27th April 2006
Here is a molecule you’ve probably used, especially if you happen to be of that type of person who never does laundry. Cyclodextrin is what’s known as an cyclic oligomer of glucose molecules. The cyclic part means that it forms a ring, and “oligomer” means a repeating series of the same molecule. It’s different from a polymer in that it’s smaller. The difference between “oligomer” and “polymer” is not well-defined, but a good general rule is: if you can count the number of monomers (glucose subunits), it’s probably better termed “oligomer.” This is the structure of glucose:
Glucose
The important thing to note here is that there are a number of hydroxyl groups (-OH) bound to carbon. The bond between carbon and oxygen is quite polar, as is that between oxygen and hydrogen. This means that it will tend to dissolve in polar liquids — water! So, this isn’t that much of a surprise: sugar dissolves in water.
Let’s take a look at the oligomer of glucose, cyclodextrin.
B-Cyclodextrin Overhead Wireframe View
This is an overhead view of beta-cyclodextrin. It is a 7-mer of glucose. Grey atoms are carbons, red atoms are oxygens, and white atoms are hydrogens. Here is another view from an angle:
B-Cyclodextrin Angled View
It’s a bit of a mess, but you can see that it forms a sort of donut shape. This compound is also quite soluble in water. It gets interesting here — notice the pore in the center. This pore still has some polar bonds in it (from the sugar hydroxyl groups, again), but the environment inside that pore is much less polar than that of water. Take a look at the space-filling view of the molecule below. This shows the actual space the atoms can be thought to take up, rather than simplifying the structure as lines as we’ve done above.
B-Cyclodextrin Overhead Spacefilling View
It’s a pretty clean circle. That pore, it turns out, can be pretty useful. Sugar and cyclodextrin are two examples of polar compounds we’ve given. Polar compounds tend to be hydrophilic, meaning they dissolve easily in water. Nonpolar compounds lack the polar groups that allow them to interact strongly with water, meaning they will tend not to dissolve. The most often used example is oil and water. The reason it’s given in chemistry class is because it’s an easy example to wrap your head around. Here are a couple more that are important to the chemistry of cyclodextrins.
Since that pore is relatively nonpolar, and the outside is relatively polar, we have a compound that can dissolve in water, but can also help nonpolar compounds go into solution. We term compounds like this “
amphipathic.” The first example (dear to the hearts of the lazy) I alluded to is Febreeze. Febreeze is a solution of cyclodextrins, plus some fragrance molecules in water. After the solution dries on your clothes (couch, dog…) you have a thin coating of cyclodextrins. These trap odor molecules (which tend to be nonpolar) and keep them from evaporating into the air. When you wash your clothes, the polar cyclodextrins (and accompanying odorants) simply go into the water.
The reverse effect can be used as well: A complex of fragrance and cyclodextrin, on, say, a dryer sheet, will release when warmed.
Another time this polarity problem comes up is in the manufacture of drugs. Lots of drugs need to be nonpolar - maybe because the enzyme they interact with requires it, maybe because they need to cross a cell membrane. Dosing people with these nonpolar compounds can be a problem, because they won’t dissolve in your stomach. If you take an insoluble drug, it will simply pass through your GI and come out in your feces. Barium, for example, is pretty toxic and will kill you quite readily.
Barium sulfate, however, is completely insoluble in water, and you can take in grams of the stuff for radiological imaging of your GI. Cyclodextrins, then, can help us out. A drug-cyclodextrin complex will be quite soluble, and cyclodextrins are completely nontoxic. The complex can then be given instead of the free drug.Citation: B. Manunza, S Deiana, M Pintore, and C Gessa. “Structure and Internal Motion of Solvated B-Cyclodextrine. A Molecular Dynamics Study.”
J. Mol. Struct. (1997). 419:133-137.
Wikipedia article on Cyclodextrins. Cyclodextrins also occur in 6- and 8-mers with different pore sizes, as you see here.Images generated with
RasTop and
CS ChemDraw.
This entry was posted on Thursday, April 27th, 2006 at 12:00 am and is filed under Drugs, Hygeine, Biology, Stinky, Perfumey.
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