THE MM3 FORCE-FIELD FOR AMIDES, POLYPEPTIDES AND PROTEINS

The potential functions for simple amides, several peptides and a small protein have been worked out for the MM3 force field. Structures and energies were fit as previously with MM2, but additionally, we fit the vibrational spectra of the simple amides (average rms error over four compounds, 34 cm-1), and examined more carefully electrostatic interactions, including charge-charge and charge-dipole interactions. The parameters were obtained and tested by examining four simple amides, five electrostatic model complexes, two dipeptides, six crystalline cyclic peptides, and the protein Crambin. The average root-mean-square deviation from the X-ray structures for the six cyclic peptide crystals was only 0.10 angstrom for the nonhydrogen atomic positions, and 0.011 angstrom, 1.0-degrees, and 4.9-degrees for bond lengths, bond angles, and torsional angles, respectively. The parameter set was then further tested by minimizing the high resolution crystal structure of the hydrophobic protein Crambin. The resultant root-mean-square deviations for the non-hydrogen atomic data, in the presence of the crystal lattice, are 0.22 angstrom, 0.023 angstrom, 2.0-degrees, and 6.4-degrees for coordinates, bond lengths, bond angles, and torsional angles, respectively.