William Tivol tivol at
Mon Dec 11 16:48:36 EST 1995

chenlong at BIO.CORNELL.EDU wrote:
: Hi,

: Can someone tell me some physical and chemical properties of polyethylene glycol 
: 4000? I mixed it with a protein at pH4.8 (at this pH this protein is slightly 
: positively charged. Its isoelectric point is 5.4). I want to know sth like the 
: charges of PEG at this pH. Can PEG complex with the protein? If they form 
: complex,what force plays the big role? Wan der waals? H-bonding? electrostatics? 
: chemical cross-linking by itself or with protein? If concentration of PEG too 
: high, protein crystal dies. What did PEG do to protein and/or protein crystal 
: and how to avert it? thank you.

: Chenlong Li.

Dear Chenlong,
	Polyethylene glycol 4000 is a polydisperse mix of (CHOH-CHOH)n with
average molecular mass of 4000 daltons.  At pH 4.8 it is uncharged.  It will
complex with a typical protein (as will glucose, glycerol and other poly-
alcohols), but I cannot tell you if it will complex with your particular
protein.  I think H-bonding is the primary mechanism of interaction, where
the OH groups on the PEG substitute for water.  The xtal could die if the
conformation with minimum energy in PEG is different from that in the xtal,
or (perhaps the same thing) if the interaction of the PEG with a part of the
protein bound to protein (rather than H2O) in the xtal becomes bound to PEG.
In this case, averting the destruction of the xtal means not putting too much
PEG in the mix.  All bets are off if the commercial prep contains compounds
which are not PEG (such as COOH-containing groups).
				Bill Tivol

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