Stability of the normal, zwitterionic neutral and anionic forms of aspartic acid in gas phase and aqueous media

Aspartic acid has a place of special importance among amino acids in view of its property to racemize from the - to the -form in living and nonliving systems which can be used to determine age. Molecular geometries of two isomeric normal neutral forms (called A and B forms), isomeric zwitterionic neutral forms (called A(Z) and B(Z) forms) and anions of these forms of aspartic acid were optimized at the RHF/6-31+G^* level. The geometries of hydrogen bonded complexes of each of the above species with one, two and three water molecules were also optimized at the same level. All these systems were solvated in bulk water using the polarized continuum model (PCM) of the self-consistent reaction field (SCRF) theory at the RHF/6-31+G^* level using the gas phase optimized geometries. It is found that in gas phase, the normal neutral A and B forms of aspartic acid would coexist while the zwitterionic A(Z) and B(Z) forms of the same would be absent. On the other hand, in bulk aqueous media, the normal neutral A and B forms of aspartic acid would not occur while the zwitterionic A(Z) and B(Z) forms would be present. Further, the abundance of the A(Z) form would be much more than that of the B(Z) form in bulk water. If the solution pH is adjusted appropriately so that a monoanion is formed, the anion of aspartic acid obtained from the neutral zwitterionic form by deprotonating O₈ would occur dominantly.