CRAMBIN - A DIRECT SOLUTION FOR A 400-ATOM STRUCTURE

The crystal structure of crambin, a 46-residue protein containing the equivalent of approximately 400 fully occupied non-H-atom positions, w as originally solved at 1.5 Angstrom by exploiting the anomalous scatt ering of its six S atoms at a single wavelength far removed from the a bsorption edge of sulfur. The crambin structure has now been resolved without the use of any anomalous-dispersion measurements. The techniqu e employed was an ab initio 'shake-and-bake' method, consisting of a p hase-refinement procedure based on the minimal function alternated wit h Fourier refinement. This method has successfully yielded solutions f or a smaller molecule (28 atoms) using 1.2 Angstrom data, and a crambi n solution was obtained at 1.1 Angstrom.