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 P-turns, which were presented by several conformers of the tetrapeptide model compound. We found that only beta-turn type IVa exhibits a 10-member pseudocycle. The intrachain H-bonds between the adjacent N-H and C=O groups in the antiparallel beta-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 C=O group at the position i and the side-chain C-H group at the position i + 3 was detected in the alpha-helical conformation of polyalanine. (C) 2007 Elsevier B.V. All rights reserved.