title: ACHESYM: an algorithm and server for standardized placement of macromolecular models in the unit cell.
abstract: Despite the existence of numerous useful conventions in structural crystallography, for example for the choice of the asymmetric part of the unit cell or of reciprocal space, surprisingly no standards are in use for the placement of the molecular model in the unit cell, often leading to inconsistencies or confusion. A conceptual solution for this problem has been proposed for macromolecular crystal structures based on the idea of the anti-Cheshire unit cell. Here, a program and server (called ACHESYM; http://achesym.ibch.poznan.pl) are presented for the practical implementation of this concept. In addition, the first task of ACHESYM is to find an optimal (compact) macromolecular assembly if more than one polymer chain exists. ACHESYM processes PDB (atomic parameters and TLS matrices) and mmCIF (diffraction data) input files to produce a new coordinate set and to reindex the reflections and modify their phases, if necessary.
authors.affiliation: Department of Organic Chemistry, Poznan University of Medical Sciences, Poznan, Poland.
authors.forename: Marcin
authors.initials: M
authors.lastname: Kowiel
authors.affiliation: Department of Crystallography, Faculty of Chemistry, A. Mickiewicz University, Poznan, Poland.
authors.forename: Mariusz
authors.initials: M
authors.lastname: Jaskolski
authors.affiliation: Synchrotron Radiation Research Section, Macromolecular Crystallography Laboratory, NCI, Argonne National Laboratory, Argonne, USA.
authors.forename: Zbigniew
authors.initials: Z
authors.lastname: Dauter
mesh_terms: D000465:Algorithms; D018360:Crystallography, X-Ray; D030562:Databases, Protein; D008958:Models, Molecular; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry*; D012984:Software
chemical_list: D011506:Proteins
keywords: Cheshire symmetry; anti-Cheshire unit cell; biological unit; crystallographic asymmetric unit; oligomer; quaternary structure; symmetry transformation