DIRECT-SPACE METHODS IN PHASE EXTENSION AND PHASE DETERMINATION .2. DEVELOPMENTS OF LOW-DENSITY ELIMINATION

The low-density elimination method for phase extension and refinement [Shiono & Woolfson (1992). Acta Cryst. A48, 451-456] has been improved by substituting a smoother density-modification procedure for the ori ginal sharp cut-off function. In addition, better criteria have been f ound for limiting the number of refinement cycles, which gives a bette r final result for much less work. The effectiveness of the process ha s been illustrated by phase refinement for a protein with high-resolut ion (1.17 angstrom) data containing 808 independent non-H atoms plus 8 3 water molecules in the asymmetric unit; the unweighted mean-phase er ror was reduced from 74 to 39.3-degrees. Phase extension and refinemen t was also demonstrated for pig 2Zn insulin starting with multiple iso morphous replacement (MIR) phases at 1.9 angstrom and extending out to 1.5 angstrom. There was a significant improvement of phases and the f inal map had a correlation coefficient of 0.540.