Food Lipids: Chemistry, Nutrition, and Biotechnology

5. Branched Fatty Acids
A large number of branched fatty acids have been identified [16]. The fatty acids
can be named according to rules for branching in hydrocarbons (Table 2). Beside
standard nomenclature, several common terms have been retained, including iso-,
with a methyl branch on the penultimate (�2) carbon, and anteiso, with a methyl
branch on the antepenultimate (�3) carbon. The iso and anteiso fatty acids are
thought to originate from a modification of the normal de novo biosynthesis, with
acetate replaced by 2-methyl propanoate or 2-methylbutanoate, respectively [16].
Other branched fatty acids are derived from isoprenoid biosynthesis including pristanic
acid (2,6,10,14-tetramethylpentadecanoic acid) and phytanic acid (3,7,11,15tetramethylhexadecanoic
acid).
6. Cyclic Fatty Acids
Many fatty acids that exist in nature contain cyclic carbon rings [27]. Ring structures
contain either three (cyclopropyl and cyclopropenyl), five (cyclopentenyl), or six
(cyclohexenyl) carbon atoms and may be saturated or unsaturated. As well, cyclic
fatty acid structures resulting from heating the vegetable oils have been identified
[27–29].
In nomenclature of cyclic fatty acids, the parent fatty acid is the chain from
the carboxyl group to the ring structure. The ring structure and additional ligands
are considered a substituent of the parent fatty acid. An example is given in Figure
10. The parent in this example is nonanoic acid (not pentadecanoic acid, which would
result if the chain were extended through the ring structure). The substituted group
is a cyclopentyl group with a 2-butyl ligand (2-butylcyclopentyl). Thus the correct
standard nomenclature is 9-(2-butylcyclopentyl)nonanoic acid. The 2 is sometimes
expressed as 2� to indicate that the numbering is for the ring, and not the parent
chain. The C-1 and C-2 carbons of the cyclopentyl ring are chiral, and two possible
configurations are possible. Both the carboxyl and longest hydrocarbon substituents
can be on the same side of the ring, or they can be on opposite sides. These are
referred to as cis and trans, respectively.
The cyclopropene and cyclopropane fatty acids can be named by means of the
standard nomenclature noted in the example above. They are also commonly named
using the parent structure that carries through the ring structure. In the example in
Figure 11, the fatty acid (commonly named lactobacillic acid or phycomonic acid)
is named 10-(2-hexylcyclopropyl)decanonic acid in standard nomenclature. An older
naming system would refer to this fatty acid as cis-11,12-methyleneoctadecanoic
acid, where cis designates the configuration of the ring structure. If the fatty acid is
unsaturated, the term methylene is retained but the double bond position is noted in
the parent fatty acid structure (cis-11,12-methylene-cis-octadec-9-enoic acid).
Figure 12 presents some examples of natural cyclic fatty acids and their trivial
and standard nomenclature.
7. Hydroxy and Epoxy Fatty Acids
Saturated and unsaturated fatty acids containing hydroxy and epoxy functional
groups have been identified [1,16]. Hydroxy fatty acids are named by means of the
parent fatty acid and the hydroxy group(s) numbered with its � location. For example,
the fatty acid with the trivial name ricinoleic (Fig. 13) is named R-12-hy-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.