Potential functions for hydrogen bond interactions. IV. Minimum energy conformation of the α-helical structure of poly-L-alanine

Abstract

Making use of the empirical potential functions for peptide NH .. O bonds, developed in this laboratory, the relative stabilities of the right and left-handed α-helical structures of poly-L-alanine have been investigated, by calculating their conformational energies (V). The value of V_min of the right-handed helix (α_P) is about — 10.4 kcal/mole, and that of the left-handed helix (α_M) is about — 9.6 kcal/mole, showing that the former is lower in energy by 0.8 kcal/mole. The helical parameters of the stable conformation of α_P are n ∼ 3.6 and h ∼ 1.5 Å. The hydrogen bond of length 2.85 Å and nonlinearity of about 10° adds about 4.0 kcal/ mole to the stabilising energy of the helix in the minimum enregy region. The energy minimum is not sharply defined, but occurs over a long valley, suggesting that a distribution of conformations (ϕ, ψ) of nearly the same energy may occur for the individual residues in a helix. The experimental data of α-helical fibres of poly-L-alanine are in good agreement with the theoretical results for α_P. In the case of proteins, the mean values of (ϕ, ψ) for different helices are distributed, but they invariably occur within the contour for V = V_min + 2 kcal/mole for α_P.