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Comparison of the intermolecular energy surfaces of amino acids: orientation-dependent chiral discrimination.

In the present work, we compare the intermolecular energy surfaces of the alanine molecule in its neutral and zwitterionic state using ab initio theory (HF/6-311++G) as a function of mutual orientation. Starting from the optimized structures of the nonbonded homochiral (l-l) and heterochiral (d-l) pairs of molecules, the energy surfaces are studied with rigid geometry by varying the distance and orientation. The potential energy surfaces of the l-l and d-l pairs are found to be dissimilar and reflect the underlying chirality of the homochiral pair and racemic nature of the heterochiral pair. The intermolecular energy surface of the l-l pair is more favorable than the corresponding energy surface of the d-l pair. The study, for the first time, reveals clear homochiral preference without use of parameters, which was unobserved in previous detailed simulations but predicted by theory. The electrostatic interaction further augments the chiral discrimination. The basis set superposition error (BSSE) corrected results show enhanced discrimination. Use of higher-level Møller-Plesset perturbation theory (MP2) and further BSSE correction do not change the conclusions made at the Hartree-Fock (HF) level. The major conclusions based on HF and MP2 level calculations remain unaltered when the calculations of the potential energy surfaces for the neutral and zwitterionic pairs are repeated using the density functional theory (DFT) (B3LYP/6-311++G). The observed orientation dependence has significance in the biological chiral recognition as well as peptide synthesis at the peptidyl transferase center where the amino terminal and peptidyl terminal undergo mutual rotatory motion.