I doubt there is a consensus on this. Your PEG solution is a mixture of
lengths of polymer and normally you can only see a small fraction of the
PEG molecule. PEG400 will be between 8 and 10 ethylene glycols. Bigger
PEGS probably vary more. My experience is that you can often only see
density for 3-6 ethylene glycol units (and this is probably not related
to whether the PEG is 400, 4K or 20K). You select the length of PEG that
matches your density rather than being a function of what is in the
crystallisation
The argument would be I guess that a hydrophobic patch makes a section
of the ethylene glycol ordered enough to be seen as density but the
extensions occupy a sufficient range of conformations as to appear
disordered. I have put short stretches of PEG as it seems to be the
best interpretation of difference density but I do always wonder if it
is real or some kind of Fourier termination effect or partially ordered
waters etc. I would be cautious about basing too much biology on it.
Nick
--
Prof Nicholas H. Keep
Executive Dean of School of Science
Professor of Biomolecular Science
Crystallography, Institute for Structural and Molecular Biology,
Department of Biological Sciences
Birkbeck, University of London,
Malet Street,
Bloomsbury
LONDON
WC1E 7HX
email n.k...@mail.cryst.bbk.ac.uk
Telephone 020-7631-6852 (Room G54a Office)
020-7631-6800 (Department Office)
Fax 020-7631-6803
If you want to access me in person you have to come to the
crystallography entrance
and ring me or the department office from the internal phone by the door
