title: "Conditional Restraints": Restraining the Free Atoms in ARP/wARP.
abstract: The automated building of a protein model into an electron density map remains a challenging problem. In the ARP/wARP approach, model building is facilitated by initially interpreting a density map with free atoms of unknown chemical identity; all structural information for such chemically unassigned atoms is discarded. Here, this is remedied by applying restraints between free atoms, and between free atoms and a partial protein model. These are based on geometric considerations of protein structure and tentative (conditional) assignments for the free atoms. Restraints are applied in the REFMAC5 refinement program and are generated on an ad hoc basis, allowing them to fluctuate from step to step. A large set of experimentally phased and molecular replacement structures showcases individual structures where automated building is improved drastically by the conditional restraints. The concept and implementation we present can also find application in restraining geometries, such as hydrogen bonds, in low-resolution refinement.
authors.affiliation: Department of Biochemistry, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands.
authors.forename: Wijnand T M
authors.initials: WT
authors.lastname: Mooij
authors.forename: Serge X
authors.initials: SX
authors.lastname: Cohen
authors.forename: Krista
authors.initials: K
authors.lastname: Joosten
authors.forename: Garib N
authors.initials: GN
authors.lastname: Murshudov
authors.forename: Anastassis
authors.initials: A
authors.lastname: Perrakis
mesh_terms: D000465:Algorithms; D018360:Crystallography, X-Ray / Q000379:methods*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry*; D012815:Signal Processing, Computer-Assisted; D012984:Software
chemical_list: D011506:Proteins