Aging PEGs

From: Jacob Keller
Date: 24 August 2011 20:18


A while ago I measured the pH's of old and new PEGs and found them
very different, and internally attributed all "old vs new PEG issues"
to pH. Upon reflection, this seems too simplistic. Are there other
known mechanisms of crystallization capacities of PEGs of various
ages?

Jacob Keller

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Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
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From: Jim Pflugrath


PEGs have small amounts of anti-oxidants added to them by the manufacturer.
I think different manufacturers may use different compounds.

And you might imagine that PEGs themselves with their -OH groups go from
alcohol to adelhyde to carboxylate.


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From: Craig A. Bingman

Commercial polyethylene glycol is contaminated with polymers that have aldehyde groups at the ends.  Other impurities include some amount of internal epoxide linkages.  The aldehyde groups can be additionally oxidized to carboxylic acids.  I would assume that oxidation to terminal carboxylic acids explains the change in PEG pH vs. time.  The pH will also change as the PEG solution comes to equilibrium with atmospheric carbon dioxide.  The aldehyde content increases with time after the PEG is dissolved in water and stored under normal atmospheric gas.  These PEGs can react with protein amine groups.

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From: <mjvdwoerd

PEG is a polymer and it can be made by anionic or cationic polymerization. Whichever you use, you go "the other way" to terminate the reaction at an appropriate time (so you have the molecular weight you want). So when you start with an acid, you terminate with a base and vice versa. If you terminate with a base, your final pH is going to be high (presumably > 7) and if you terminate with an acid, the pH is going to low.

It is therefore important to keep track of your lot number (because as long as the lot number is the same, the treatment was the same). For crystallization recipes that do not involve buffers (there are some!) this is essential, because PEG and PEG are not the same thing (and you should always pH the solution before you use it, so you have a reference point - remember we do not know what we have in our crystallization drops, but we do know what we put into them to make them). Even if the PEG was made by the same process, the manufacturer is concerned with stopping the polymerization at the right time, but not "how hard" they stop it. In other words, the solution might be pH 8 or pH 11 when it is done. 

So when you say that you measured different PEGs and found the pH to be different, that might be accounted for by the way the PEG was made and they may always have been different, irrespective of age.

It is probably not known if "acid PEG" vs "basic PEG" ages at a different speed and with a different mechanism. As you know, the first thing to observe is whether PEG solutions are clear ("water") or slightly colored ("dilute lemonade" :-) because this is a sign of aging. Remember that PEGMME is much more sensitive to aging than PEG itself (you should store MME solutions in the dark). And if the PEG is stored in dry form, it is not very easy to age it by chemical reaction, because nothing is "swimming", but "not easy" is not the same as "impossible".

My 2 cents worth.

Mark

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From: Prince, D Bryan 

For those of us truly controlling types :), I used to make the PEG
solutions and filter them over a Bio-Rad resin that filtered out all the
junk added to stabilize the PEG solution. Then, of course I had to
freeze all my PEG solutions in aliquots, or wrap them in foil and store
at 4C in the dark. This would take several days, depending on the FW of
the PEG. If you are really sensitive about what is in your PEG
solutions, try GC-grade PEG's. The FW profile is much more restricted
around the reported value (i.e. PEG 3350 molecular biology grade has a
broad peak centered on Mr=3350. PEG 3350 GC-grade has a much tighter
peak profile.) Back before you could buy Crystal Screen I, II or HT, you
had to make the stock solutions, then make the screen. But at least when
you did that, you had all the stocks. Now, I just buy pre-made
solutions, and keep them in a drawer with a date opened written on the
bottle. Isn't progress grand? :)

Bryan


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From: Frank von Delft

And now, ....  does anybody know of systematic data indicating how consistently all this matters?
phx

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From: Van Den Berg, Bert

Is there anything that consistently matters in crystallography? ;-)

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From: R. M. Garavito

Time to start digging in the archives.  Try looking at work by Fran Jurnak in 1986 (J. Cryst. Growth 76, 577) and Bill Ray's work in 1985 (Analytical Biochem 146, 307), and then the works that cite them.  I thought this was common knowledge, but I guess it goes in phases.  

Aging of poly(oligo)-oxyethylene-based compounds is well known in the surfactant field as it changes the chemical properties of common detergents (Brij, Triton, C10E6, etc.), not only by adding aldehydes and carboxylates to the system, but also by increasing metal binding.  It is a sobering sight to see old PEG cross-link your 3-month old crystal:  the damn thing wouldn't dissolve after 2 hours in plain buffer, but when I poked it with a glass fiber, the protein oozed out like the center of a cherry cordial, leaving a sad looking deflated shell of a crystal.

Cheers,

Michael

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R. Michael Garavito, Ph.D.

Professor of Biochemistry & Molecular Biology

513 Biochemistry Bldg.   

Michigan State University      

East Lansing, MI 48824-1319

From: Hargreaves, David
Date: 5 September 2011 14:43

I went through  the peg solid stocks in our cupboard and noticed some of them smelled acrid. I probably won’t use them. Not sure whether they’re chemically decomposing. Maybe fossilising?

Dave

David Hargreaves

Associate Principal Scientist

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AstraZeneca