Reliability of MNDO in determining the equilibrium structures of unionized amino acids

Abstract

The object of this report is to estimate the reliability of MNDO in determining the equilibrium structures of unionized amino acids. For this purpose, the optimized structures for the three conformations of unionized glycine, the two conformations of unionized alanine and the two conformations of unionized serine obtained using MNDO are compared with those obtained with ab initio 4-21G. The optimized structural parameters of these conformations by MNDO are generally acceptable except for the C αC β bond lengths of the two conformations of serine (=1.58 Å). The mean absolute difference between the optimized bond lengths by MNDO and by 4-21G is 0.022 Å, the difference in bond angles is 2.5° and the difference in torsional angles is 6.1°. The correlation coefficient between the optimized bond lengths by MNDO and by 4-21G is more than 0.99, the coefficient for the bond angles is greater than 0.75 and the coefficient for the torsional angles is greater than 0.99. These results suggest that MNDO is reliable in determining the equilibrium structures of unionized amino acids.