Cyclodextrin: A Nonpolar Cage for Cholesterol

Opening image: cholesterol. The large nonpolar surface means that cholesterol is insoluble in aqueous solutions. (The limit of detection of free cholesterol molecules in solution is 10^-8 M.)

You may be surprised to learn that a molecule, β-cyclodextrin, built from glucose, is able to extract substantial amounts cholesterol from mammalian cell membranes into a water-soluble cyclodextrin-cholesterol complex.

Ball-and-stick model of β-D-glucose. The six-membered glucopyranose ring lies in the plane of the screen. The nonpolar C–H bonds are parallel to the axis of the molecule, whereas the polar O–H groups form a belt around the equatorial rim of the disk.

Toggle space-filling model. Like the Roman god Janus, the glucose molecule has two faces. The axially oriented C–H bonds provide a flat localized nonpolar environment. Various proteins which bind glucose specifically contain a nonpolar aromatic ring in the binding pocket which interacts with the flat nonpolar surface of glucose, but not other hexoses, all of which have one or more axial hydroxyl groups.

Cyclodextrins are composed of glucose units connected by glycosidic linkages to form a series of oligosaccaride rings comprised of 6, 7 and 8 glucose units (α-, β- and γ-cyclodextrin, respectively). The cyclodextrins are shaped like donuts, with a very hydrophobic cavity in the center. While the height of the cyclodextrin cavity is the same for all three types, the number of glucose units determines the internal diameter of the cavity.

β-Cyclodextrin crystallizes as a dimer but only one monomeric unit is shown here. Toggle between wireframe and spacefilling models. View the model along the axis of the cylinder. What is the orientation of the axial C–H bonds lining the central cavity?

Cyclodextrin-caged cholesterol. The cholesterol finds itself in a hydrophobic cavity formed by two cyclodextrin molecules. Toggle removal of cholesterol.

Ternary complexes, like (cyclodextrin)₂-cholesterol, must form stepwise (a collision of three molecules in the correct orientation at the same time is improbable). Clearly the first step is formation of a cholesterol-cyclodextrin binary complex.

A binary complex is formed when cholesterol slips into the hydrophobic cavity of a β-cyclodextrin molecule. Then a second cyclodextrin binds to the binary complex to form the final ternary complex.

Toggle surface model off. Note the hydrogen bonds between the two cyclodextrin molecules. Would you expect to find cyclodextrin dimers in solution in the absence of cholesterol? Explain.

The hydrophobic effect provides the thermodynamic driving force stabilizing the caged cholesterol. During formation of the ternary complex, water molecules comprising the solvent cage around cholesterol are released to the bulk solvent, along with five ordered water molecules from within the central cavity of each cyclodextrin. The number of waters in the central cavity is known from the crystal structure.

Summary

1. The high cohesiveness of water, which is due to hydrogen bonding, is central to the hydrophobic effect.
2. Water is more structured near nonpolar solutes, losing entropy to avoid the loss of hydrogen bonds.
3. The structure of water around nonpolar solutes at room temperature is surprisingly similar to that of bulk water, in that there is a tendency for water molecules to form pentagons around nonpolar groups.
4. The hydrophobic effect follows from the fact that water must be displaced from the surfaces of nonpolar (hydrophobic) molecules during association into complexes.
5. The central hydrophobic cavity of β-cyclodextrin shows a high binding affinity toward cholesterol because the size, shape, and nonpolar character of the central cavity and cholesterol are a good match.
6. Usually, molecular association is made possible not by a single weak interaction but through the simultaneous cooperation of several weak interactions.