Intrinsic torsional potential parameters for conformational analysis of peptides and proteins

New sets of intrinsic torsional parameters, obtained by fitting to experimental and/or theoretical values of torsional barriers and relative conformational energies of various uncharged model organic compounds, are presented. They are intended for use in conformational energy computations on peptides and proteins. A three-term Fourier series expansion is used to represent the intrinsic torsional energy. Each set of intrinsic torsional parameters, obtained from a single model compound, reproduces experimental torsional barriers, relative conformational energies, and torsion angles of related molecules not used for the parametrization. The sets of parameters of a new potential function, including electrostatic interactions based on partial atomic charges, and nonbonded, hydrogen-bond, and intrinsic torsional energies, are tested in conformational energy calculations on a model peptide N-acetyl-N′-methylalanineamide. The electrostatic energy component plays a significant role in the total conformational energy and leads to a high relative energy of the α_R (A) c onformation compared to the C₇ ^eq (C) conformation, although the latter is still the global minimum. These results differ from those with ECEPP/3, CHARMM, and AMBER, but are reasonably consistent with those from recent ab initio studies.