A number of studies have shown that the main-chain conformations (phi and psi torsional angles) of a residue can be influenced by the orientation of its side chain (kappa1 torsional angle), and vice versa. None of these reports, however, have focused on the effect of the systematic variations of the kappa1 torsions that exert on the phi and psi torsions. Using ab initio quantum mechanics calculations, a systematic torsional scan was carried out on kappa1, phi, and psi torsions of Leucine for every 30°. Among 1728 conformers, the energy contour maps of (phi and psi torsions) for each kappa1 have shown that conformers with kappa1 at the range of (30°, 90°) have only one low energy region (less than 5 kcal/mol above the global minimum), with phi at (-130°, -50°) and psi at (-25°, 200°); while the conformers with kappa1 at the range of (-90°, -30°) display two low energy regions: region one with phi at (-130°, -50°) and psi at (-50°, 200°); region two with phi at (50°, 90°) and psi at (-120°, 20°) with the exception of the region two for kappa1 at -30°, which shows a narrower yet longer region with phi at (50°,80°) and psi at (-170°, 40°). The energy contour map for anti conformations (where kappa1 at about 170°) shows a similar pattern of those with kappa1 at the range of (60°, 90°). Insights gained from this study would be useful in the development of new algorithm for ab initio protein modeling or protein folding as well as in our understanding of protein structure and functions.
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