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1. Does anyone actually test their chemicals for peroxides?
No.
2. What tests would be useful for PEGs, dioxane, glycerol, and the
likes?
Not applicable.
3. Has anyone bothered to define a threshold for 'good' vs. 'bad'?
No. But I used to work in a lab run by the Federal government and they
have a (safety) rule that you cannot keep chemical beyond the
manufacturer's "chemical lifetime" - this is 1 year for ethers, I
think. Beyond that we defined our own criteria: obvious things such as
accumulation of water (i.e. chemical no longer solid), or color change
(which does happen to PEGMME). Defining your own criteria is a way
around being told that you must re-purchase all your chemicals after X
amount of time.
4. Is peroxide formation a problem only for liquid PEGs, or does it
also happen to a significant degree in solid PEGs?
Do not know. All I know is that solid (frozen and/or crystalline)
proteins sometimes do degrade. Example: insulin.
5. What is the best storage method for (liquid) PEGs? Does storage at
4°C or -20°C prolong the lifetime?
It has been my understanding that *some* chemicals (PEGMME included)
might last longer if they are protected from light.
6. Any other thoughts?
A story to serve as a caution that maybe you should not worry and just
test for crystallization.
Quite some time ago I crystallized and solved the structure of a
restriction enzyme-DNA complex. I used the Hampton screen to get
initial conditions and the critical component to get crystals was
dioxane. After getting the initial hit, I tried to reproduce the
condition with chemicals from the lab. No luck. Went back to the
Hampton screen and got crystals. Then I borrowed a couple of mLs of
dioxane from every lab in the department, I had 6 or 7 samples and
repeated the experiment. ONE sample gave crystals, the rest did not. So
I looked up the manufacturer and grade of reagent and ordered my own
bottle. Tried the crystallization and it did NOT work. Called technical
support for the supplier and was heavily chastized for keeping an ether
beyond its expiration date. Asked what the difference was and if I
could have evaluations of the chemical lots from the factory. No good,
no apparent difference. I pursued optimization with the one sample of
dioxane that worked (and bought the owner a new bottle).
It was my hope that after solving the structure (in which several
dioxane molecules were visible) it would become clear what was present
or absent in the dioxane that was absent or present in the other
samples, explaining how this helped crystallization.
No luck. There never was an apparent reason why one (very old) sample
worked and other (old and new) samples did not work.
Lesson re-learned: crystallization is not a science, but witchcraft and
you must always have a good supply of everything.
Mark
Mark van der Woerd
Research Scientist
Dept. of Biochemistry and Molecular Biology
Colorado State University
Fort Collins, CO 80523
Phone (970) 491-0469
-----Original Message-----
From: Mischa Machius <[EMAIL PROTECTED]>
To: Bulletin Board CCP4 <[email protected]>
Sent: Wed, 5 Jul 2006 16:15:32 -0500
Subject: [ccp4bb]: How to test for PEG contamination?
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Hi y'all - We are trying to figure out how "good" some of our PEGs
still are and would appreciate if anybody had any advise for a proper
assay.
Specifically, PEGs, as well as other organics, such as dioxane,
glycerol, etc., over time accumulate peroxides that are harmful to
proteins. The good old iodide test (mixing the sample with potassium
iodide/acetic acid) gives some hints about peroxides in a sample.
However, I would like to be more quantitative. Essentially, I'd like to
define a threshold value at which a chemical should be discarded, and
I'd like to have a reliable test for that. There are quantitative kits
available that use Fe++/xylenol orange. Here are the questions:
1. Does anyone actually test their chemicals for peroxides?
2. What tests would be useful for PEGs, dioxane, glycerol, and the
likes?
3. Has anyone bothered to define a threshold for 'good' vs. 'bad'?
4. Is peroxide formation a problem only for liquid PEGs, or does it
also happen to a significant degree in solid PEGs?
5. What is the best storage method for (liquid) PEGs? Does storage at
4°C or -20°C prolong the lifetime?
6. Any other thoughts?
Many thanks in advance.
Best - MM
-------------------------------------------------------------------------
-------
Mischa Machius, PhD
Associate Professor
UT Southwestern Medical Center at Dallas
5323 Harry Hines Blvd.; ND10.214A
Dallas, TX 75390-8816; U.S.A.
Tel: +1 214 645 6381
Fax: +1 214 645 6353
