Uma's use of quotes around "di" suggests a related question about PDB convention. It was my (perhaps not very good) understanding that ligands should be identified by what is actually present in the crystal, and not by what can be modeled. For example endogenous ubiquinone is likely to be UQ50 (depending on the species) but most of that 50-carbon side chain is hanging out in the lipid or detergent and completely disordered. Still we should use the ligand identifier for UQ50, even though codes exist for UQ with 5 or 10-carbon side chains that are much better accommodated by the density.
If that is the case, one should not use the pdb identifier for diethylene glycol (PEG) when PEG4k was the precipitant, unless you believe that the binding site has specifically selected diethylene glycol from an extremely broad range of polymer lengths in the added material. Using the identifier for a much longer PEG will result in a large number of "missing atoms" listed in the report, but would eliminate the unreasonable assumption that PEG fragment models must always end with a terminal oxygen. Even if that is the rule, I would agree that PEGs would be a good place to ignore the rule. Since PEGs have a MW distribution, it is impossible to know exactly what is bound and it may be different in different unit cells. If you are not going to get it right no matter what you put, you might as well put something that fits. eab On 01/25/2017 09:51 AM, Uma Gabale wrote:
Dear all, Thank you very much for your replies. It is a PEG, a "di"ethylene glycol to be precise, in most chains. Best regards, Uma. -- Uma Gabale, PhD Research Associate Molecular and Cellular Biochemistry Indiana University Bloomington
