Why is glycine a zwitterion in aqueous solution? A theoretical study of solvent stabilising factors

In this paper an ab initio study of specific and non-specific solvent effects on the glycine zwitterion energetics and formation mechanism is presented. Calculations are carried out at the HF an MP2 levels with the 6–31 + G∗∗ basis set. The description of glycine in solution requires the use of continuum models or the inclusion of several discrete water solvent molecules into the calculations. Zwitterion formation in solution occurs by means of an intramolecular proton transfer from oxygen to nitrogen. An analysis of the intermolecular mechanism shows that the addition of one water molecule does not favour the process either geometrically or energetically.