Density modification tries to improve the map by introduction restrictions on the flattness of the bulk solvent region and the details of the shape of the density in the protein region (histogram matching). Both kinds of information are included in normal crystallographic refinement. After one has constructed a model that covers much of the structure one goes into refinement and examines the 2DFo-mFc map to see if the previously uninterruptible bits of the map have become better. This is a much more powerful technique than DM alone. Personally, I would refine the model without any ligand built to get the best possible map before making any interpretation of the bound compound. I would also do several rounds of rebuilding of the protein and add the obvious water. Not only will the result be the best possible (unbiased) image of your ligand the result will also be an "omit" map that you can use in your publication to justify your final interpretation. Dale Tronrud On 6/3/2017 11:33 AM, Sam Tang wrote:
Dear community
I guess it may not be a standard practice but I wonder if anyone has tried to use a density modified map for ligand building.
My situation is that I am trying to fit a single strand DNA ligand into an enzyme model. The dataset is at 3 Ang, P1 space, with overall completeness of only about 88%. MR was done by the apo-enzyme and the density for the first two-third of the ligand is very good and could easily be modelled. Towards the last one-third the density becomes much weaker so nucleotides cannot be easily put. This is the only dataset so far, so multi-crystal averaging / map averaging is not possible at this stage. NCS averaging is also not possible. We did try to sharpen map during refinement.
I understand that DM is usually used to improve phase at the initial stage esp. with experimental phasing. With MR would you think density modification could produce a better map to aid the modelling of the last one-third ligand? Can I use the denmod.mtz directly for manual model building?
Any input would be very much appreciated.
Best
Sam Biochemistry Programme, CUHK
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