Biology Reference
In-Depth Information
that at physiological temperature (37
o
C) fatty acids of 10-C or less are fluid (their T
m
is
below 37
o
C) while those of 12-C or more are solid (their T
m
is above 37
o
C). The very
short chain fatty acids (C-2 and C-4) are completely miscible with water, while the
very long chain fatty acids (C-20, C-22 and C-24) are essentially insoluble in water. These
properties will be important in later discussions of membrane 'fluidity' (order), thick-
ness, interdigitation, and permeability (Chapter 9). The three most common saturated
fatty acids found in membranes are myristic (14-C), palmitic (16-C), and stearic (18-C).
These three fatty acids have been known for a long time as they were discovered before
1850! Stearic acid, for example, was first identified in 1823 by the early French biochemist
Chevreul, a full century before Sumner in 1925 isolated the first enzyme, urease, from
jackbean meal.
All long chain fatty acids, those important in membrane structure, have very limited
water solubility, but are soluble in organic solvents. The solubility of palmitic acid (16:0)
in various organic solvents is presented in
Tabl e 4 . 4
. The best solvent, chloroform, has histor-
ically been the solvent of choice for efficiently dissolving lipids. In fact, mixtures of chloro-
form/methanol/water have been the primary solvents used for membrane lipid extractions
since 1957
[8]
. The large differences in lipid solubility for different organic solvents are an
important property in separation of the various fatty acids for compositional analysis
(Chapter 13).
Double Bonds
Membrane acyl chain double bonds are primarily cis (also designated Z) and are almost
never conjugated. Normally, the double bonds are separated by intervening methylene
groups that result in exceptional chain flexibility characteristic of fatty acids with multiple
double bonds (Chapter 9). The intervening methylene group is also a major target for lipid
peroxidation. Many common membrane fatty acids are therefore readily oxidized and
must be effectively protected by membrane anti-oxidants (e.g. vitamin E).
TABLE 4.4
Solubility of Palmitic Acid in Organic
Solvents (g/l, 20
o
C).
Chloroform
151
Benzene
73
Cyclohexane
65
Acetone
53.8
Ethanol (95%)
49.3
Acetic acid
21.4
Methanol
37
Acetonitrile
4
(water)
0.007
