Dear CCP4 colleagues.
I'm just finishing up a refinement, but am left with one little curio that I just can't seem to solve.
One aspartic acid residue is associated with some extra, unexplained electron density.
--> please see:
http://i.imgur.com/vCYOqam.png
Where, the Fo-Fc map is contoured at 3.78 rsmd in Coot.
I have tried a number of different modelling scenarios, but as yet can't reach a wholly satisfactory conclusion; waters, alternate conformers, really don't seem to cut it. I though about some radiation-induced phenomena, but this data set was collected on
a home-source, so I guess this is unlikely.
So, I would really appreciate some ideas and suggestions. Hopefully it is blindingly obvious to someone.
Random Thought: could it be PEGylation of the side-chain?
Some other hopefully useful background information:
* I'm sure it is/was an ASP, because the same protein (made from the same construct) has been used in previous crystallisations, and the resultant structures have clear, unambiguous electron density for the side chain.
* the crystallization condition is PEG 200, with some Na/K phosphate at pH 5.8, and NaCl. The protein itself contains HEPES buffer.
With many thanks,
Tony.
---
Dr Antony W Oliver
Senior Research Fellow
CR-UK DNA Repair Enzymes Group
Genome Damage and Stability Centre
Science Park Road
University of Sussex
Falmer, Brighton, BN1 9RQ
email:
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