Biology Reference
In-Depth Information
Conjugated double bonds
C
C
C
C
C
C
¼
¼
Methylene-interrupted double bonds
C
C
C
C
C
C
C
If a single double bond is present, the fatty acid belongs to the monoenoic or mono-
unsaturated class (MUFA), while fatty acids with more than one double bond are referred
to as polyenoic or polyunsaturated fatty acids (PUFA).
¼
¼
Fatty Acid Nomenclature
In order to define the structure of fatty acids, it is necessary to describe the number of
carbons in the chain, as well as the number, type (cis or trans), and position of each double
bond. The nomenclature employed in this topic will first list the number of carbons in the
chain, followed by a colon, and then the number of double bonds. Next the position (super-
script
followed by the carbon position of the double bond from the carboxyl terminus) is
listed. It is assumed the double bond is cis, unless a t indicating trans is added. For example,
elaidic acid, an 18-carbon trans monoeonic acid with the double bond between carbons 9 and
10 is written 18:1
6
9t
, while the homologous 18-C cis fatty acid, oleic acid, is designated
18:1
6
9
. The homologous sequence of 18-C fatty acids is shown in
Table 4.5
with the alternate
omega designation (discussed below) included.
Melting points (T
m
s) are an important general property of fatty acids that will have
significant implications for later discussions of membrane 'fluidity' (order), thickness,
and permeability (Chapter 9). Using the 18-C sequence as an example, upon adding a first
double bond (stearic
6
53.4
o
C(
Tabl e 4. 6
). Addition
oleic) the T
m
decreases by a very large
/
21.2
o
C.
of a second double bond (oleic
linoleic) substantially further decreases T
m
,by
/
The addition of a third double bond (linoleic
/
a
-linolenic) decreases T
m
, but only by an
6
o
C. Compare this to the saturated fatty acid series listed in
Tabl e 4 . 3
.Saturated
fatty acid T
m
s increase with every additional 2-carbons in chain length, but only by a small
amount, similar to that noted for the addition of a third double bond in the 18-C sequence.
T
m
increases by
additional
6.5
o
Cin
going from palmitic to 18-C stearic. These T
m
changes indicate that the effect of double
bonds, particularly the first two double bonds, is much greater than changing carbon
chain length.
9.2
o
C in going from 14-C myristic to 16-C palmitic, and by
þ
þ
TABLE 4.5
T
m
s of the 18-Carbon Series of Fatty Acids.
Name
Structure
T
m
Omega
Stearic
18:0
69.6
0
18:1
6
9
Oleic
16.2
9
18:2
6
9,12
Linoleic
5
6
18:3
6
9,12,15
a
-Linolenic
11
3
18:3
6
6,9,12
g
-Linolenic
6
18:1
6
9t
Elaidic
43.7
9
