When default Sir2019 fails: some advices

Sir2019 has developed automatic strategies to find the correct solution among the various trials. In addition the user can adopt several options to choose his own phasing pathway.

    • A different phasing approach can be used.
    • An alternative space group should be carefully considered.
    • The number of reflections (Nlarge) actively used in the MDM or SDM phasing process is fixed by the program. For some structures, the ratio “number of active triplets/Nlarge” is too small (e.g., less than 15). Larger values of Nlarge (NREF directive) may improve the phasing procedure. The same result may be achieved modifying (decreasing) the G threshold (GMIN directive) for triplets reliability.
    • High (or low) resolution reflections may occasionally play a too important role in the first steps of the phasing process. Fixing an overall thermal factor lower or larger (BFAC directive) than that provided by the normalization routine may successfully change the phase extension and refinement procedures.
    • Experimental data of good quality allow the success of structure solution. If in some extreme (very low or very high) resolution shells, equivalent reflections show large internal R(F) values (Rmerge > 25-30%), the user can truncate (MINRES o RESMIN directives) the data submitted to the phasing process.
    • The user can modify the DSR strategy (DSR directive) to improve the efficiency of the phasing procedures and of the structure refinement process.
    • Low data resolution is an unavoidable feature in macromolecular crystallography: it may limit the efficiency of crystal structure determination. Extrapolating the moduli and phases of unobserved reflections beyond and behind the experimental resolution limit (GENERATE directive) makes the ab initio phasing procedures and structure solution process more efficient.
Previous page                        Next page