A geometry force field which converts low-resolution X-ray models to structures with properties found at ultra high resolution.

A geometry optimization force field was developed using ultra high-resolution structures and tested using high- and low-resolution X-ray structures. Protein and small molecule X-ray data was used. When applied to ultra high-resolution structures the force field conserves the internal geometry and local strain energy. When applied to low-resolution structures there is a small change in geometry accompanied by a large drop in local strain energy. Although optimization causes only small structural changes in low-resolution X-ray models, it dramatically modifies profiles for hydrogen bonding, Van der Waals contact, bonded geometry, and local strain energy, making them almost indistinguishable from those found at high resolution. Further insight into the effect of the force field was obtained by comparing geometries of homologous proteins before and after geometry optimization. Optimization causes homologous regions of structures to become similar in internal geometry and energies. Once again, the changes only require small atomic movements. These findings provide insights into the structure of molecular complexes. The new force field contains only short-range interatomic potential functions. Its effectiveness shows that local geometries are determined by short-range interactions which are well modeled by the force field. Potential applications of this study include detection of possible structural errors, correction of errors with minimal change in geometry, improved understanding and prediction of the effects of modifying ligands or proteins, and computational addition of structural water.