CRYSTALLOGRAPHIC STRUCTURE REFINEMENT USING MOLECULAR DYNAMICS CONSTRAINED TO TORSION ANGLES. Luke M. Rice and Axel T. Brünger, Howard Hughes Medical Institute and Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut, 06520, USA

A reduced variable conformational sampling strategy based on molecular dynamics constrained to torsion angles has been implemented and applied to crystallographic refinement (L. M. Rice and A. T. Brünger, PROTEINS 19:277-290, 1994). This formulation reduces the number of adjustable parameters by approximately tenfold, and allows for significantly higher simulation temperatures by eliminating high frequency bond and angle vibrations. Refinement protocols using torsion angle dynamics with constant temperature searching typically have a greater radius of convergence compared to conventional refinement stratergies. Applications to refinements of very poor initial models and to refinements at low resolution will be discussed. Preliminary results suggest that this reduced variable method will allow refinement at lower resolution than is currently possible with existing approaches.