Biology Reference
In-Depth Information
A. DETERGENTS
It is hard to overstate the importance of detergents in life science studies, particularly those
involving biological membranes. At present, detergents form the cornerstone of membrane
studies at the molecular level. As a result, a large detergent industry has flourished,
producing well over a thousand different products. Detergents are amphipathic molecules,
simultaneously containing polar and non-polar ends (discussed in Chapter 3)
[1]
. The first
detergent was also the first soap (see Chapter 5), namely the potassium salt of free fatty acids
resulting from the saponification of animal fat.
Detergents are a class of molecules that have the ability to disrupt hydrophobic/hydro-
philic interactions like those that drive the stability of membranes (see the hydrophobic
effect, Chapter 3)
[1]
. Detergents are used to lyse cells and to extract membrane proteins
and lipids for reconstitution experiments. Many additional functions have been assigned
to detergents including protein crystallization, immunoassays, and electrophoresis. While
detergents' amphipathic structure may grossly resemble that of membrane phospholipids,
a closer examination shows that detergents are much more water soluble (
Table 13.1
). For
example, solubility of the phospholipid DPPC is only ~4.7 x 10
7
mM, many orders of magni-
tude lower than for membrane detergents. This trait of resembling a phospholipid while
being much more water soluble is the reason detergents can be successfully employed to
extract and purify integral proteins from membranes.
Detergents that are used in membrane studies can be divided into 5 basic categories:
anionic, cationic, non-ionic, zwitterionic, and bile salts. The structures of some of the more
commonly used detergents in membrane studies are depicted in
Figure 13.1
. The first 4 of
these classes are synthetic detergents and are classified by the net charge residing on their
polar moiety. The 5
th
classification is based on it being a biologically produced type of
TABLE 13.1
Detergents Commonly Used in Membrane Studies with their HLB and
CMC Values.
Type
Detergent
HLB
CMC (w/v%)
Anionic
SDS
0.17
Cationic
Cetylpyridinium
Bromide
16.7
0.042
Non-Ionic
Brij-35
16.9
0.09
Octyl-Glucoside
13.1
0.7
Triton X-100
13.6
0.0155
Tween 20
16.7
0.0987
Zwitterionic
CHAPS
13.1
0.5
Bile Salt
Cholate
18
0.388
Deoxycholate
16
0.083
~3.5 x 10
7
g/l)
(phospholipid
DPPC
solubility
