5.22 Find Waters

As with finding ligands, you are given a chose of maps, protein (masking) atoms. A final selection has to be made for the cut-off level, note that this value is the number of standard deviation of the density of the map before the map has been masked. The default sigma level (water positions must have density above this level) is set for a “2Fo-Fc”-style map. If you want to use a difference map, you must change the sigma level (typically to 3 sigma) otherwise you run the risk of fitting waters to difference map noise peaks.

Then the map is masked by the masking atoms and a search is made of features in the map about the electron density cut-off value. Waters are added if the feature is approximately water-sized and can make sensible hydrogen bonds to the protein atoms. The new waters are optionally created in a new molecule called “Waters”.

You have control over several parameters used in the water finding:

(set-write-peaksearched-waters)

which writes ligand-waters-peaksearch-results.pdb, which contains the water peaks (from the clusters) without any filtering and ligand-waters.pdb which are a disk copy filtered waters that have been either added to the molecule or from which a new molecule has been created.

(set-ligand-water-to-protein-distance-limits min-d max-d) sets the minimum and maximum allowable distances between new waters and the masking molecule (usually the protein). Defaults are 2.4 and 3.2Å.

(set-ligand-water-spherical-variance-limit varlim) sets the upper limit for the density variance around water atoms. The default is 0.12.

The map that is marked by the protein and is searched to find the waters is written out in CCP4 format as "masked-for-waters.map".

5.22.1 Refinement Failure

Sometimes as a result of water fitting, you may see something like:

 WARNING:: refinement failure
           start pos: xyz = (      17.1,     34.76,     60.42)
           final pos: xyz = (     17.19,     34.61,     60.59)

When Coot finds a blob, it does a crude positioning of an atom at the centre of the grid points. It then proceeds to move to the peak of the blob by a series of translations. There are a certain number of cycles, and if it doesn't reach convergence by the end of those cycles then you get the error message.

Often when you go to the position indicated, you can see why Coot had a problem in the refinement.

5.22.2 Blobs

After a water search, Coot will create a blobs dialog (see Section sec_blobs).