Abstract
Allinger's MMP2(85) program has been converted to an IBM environment, and the dimensions expanded to a current maximum of 999 atoms. Substantial additional expansion will be possible. An all-atom set of parameters, which permit Allinger's comprehensive force field to be applied to the molecular mechanics treatment of peptides, has been determined. These parameters, termed MMPEP, contain 21 atom types: 5 for carbon, 6 for hydrogen, 5 for nitrogen, 4 for oxygen, and 1 for sulfur, and are based on crystallographic heavy atom bond lengths and bond angles, vibrational and microwave spectra, and ab initio calculations. To minimize the conformational energy of a peptide from an initial starting geometry, all internally stored parameters are released, and replaced by PEPCON, a 360-line external file containing the MMPEP parameters.The ability of the MMPEP parameterization of MM85 to reproduce experimental crystal structures has been tested on several peptides and polypeptides, and the use of a dielectric constant ε = 78.5 D leads to the following results: Ala-Ala-Gly (rms = 0.261); Gly-Gly-Val (rms = 0.349); glutathione (rms = 0.417); crambin (327 heavy atoms; rms = 0.310 for all heavy atoms); insulin (389 heavy atoms; rms = 0.646 for all heavy atoms); the origins of deviations can be interpreted. No problems have been encountered in the application of the Newton–Raphson minimization procedure to such large molecules as crambin and insulin, even though all possible nonbonded interactions have been retained. On the IBM 3081 computer, real time minimization of trip)eptides requires 1–2 min, crambin requires 250 min, and insulin 200 min. Since hydrogen bonding in Allinger's force field is a natural result of electrostatic and van der Waals interactions, in MMPEP hydrogen bonding is taken into account through the large number of hydrogen atom types and their different bond moments and van der Waals radii.
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