Ab initio studies of the relative energetics of glycine and its zwitterion

Ab initio molecular orbital calculations using the 4-31G atomic orbital basis set have been carried out for various conformations of the glycine zwitterion. The equilibrium conformation based on standard bond lengths and bond angles is characterized by a planar framework involving an intramolecular NH...O hydrogen bond and lies 29 kcal/mol above the equilibrium conformation for the nonzwitterionic form, implying a hydration energy of approximately 50 kcal/mol for the gas-phase zwitterion. Examination of the energetics of intramolecular proton transfer indicates that the zwitterionic form is associated with a very shallow local minimum characterized by an elongated NH bond (1.075 A) and separated from the nonzwitterion by a very small barrier (approximately <0.5 kcal/mol). Thus the gas-phase zwitterion is not expected to have an observable lifetime, and it is not even clear that a vibrational state would exist within the local minimum which defines it