Echoing some previous comments, restraints on backbone torsion angles can give substantial improvements in low resolution refinements.  I've seen it in several virus structure refinements  3.25A to 3.75A.  Just have to make sure that you then don't use phi/psi angles as a validation.

There is a very easy / transparent way to restrain backbone torsion angles in the refinement program TNT by Dale Tronrud & colleagues.  Restraints are available, but given zero weight by default.   The restraint on each torsion angle is specified as a periodic set of equal optima, such as {0, 180} or {60, 180, 240} just like many molecular mechanics programs.  From decade-old memory, the defaults for phi & psi are both {60, 180,.240} which can be modified in the topology files to better target the known optima of a Ramachandran plot.

A very similar restraint / potential is also available in X-Plor, and I'm guessing is available in CNS and Phenix, but it might be a little more work to invoke.

Different design criteria have been used in writing different refinement programs.  There isn't much call for backbone torsion restraints in typical Shellx or Refmac refinements.  If you are working at unusually low resolution, other programs might be more suitable.

Michael S. Chapman, R.T. Jones Professor of Structural Biology
Dept. Biochemistry & Molecular Biology; School of Medicine, Mail Code L224
Oregon Health & Science University
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