INTRINSIC TORSIONAL POTENTIAL PARAMETERS FOR CONFORMATIONAL-ANALYSIS OF PEPTIDES AND PROTEINS

New sets of intrinsic torsional parameters, obtained by fitting to exp erimental 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 com putations on peptides and proteins. A three-term Fourier series expans ion is used to represent the intrinsic torsional energy. Each set of i ntrinsic torsional parameters, obtained from a single model compound, reproduces experimental torsional barriers, relative conformational en ergies, and torsion angles of related molecules not used for the param etrization. The sets of parameters of a new potential function, includ ing electrostatic interactions based on partial atomic charges, and no nbonded, hydrogen-bond, and intrinsic torsional energies, are tested i n conformational energy calculations on a model peptide N-acetyl-N'-me thylalanineamide. The electrostatic energy component plays a significa nt role in the total conformational energy and leads to a high relativ e energy of the alpha(R) (A) conformation compared to the C-7(eq) (C) conformation, although the latter is still the global minimum. These r esults differ from those with ECEPP/3, CHARMM, and AMBER, but are reas onably consistent with those from recent ab initio studies.