COMPARISON OF MODEL WITH ELECTRON DENSITY: REFINEMENT, MODEL-BUILDING, QUALITY ASSESSMENT IN CRYSTALLOGRAPHY & EM. Xie, Q.; Blanc, E., Zhou, G., Tong, J. & Chapman, M.S., Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306-3015, USA

A function has been derived through which it is possible to calculate from an atomic model, the appearance of an electron density map at any arbitrary experimental resolution [Chapman (1995) Acta Crystallogr. A51: 69-80]. This serves as the basis of a stereochemically restrained refinement protocol that is very fast when applied to local regions and overcomes many of the problems with previous implementations of real-space refinement. This method has been used in the refinement of 3 virus structures [Chapman & Rossmann (1996), Acta Crystallogr. in press; Balaji & Caspar, in prep.; Blanc et al., IUCR abstract]. The method shows promise for protein crystallography in bridging between model building and reciprocal space refinement, to help bring an initial model within the convergence radius of conventional refinement. The results of ongoing systematic tests with maps of various qualities will be presented. The same mathematical models of electron density are being used for the improvement of indices that measure the quality of a model on a residue-by-residue basis. They are also being used at low (~20 Å) resolution in the development of methods to orient and position domains of known structure within electron micrograph images of large assemblies. The authors will summarize the theoretical foundation common to all of these applications, and present some of the recent results that demonstrate the success of this approach.