BioPHYSICAL chemistry

At room temperature, a solution of pure
water should give a surface tension of
72.8 mN m−^1. When a detergent is added to
water the presence of the detergent disrupts
the interactions between water molecules
and the surface tension decreases. Thus, as
the concentration of a detergent is increased,
the surface tension will usually decrease until
it reaches a specific concentration, above
which there is no longer any change in sur-
face tension (Figure 4.10). This is interpreted
as the concentration at which it is thermo-
dynamically favorable for micelles to form.
The detergent concentration at which micelles
form is called the critical micelle concentration,
or CMC.
Proteins present in membranes can be
purified by the use of detergents. The lipid
environment surrounding the hydrophobic
surface of the protein is replaced by deter-
gents, allowing extraction from the cell as seen in Figure 4.11. The pro-
teins can then be further purified by conventional techniques provided
the presence of detergents is maintained in solutions. Once isolated and
purified, the action of the protein in the cell membrane can be studied
by inserting the proteins back into membranes. One traditional approach
was to make synthetic bilayers by placing a small septum between two
chambers. Lipids present in the solutions will spontaneously form a
bilayer across the septum. If membrane proteins are present in solution
they will also be incorporated into the bilayer. Another approach is to
introduce detergent-solubilized proteins into a solution that can form
liposomes (Figure 4.11). The detergent is removed from the protein
by dialysis and the protein is spontaneously inserted into the lipsosome

CHAPTER 4 PHASE DIAGRAMS AND MIXTURES 81

CMC

Surface tension

Detergent concentration

Phospholipids

Detergent-solubilized

calcium pump

Calcium pump in

reconstituted membrane

ATP

and

Ca^2 

Ca^2 

Ca^2 

Ca^2 

Ca^2 

ADP

Dialysis

ATP

Figure 4.10The dependence of surface

tension on the presence of detergent can be

used to determine the CMC, or critical micelle

concentration.

Figure 4.11Membrane proteins can be embedded in bilayers by insertion
into liposomes.