Ab initio direct methods: practical advice for getting beyond the first 300 atoms.
Not all crystallographic structural investigations are amenable to a phasing solution by direct methods alone. Guideline procedures are outlined which are intended to help the evaluation of whether direct-methods procedures may be expected to phase diffraction data for large molecular structures. This analysis is directed at three separate levels of inquiry: (1) How good are the primary data and can E values be derived to represent a point-atom structure. (2) How well do the data interact through phase relationships and may they be expected to produce a stable phasing solution. (3) What is the prognosis for finding recognizable solutions. Data are presented from the post-mortem analyses of a number of large, difficult-to-solve, structures to illustrate each of these points. Direct-methods practitioners are to be encouraged that crystal structures having more than 300 atoms per asymmetric unit may occasionally be determined utilizing present methodologies provided that an a priori prognosis for obtaining a solution is favorably high, adequate computational resources are available, and sufficient persistent effort is applied.
Langs
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Medical Foundation of Buffalo, NY 14203, USA.