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Hello, everybody, I have here Proceedings of the First International Conference on Protein Crystal Growth from 1985 (when I was younger), published as a special issue of Journal of Crystal Growth (1986). J.Cryst.Growth 76 (1986), 577-582 by Fran Jurnak discusses impurities in PEG. There's a reference to Ray and Puvathingal, Anal.Biochem. 146 (1985) 307, discussing how to purify PEG (borohydride reduction follwed by de-ionization). The one time somebody here tried to purify their own PEG, they measured concentration of the final product by comparison of its index of refraction to a table of such things that they found somewhere. You could measure conductivity of your PEG solution to get a clue how much ionic stuff is present. We have small PEG's in plastic bottles, and they gradually collapse their containers, I assume by absorbing oxygen. (I think) Fluka specifies "percent ash" which is a measure of non-PEG non-flammable inorganic components. Fluka "microselect" and the gas chromatography grade PEG4000 from EM (EMD?) work well for us. -Dan Anderson On Wed, 5 Jul 2006, Mischa Machius wrote: > *** For details on how to be removed from this list visit the *** > *** CCP4 home page http://www.ccp4.ac.uk *** > > > 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 > > >
