Accurate geometries for `mountain pass` regions of the Ramachandran plot using quantum chemical calculations (Proteins: Structure, Function, and Bioinformatics)

Unusual local arrangements of protein in Ramachandran space is not well represented by standard geometry tools used in either protein structure refinement using simple harmonic geometry restraints or in protein simulations using molecular mechanics force fields. By contrast, quantum chemical computations using small poly-peptide molecular models can predict accurate geometries for any well-defined backbone Ramachandran orientation. For conformations along transition regions - ? from -60 to 60° - a very good agreement with representative high-resolution experimental X-ray (