title: An introduction to stereochemical restraints.
abstract: At the resolution available from most macromolecular crystals, the X-ray data alone are insufficient to lead to a chemically reasonable structure, so stereochemical restraints are essential. These usually restrain bond lengths, bond angles, planes and chiral volumes. The definition of these restraints and where the values come from are described. A dictionary entry contains information about the atom types, their connectivity and all the appropriate restraints. Torsion angles are not usually restrained, but they do have optimum values. In the special case of flexible five- and six-membered rings, including pentose and hexose sugars, the ring pucker is defined by combinations of torsion angles and the pucker affects the position of substituents.
authors.affiliation: MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, England. pre@mrc-lmb.cam.ac.uk
authors.forename: Philip R
authors.initials: PR
authors.lastname: Evans
mesh_terms: D002244:Carbon / Q000737:chemistry*; D019295:Computational Biology; D003460:Crystallization; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008024:Ligands; D046911:Macromolecular Substances; D008958:Models, Molecular; D015233:Models, Statistical; D008968:Molecular Conformation; D011487:Protein Conformation; D017510:Protein Folding; D011506:Proteins / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D008024:Ligands; D046911:Macromolecular Substances; D011506:Proteins; D002244:Carbon