Potential energy surface study on glycine, alanine and their zwitterionic forms

The conformational stability of Glycine, Alanine and their zwitterionic forms have been studied using density functional theory (DFT) methods. The conformers have been predicted by the Potential energy surface scan employing the DFT methods, B3LYP, B3PW91 and B3P86 implementing 6-311 + + G** basis set. The rotational potential energy curves for neutral molecules and zwitterionic form have been determined at the above levels of theory. The Fourier decomposition potentials were analysed for the above molecules. The B3LYP level of theory predicts conformer 1 as the most stable one, but B3PW91 and B3P86 levels of theory have predicted conformers 2n, 2b for glycine and alanine respectively. The chemical hardness calculated at the HF/6-311+ + G** level of theory shows that the Maximum hardness principle fails to predict the conformational stability for hydrogen bonded systems. (C) 2004 Elsevier B.V. All rights reserved.