QTAIM study of the closed-shell interactions in peptide secondary structures: A cluster treatment of oligo- and polyalanines

The closed-shell interactions in oligo- and polyalanines are studied by the quantum theory of atoms in molecules (QTAIM) using electron densities derived from the B3LYP/6-31+G∗∗ ground-state electronic wave-functions. The QTAIM enabled us to identify a large number of the intraturn closed-shell stabilizing interactions in the β-turns, which were presented by several conformers of the tetrapeptide model compound. We found that only β-turn type IVa exhibits a 10-member pseudocycle. The intrachain H-bonds between the adjacent N–H and CO groups in the antiparallel β-sheet conformation of polyalanine have not been found. At the same time, these interactions do exist in the parallel conformation and are even stronger than the interchain N–H…O bonds. A weak interaction between the CO group at the position i and the side-chain C–H group at the position i + 3 was detected in the α-helical conformation of polyalanine.