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 ________________________________ From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Israel Sanchez Sent: Thursday, December 08, 2011 2:40 PM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] The stupid/naive question 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.
