Friday, June 1, 2007

PEG and me

Poly(ethylene glycol) is an interesting molecule. It is a polymer of ethylene glycol made by the polymerization of ethylene oxide.

Ethylene glycol is a dialcohol:

It is used in antifreeze and it toxic.

Recently it was discovered that a number of toothpastes made in China contained high amounts of diethylene glycol. mmmmmm minty deathpaste. DEG looks like:

When ethylene glycol is polymerized with itself it is know as poly(ethylene glycol) or PEG for short. PEG takes on a lot of different properties. The properties vary based on how big the molecule is. Ψ*Ψ at Carbon-based Curiosities recently posted about poly(ethylene glycol) (PEG) and it use in a hot bath.

I have been exposed to PEG in a number of interesting ways. In every instance it have been useful. I will relate three occurrences.

My Ph.D. work in grad school dealt with the use of carbohydrate-functionalized dendrimers to study protein-carbohydrate interactions. A dendrimer is essentially a spherical polymer. I made dedrimers of different sizes with varying numbers of carbohydrates on the surface to see how they interacted with proteins. The goal was to understand more about how these important signaling processes occur.

OK, long story short time. No need to cover everything and bore you, the kind reader.

My first attempt used a phenyl ring as a linker between the dendrimer and mannose. It look like this:

This molecule shown had a molecular weight of about 28,000, and it had the solubility of brick dust. I needed a functionalized dendrimer that dissolved in water. This one did not. The solution turned out to be PEG. I replaced the phenyl group with a PEG group and voila, it was soluble in water. The structure looked a bit like this:

Granted, it's not really a *poly*ethylene glycol, but it is part of a larger molecule.


Since I had laser eye surgery I have needed to use eye drops to keep my peepers moist. One of my favorite brands is Systane. The active ingredient is polyethylene glycol 400. The 400 means there PEG has an average molecular weight of 400 daltons, which translates to a molecule that contains 9 ethylene glycol molecules.


A while ago my daughter was constipated. The doctor prescribed GlycoLax. GlycoLax is polyethylene glycol 3350. It's average molecular weight is 3350, which means it is made up of about 76 ethylene glycol molecules.

In all of these, the function is the same: hold on to water. This is done through hydrogen bonds between the water and the oxygens on the ethylene glycol molecules. It seems to work quite well.


elderchemgeek said...

You mentioned your molecule had the solubility of brick dust. Well another great use of PEG is to formulate (i.e., help solvate) highly flat organic molecules (read kinase inhibitors) for introduction into some unsuspecting rodent. It’s just a matter of how much PEG it takes to get a homogeneous solution…

Matt Jenks said...

I also used some PEG (360, I think?) as a solvent a couple of months ago to help displace a somewhat unreactive chlorine with a cyano group. It worked sort of well, but clean up was a bit of a hassle. Trying to extract the desired compound out of water with a bunch of PEG in it required a lot of DCM.

Jordan said...

During my PhD oral defense (like you, I had used some PEG linkers in my thesis work) I was asked to explain why polyoxyethylene (i.e., PEG) is a soft, highly stable, highly soluble substance, while polyoxymethylene is brittle, poorly soluble, and very unstable.

milkshake said...

Some companies that make more expensive brands of cream sodas and cherry pops etc use gum arabica additive as a thickener, something to increase the "mouthfeel" without adding more calories.

I wonder how would PEG-enhanced beer taste like.

Anonymous said...

Hi there. I was browsing through the internet about PEG and came across this site. I would just like to ask if there are any particular solvents that are unreactive with PEG? The PEG that i'm working with is a cured solid wax-like PEG. I would be most grateful if you can enlignten me on this one. thanks in advance