W0285

Model-free High Resolution Maps. George M. Sheldrick, Lehrstuhl für Strukturchemie, Univ. Göttingen, Germany.

Since the program SHELXD (Usón & Sheldrick, 1999; Schneider & Sheldrick, 2002) is often able to locate the heavy atom sites from SAD or MAD data in a few minutes, there was a need for a program that is able to produce native phases starting from these sites in a fast, simple and robust manner so that, for example, it could be seen whether the structure had been 'solved' before taking the crystal off the beamline. The program SHELXE (Sheldrick, 2002) that performs density modification by the sphere of influence method was written for this purpose.

Under special circumstances, i.e. (a) when high quality MAD data are used, (b) when the solvent content is high (>0.6) and especially (c) when the data extend to very high resolution (<1.5Å), the maps obtained by SHELXE showed unexpectedly high map correlation coefficients (>0.95) compared to the final anisotropically refined structures. These model free maps, unlike the maps from the final refinement, should be free from model bias. The characteristics of these model free maps, and possibilities of improving them further by incorporating chemical knowledge in a general way, will be explored and ideas to incorporate them into the structure refinement procedure discussed.

Schneider, T.R. & Sheldrick, G.M. (2002). Acta Cryst. D58, 1772-1779.

Sheldrick, G.M. (2002). Z. Kristallogr. 217, 644-650.

Usón, I. & Sheldrick, G.M. (1999). Curr. Opin. Struct. Biol. 9, 643-648.