BioPHYSICAL chemistry

also possible. At an air/water interface the lipids will form monolayers

with the polar head groups directed towards the water and the fatty acid

chains exposed to air (Figure 4.4).

Detergent molecules are also amphipathic compounds and usually

contain a hydrophilic polar head group and a hydrophobic hydrocarbon

chain. It should be noted that detergents usually have only a single carbon

chain and can pack into other arrays, micelles, and liposomes, as shown

below. In a micelle, the hydrophobic chains are in the interior and the

polar head groups are exposed to the water. Micelles can be quite large

and can contain hundreds of detergent molecules. Liposomes have a bilayer

arrangement with a central aqueous compartment lined by the polar groups

of the inner lipids (Figure 4.5). Since liposomes and micelles have no

hydrophobic edges, both structures are relatively stable in an aqueous

environment.

The ability of detergents and lipids to form micelles and liposomes can

be described with the use of phase diagrams. The formation of a micelle

76 PARTI THERMODYNAMICS AND KINETICS

(a) Water

Water

(b) Air

Water

(a)

Bilayer membrane

Inner aqueous

compartment

(b)

Figure 4.4Lipids can form both (a) bilayers and (b) monolayers.

Figure 4.5Detergent molecules can form (a) micelles or (b) liposomes.