12.3.2 Proline Constructor File

The construction of prolines is more involved than the other amino residues because of the ring. The approach used in CONGEN is to precompute a table of proline geometries based on the particular value of the phi torsion angle. When a ring with a particular phi angle is required, the table is linearly interpolated to find the geometry to use.

The geometries are determined using a model of just the ring with two terminating methyl groups, specifically, 1,2-dimethyl pyrrolidine. The geometry and energies for particular phi angles are computed by constructing the ring with the given phi angle, setting a torsion angle constraint (see Harmonic Dihedrals), with a high force constant. Then, the ring is minimized, and the actual value of phi, and the bond lengths, angles, and torsions needed to construct CB, CG, and CD are used. In addition, the energy of system e, the van der Waals energy evdw, and constraint energy ec are stored in the table.

This table is stored in the proline constructor file and has the following format:

     title
     count                                          (I5)
     phi,e,evdw,ec,(bond(i),theta(i),phi(i),i=1,3)  (13F10.5)

where count is the number of table entries. phi is the phi backbone torsion angle after minimization. e, evdw, and ec are given above. The triples of bonds, angles, and torsions given the construction data needed to construct CB, CG, and CD, using internal coordinates, C-N-CA-CB, N-CA-CB-CG, CA-CB-CG-CD, respectively.

A proline constructor file good for general use is stored in CGDATA:PRO.CNS. See Emap, for a description of its construction. The subroutine, RDPROCONS, is used this file.