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
-----Original Message-----
From: Jacob Keller <j-kell...@fsm.northwestern.edu>
To: CCP4BB <CCP4BB@JISCMAIL.AC.UK>
Sent: Wed, Aug 24, 2011 1:18 pm
Subject: [ccp4bb] Aging PEGs
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
*******************************************
Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
cel: 773.608.9185
email: j-kell...@northwestern.edu
*******************************************
