A comparison of the CHARMM, AMBER and ECEPP potentials for peptides. II. Phi-psi maps for N-acetyl alanine N'-methyl amide: comparisons, contrasts and simple experimental tests.

phi-psi maps of N-acetyl alanine N'-methyl amide have been computed using the CHARMM potential, the all-atom AMBER potential, and the ECEPP/2 potential, before and after adiabatic relaxation. Maps using the CHARMM and AMBER potentials were determined with values of 1.0 and 4.0 for the dielectric constant epsilon, and with a distance dependent dielectric constant. Adiabatic relaxation was carried out using flexible geometry for the CHARMM and AMBER potentials, and using rigid geometry for the AMBER and ECEPP potentials. In all cases, the lowest energy was found in the C7eq region (phi approximately -70 degrees, psi approximately 70 degrees). The maps with CHARMM and AMBER with epsilon = 4.0 and with ECEPP, without adiabatic relaxation, were broadly similar but differed in the relative energies allotted to high-energy regions of the map. After adiabatic relaxation with rigid geometry, the map with ECEPP, and the map with AMBER using a distance-dependent dielectric constant, agreed fairly well apart from differences in the relative energies of the alpha R, alpha L, and C7ax regions. After adiabatic relaxation with flexible geometry, the maps with CHARMM and AMBER became very similar; the lowest energies were observed in the C7eq region, the C5 region (phi approximately -150 degrees, psi approximately 150 degrees) and the C7ax region (phi approximately 70 degrees, psi approximately -70 degrees). Breakdown of the energies, after adiabatic relaxation, into electrostatic, nonbonded, and geometric (including torsional) contributions, showed that (1) with fixed geometry, the nonbonded and torsional contribution to the ECEPP and AMBER potentials were very similar, but the electrostatic contributions were markedly different; (2) with flexible geometry, the nonbonded contribution to the CHARMM and AMBER potentials did not vary greatly over the whole map. The phi-psi maps were subjected to three simple comparisons with experiment. (1) The maps were used to predict the characteristic ratio for poly-L-alanine, and the results were compared with experimental findings (D.A. Brant and P.J. Flory, J. Amer. Chem. Soc. 87, 2788-2791, 1965). The agreement with experiment was acceptable for ECEPP, and for CHARMM after adiabatic relaxation, marginal for AMBER after adiabatic relaxation, and unsatisfactory for CHARMM or AMBER without adiabatic relaxation. (2) Deviations of bond angles from their equilibrium values, in energy-minimized conformations, were compared with values deduced from crystals of terminally-blocked amino acids. With both the CHARMM and AMBER potentials using flexible geometry, one or more excessive deviations was observed in the C7ax local minimum.(ABSTRACT TRUNCATED AT 400 WORDS)