That's a good question ! Here is my take on
it:
We are talking of say 5% PEG 2K weight per weight. So 5% PEG
2K and 5% PEG 20K contains ~ the same weight of polymer per liter of solution,
and therefore the ~ same molarity of the ethylene oxide motif. Hence
neglecting the effect of the ends of the polymer one would expect 5% PEG 2K and
5% PEG 20K to be nearly equivalent.
In practice there are some variations in the required PEG
concentration, but it certainly does not vary say 5-fold between PEG 1K and PEG
5K.
Also the following is often true: small PEGs < 800 trend
all the behave the same, but as a different class of precipitant than
medium-sized PEG, which themselves are different than the very large PEG (10K
and above). So if a protein crystallizes with PEG 4K as precipitant it *usually*
does not crystallize with PEG 400, and the same can be said for PEG 20K vs. PEG
2K. The small PEGs are much better cryoprotectants.
Thierry
Hello folks,
After a discussion with a colleague a question
aroused regarding precipitants in crystallisation conditions. I must confess
that I do not know if it is a really naive question or just a stupid one (guess
the second option thought...), anyhow there we go:
Imagine you have
crystallisation condition that is more or less successful but not so great, that
is, you are looking for some optimisation. Your condition let say is 5% PEG 20K.
Assuming a world where relationships are linear (and I know this is not
true, by the way...), I would say that I can change 5%-PEG20K by
50%-PEG2K.
So the question is, does anyone know if someone has invest some
time in discovering this kind of relationships between main precipitants used in
macromolecular crystallography?
The reason for this kind of weird
question is that currently one of our cryo-protection protocol really damage our
crystals and I was wondering if the same crystal could grow in a cryo-condition
or something closer to it.
Thank you for investing some time in such a
awkward question!!!
--
Israel
Sanchez Fernandez PhD
Ramakrishnan-lab
MRC Laboratory of Molecular
Biology,
Hills Road, Cambridge, CB2 0QH, UK
Notice: This e-mail message, together with any attachments, contains
information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station,
New Jersey, USA 08889), and/or its affiliates Direct contact information
for affiliates is available at
http://www.merck.com/contact/contacts.html) that may be confidential,
proprietary copyrighted and/or legally privileged. It is intended solely
for the use of the individual or entity named on this message. If you are
not the intended recipient, and have received this message in error,
please notify us immediately by reply e-mail and then delete it from
your system.