Some theoretical observations on the 1:1 glycine zwitterion-water complex

The structural transformation of three minima on the HF/6-31G* glycine zwitterion-water complex potential energy surface have been investigated at the Hartree-Fock (HF), second order Mcircle divideller-Plesset perturbation theory (MP2)and the density functional theory using the B3LYP hybrid exchange-correlation functional employing several Pople's standard basis sets. Contrary to the previous computational results, our results clearly confirm that the zwitterionic structure of glycine does not exist in the 1:1 complex with a water molecule in the gas phase. Several above mentioned computational methods were examined. HF calculations performed at various levels (up to 6-311++G(3df,3pd)) demonstrated the difficulty of obtaining reliable results. Generally, the agreement between B3LYP and MP2 calculations was good. With accurate basis sets, they got the proper results. At the same time, the results at different levels from CP-corrected gradient optimization and CP-uncorrected were also compared. (C) 2003 Elsevier Science B.V. All rights reserved.