W0315

Local Squaring Functions for Multi-Atom Fragments. Jeffrey Roach, Charles W. Carter, Jr., Biochemistry, Univ. of North Carolina, Manning Dr., CB#7260, Chapel Hill, NC 27599 USA.

Local squaring functions for single atoms were introduced as a formulation of the Sayre squaring method equations specialized for effective phase refinement and initial atomic model construction. Unfortunately the requirement of atomic resolution data severely limits the scope of this technique. Generalized local squaring functions that represent the likelihood that a multi-atom fragment occupies a given position at a given orientation relax this requirement thereby making it applicable at lower resolution. Facilitating both interpolation and interpretation, the special unitary 2-group (or quaternion group) parameterizes the orientation space. An example of phase refinement and computer-aided model construction is given by the local squaring functions of several backbone fragments applied to a 2.1 Å experimental rusticyanin electron-density.