealed that the stabilization effects are closely related with the proton-capturing capacities of the anions. In order to stabilize the glycine zwitterions, the proton affinity (PA) of the anionic species should fall within the range of 1567.0-1983.6 kJ mol (-1) and, meanwhile, the proton-affinity differences of the two binding sites (DeltaPA) should be less than 104.2 kJ mol (-1). The present results can be used to direct the efficient designs of the stabilizers to other amino acid zwitterions as well as other types of zwitterions. In addition, the density functional theory was used and compared with the default MP2 theory, with the details given in the discussions.