Conformational study of cationic, zwitterionic, anionic species of aspartic acid, water-added forms and their protonation. A DFT method

The potential energy surfaces of various species of aspartic acid have been computed at the DFT/B3LYP/6-31G(d) level of theory. Conformations of 34, 11, 9 and 3 were found from 324, 162, 81 and 81 possible conformers for species H3asp+, H2asp, Hasp− and asp2−, respectively. The most stable conformation for species H3asp+, H2asp, Hasp− and asp2− are γD, αL, εL and βL conformers, respectively. Three protonation steps due to the tetrahydrated forms of species asp2−, Hasp− and H2asp of aspartic acid computed at the B3LYP/6-31+G(d, p) level of theory with zero-point vibrational energy corrections are exothermic reactions and their stabilization energies of the stepwise protonation are −244.68, −308.57 and −379.97 kcal mol−1, respectively.