Delta G Project One -- Amphipathic Molecules

Amphipathic molecules are a biologically important class of molecules. These schizophrenics have both hydrophobic (not liking water) and hydrophilic regions (liking water). As such they are termed amphipathic (also known as amphiphilic) molecules. The structure of amphipathic molecules usually revolves around a non-polar hydrocarbon region and a region that is polar, ionic, or both. The non-polar hydrocarbon region -- or tails -- tend to lie in parallel arrays reacting to van der Waals forces. The water around them hydrates the polar or ionic head groups.

When dispersed in water the hydrocarbon chains self-aggregate from the water. The aggregated results in micelles (for those aggregates dispersed in water) and monolayers (at the air water boundary). The tendency for the hydrocarbon chains to become remote from the polar, solvent, water, is known as hydrophobic effect. Hydrocarbons form no hydrogen bonds with water, and a hydrocarbon surrounded by water facilitates the formation of hydrogen bonds between the water molecules. The bulk water is more structured than it is in the absence of the hydrocarbon; it has lostentropy and as such is in a thermodynamically less favorable state. With the hydrocarbons organized such that they are remote from water this renders the water molecules less ordered. The hydrophobic effect is considered to be entropically driven.

Only a small quantity of an amphiphilic lipid in water will form a monolayer. The tendency is the formation of micelles. Micelles can be formed as spherical, ellipsoidal, diskoidal, and cylindrical variations. There are two types of micelles formed by amphiphilic lipids, small and large. Both types form through the two opposing forces: (1) attractive forces between hydrocarbon chains (van der Waals) caused by the hydrophobic effect forcing the chains together and (2) repulsive forces between the polar head groups.

COMMON QUESTIONS ANSWERED

Question 1. What is another form of a micelle?

A bilayered structure in the form of a closed hollow sphere is possible. This type of micelle is called a vesicle. The concept of a vesicle is two sheets of lipids with their hydrocarbon chains opposed (a bilayer). The sheets then curving to form a self sealed hollow sphere. More commonly known as a cell wall.

Question 2. What determines the lower limit of micellar size?

The hydrophobic effect does; a minimum number of hydrocarbon chains must associate before the water-hydrocarbon interface is eliminated. Micelles therefore have a minimum size.

Question 3. What determines the upper limit of micellar size?

The repulsion of the polar heads does. If there are two hydrocarbon chains per polar head group, the nonpolar volume per head group is twice that of an amphiphilic lipid with one hydrocarbon chain. The greater repulsive force in the latter prevents the lipid molecules from coming too close and keeps micellar size small. The weaker repulsive force and large hydrocarbon volume in the former allows very much larger micelles to form, particularly bilayers and vesicles.

Question 4. Which lipids form small micelles and which form vesicles and bilayers?

The length of the hydrocarbon chain relative to the polar head group determines the formation of micelles or vesicles. Acylate ions and lysophosphoglycerides form small micelles; phosphoglycerides, phosphosphingolipids, plasmalogens, glycoglycerolipids, and glcosphingolipids form vesicles and bilayers.

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