Constrained-restrained least-squares (CORELS) refinement of proteins and nucleic acids.

Publisher Summary The CORELS (COnstrained-REstrained Least-Squares) program is developed for proteins and nucleic acids to take advantage of the intrinsic rigid groups found in the molecules and to overcome the relatively low resolution of the X-ray data from their crystals. CORELS combines Scheringer's rigid groups constraints, extended to allow for variable torsion angles, with distance restraints to maintain stereochemistry between groups within a specified error limit. Even though allowing variable internal dihedral angles introduces torsional degrees of freedom within the otherwise constrained group, it can reduce the total number of structural parameters in the structure by decreasing the number of groups. The advantages of this approach include a large increase in the data-to-parameter ratio over the restrained refinement methods; automatic maintenance of group stereochemistry: within the group, all bond lengths and bond angles are constrained. The use of a constrained-restrained least-squares procedure has proven to be extremely useful in refining macromolecular structures, especially when the initial model has severe errors. This method inherently has many fewer degrees of freedom than restrained refinement procedures and therefore is applicable at extremely low resolution with a very large radius of convergence.

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