Substituent effects on long-range interactions in the β-sheet structure of oligopeptides

The energetic stabilization and geometrical parameters of the β-sheet structure of three oligopeptides L-Ala_n, selected conformers of L-Val_n and L-Ser_n (n=2-10) have been investigated by DFT computations. The data reveal the important effect of long-range interactions operating through hydrogen bonding and dipole-dipole interactions. Compared with related data on Gly_n oligopeptides determined in our previous study (V. Horváth, Z. Varga, A. Kovács, J. Phys. Chem. A, 2004, 108, 6869-6873), the methyl, i-propyl and hydroxymethyl substituents lead to a decrease of the long-range interactions. While the energy gain from these interactions of a Gly unit in Gly₁₀ was 2.4 kJ/mol, the contribution decreased to 2.1, 1.8, and 0.4 kJ/mol in Ala₁₀, Val₁₀, and Ser₁₀, respectively. Variations of the geometrical parameters (most important being the lengths of H⋯O hydrogen-bonds and the C-N bonds) are in agreement with the decrease of the stabilization energies. Another important effect of the substituents is that their steric interactions introduce irregularities in the build-up of energetic and geometric trends, in contrast to their monotonous build-up in Gly_n.