Abstract
In isolation, both weak isomorphous/anomalous difference signals from heavy-atom derivatization and phases from partial molecular-replacement solutions for a subset of the asymmetric unit often fall short of producing interpretable electron-density maps. Phases generated from very partial molecular-replacement models (if generated carefully) can be used to reliably locate heavy-atom sites, even if the signal is not sufficiently strong to allow robust finding of the sites using Patterson interpretation or direct methods. Additional advantages are that using molecular-replacement phases to define the heavy-atom substructure avoids the need for subsequent hand determination and/or origin-choice reconciliation and that the partial model can be used to aid the mask determination during solvent flattening. Two case studies are presented in which it was only by combining experimental and molecular-replacement phasing approaches that the crystal structures could be determined.
Highlights
Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, England
Combining experimental phases in refinement is by no means a new concept: for a review of the advantages that phase combination brings to macromolecular refinement, see Pannu et al (1998), Adams et al (2009) and references therein
BUSTER-TNT has the advantage that its low-resolution Fcalc can include a contribution from the part of the structure that is ordered but is yet to be modelled; this in turn makes the scaling more accurate and decreases the model bias that is normally introduced by the refinement of partial structure parameters against data that contain scattering from the entirety of the asymmetric unit
Summary
When we learn how to determine crystal structures, we traditionally think of there being two major routes to phase determination: ‘experimental’ or ‘molecular replacement’. It has previously been demonstrated that where molecular-replacement models are highly partial (i.e. less than 60% complete), modelling the ‘missing atoms’ as part of refinement significantly improves the quality of the maps obtained (Blanc et al, 2004). In this way, BUSTER-TNT has the advantage that its low-resolution Fcalc can include a contribution from the part of the structure that is ordered but is yet to be modelled (the ‘missing atoms’); this in turn makes the scaling more accurate and decreases the model bias that is normally introduced by the refinement of partial structure parameters against data that contain scattering from the entirety of the asymmetric unit. The latter was accomplished by using a combined BUSTER-TNT/SHARP solvent-flattened map (the phases were combined in SHARP and solvent flattening was carried out with SOLOMON launched from within the SUSHI interface) to define the missing-atoms envelope during refinement
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
