Commission on Crystallographic Computing

Direct-method SAD phasing and dual-space model completion

Hai-fu Fan
Institute of Physics, Chinese Academy of Sciences, P.R. China

An important step of SAD phasing is to break the intrinsic phase ambiguity, which appears as a bimodal distribution with peaks symmetrically related by the phase contributed from the imaginary-part scattering of the heavy-atom substructure. The Sim distribution can be used to break the phase ambiguity. However this will always strengthen the peak with phase angle smaller than that of the average of the phase doublet. On the other hand the Cochran distribution from direct methods could be peaked anywhere and thus has the chance of enhancing a peak anywhere within 0 and 2pi. Direct-method SAD phasing is based on the combination of the bimodal SAD-phase distribution, the Sim distribution and the Cochran distribution.

It is seen from basic formulae of direct methods that they may be more efficient in phase extension/refinement than in ab initio phasing. A direct-method aided dual-space fragment  extension procedure has been developed, which is very efficient for model completion in the SAD/SIR case and the MR case (without SAD/SIR information).

In my talk, examples of direct-method SAD phasing and dual-space model completion will be given in detail. In the tutorial, demonstration will be given on using the new GUI (2008 version) of OASIS. Students are welcome to have their own data trying with OASIS.

These pages are maintained by the Commission Last updated: 15 Oct 2021