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6.3.2.2 Unsaturated Fatty acids
This group of fatty acids are characterized by having one (monounsaturated) or more double bonds (polyunsaturated). In most of the unsaturated fatty acids, there is a double bond (∆9) between carbon atom 9 and 10 . e.g. Structure of oleic acid C18:1, ∆9 10 9 CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH=CH-CH2-CH2- CH2-CH2-CH2-CH2-CH2-COOH Oleic acid (n9)
The additional double bonds are usually present between the ∆9 double bond and the methyl terminal end of the chain.
In fatty acids containing two or more double bonds, the double bonds are separated by a methylene group:–CH=CH–CH2–CH=CH–The double bonds of nearly all the naturally occurring unsaturated fatty acids are with the cis geometrical configuration, which produces a rigid bend in the aliphatic chain. Unsaturated fatty acids may be further subdivided as follows: a) Monounsaturated (monoethenoid, monoenoic) acids: A monounsaturated fatty acid is a fatty acid with a carbon chain in which one carbon–carbon double bond is present. e.g. Oleic acid: It is fatty acid with18 carbon atoms and a single double bond. The structure is represented as follows Oleic acid C18:1; ∆ 9 18 10 9 1 CH3 (CH2)7CH=CH (CH2)7 COOH
or C18:1 ω9 or 18:1 n-9
ω1 2 3 4 5 6 7 8 9 10 CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2=CH - (CH2)7COOH Structure of Oleic acid b) Polyunsaturated (polyethenoid, polyenoic) acids (PUFA): The polyunsaturated fatty acid is a fatty acid with a carbon chain in which two or more carbon-carbon double bonds are present. They are classified as follows according to the number of double bonds present.
i) Dienoic acid: fatty acid with 2 double bonds e.g. Linoleic acid ω6, C18:2 or 18:2 n- 9,12
ω1 2 3 4 5 6 7 8 9 10 18 CH3-CH2-CH2-CH2-CH2-CH=CH-CH2-CH = CH-(CH2)7COOH n18 12 9 1
ii) Trienoic acid: fatty acid with 3 double bond e.g. Linolenic acid ω3, C18:3 or n-3, 6, 9
ω1 2 3 4 5 6 7 8 9 10 18 CH3-CH2-CH=CH-CH2- CH= CHCH2CH = CH (CH2)7COOH n18 15 12 9 1
iii) Tetraenoic acid e.g. Arachidonic acid : ω6, C 20:4 or ω6 6 9, 12, 15 CH3CH2CH2CH2CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3 COOH n 20 14 11 8 5
iv) Pentaenoic acid e.g. Eicosa pentaenoic acid (EPA) ω3
ω3 CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3 COOH 20 17 14 11 8 5
v) Hexoenoic acid e.g Docosahexa enoic acid (DHA) ω3
n22 19 17 14 11 8 5 CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3 COOH ω3 |