Second order Moller-Plesset (MP2) perturbation theory with 6-311 + +G (d, p) as well as aug-cc-pvtz basis sets are applied to find the minimum number of water molecules necessary to transform the neutral form of amino-acids into the zwitterionic form by inhibiting infra-molecular proton transfer. Electrostatic and polarization interactions are responsible for the stability of the zwitterionic form in water. It is observed that three to six water molecules serve as the minimum number required to transform the neutral form into the zwitterion of all the amino acids studied here. The vibrational spectra of neutral and zwitterionic forms are clearly distinguishable.