LALS: a linked-atom least-squares reciprocal-space refinement system incorporating stereochemical restraints to supplement sparse diffraction data

LALS is a computer program for the refinement of molecular structures. It is primarily intended for helical macromolecules but has applicability in other fields. The refinement uses as data, X-ray structure factors, usually from fibre diffraction studies, and/or stereochemical information, including a comprehensive short-contact search. Various other geometrical constraints and restraints may be placed on the molecular conformation. As parameters of the refinement, LALS uses primarily dihedral angles about single bonds, assuming bond lengths and angles to be fixed and known. This greatly reduces the number of parameters from conventional atom-position refinements, as is necessary in systems where data are sparse. LALS has been used successfully to investigate the structures of a large number of more or less ordered polynucleotides, polysaccharides and other fibrous materials.