Because they contain only single bonds, saturated fatty acids possess an enormous amount of flexibility, which allows them to bend into all kinds of shapes.

• Nomenclature of Saturated Fatty Acids

You might see a notation such as 16:0 (Figure 10.3).

• The methyl group at the end of the fatty acid is known as omega (x).

• The number of carbon atoms is noted from the carboxyl end.

• The double bonds in fatty acids are numbered from the methyl or omega (ω) end

• The first number indicates that the fatty acid has 16 carbon atoms.

• The second number indicates that there are no double bonds.

Figure 10.3 A fatty acid with the notation 16:0.

The cis conformations (see Figure 10.2) of the double bonds of unsaturated fatty acids have a bend in their structure of about 30 degrees. This creates significant structural features, particularly for membranes, and as components of membranes the fatty acids tend to have phosphate groups added to them to make phospholipids.

Cis fatty acids can be turned into trans forms by an industrial process. This creates fats with a much higher melting point, which are solid at room temperature and can be made into a spread (e.g. margarines). The natural formation of the bonds has been changed.

• Unsaturated Fatty Acids and the Relevance of its Nomenclature

Oleic acid is designated as 18:1 (Figure 10.4)

• The numbering runs from left to right, i.e. from the carboxyl group to the methyl group.

• The 1 indicates one double bond.

• The more precise designation is 18:1(n-9), which means that the last double bond is 9 carbon atoms from the terminal methyl group (numbers in bold and underlined).

Figure 10.4 A fatty acid with the notation 18:1.

This short-hand provides quite a bit of information at a glance to a pharmacist.

Linoleic Acid

• Linoleic or all-cis-9, cis-12-octadecadienoic acid – 18:2(n-6) – (see Figure 10.5).

• Has two cis double bonds: one 9 carbon atoms from the terminal methyl group (9 carbon atoms from the terminal carboxyl group), the other 6 carbon atoms from the terminal methyl group (12 carbon atoms from the terminal carboxyl group).

• Omega 6 fatty acid: this is where the term “omega 6 fatty acids” comes from. The first double bond is at the sixth carbon from the methyl terminal group.

Linolenic Fatty Acids

There are two types (see Figure 10.6):

• alpha linolenic

• gamma linolenic.

Alpha linolenic acid or all-cis-9,12,15-octadecatrienoic acid:

• 18:3 (n-3)18 carbons.

• Three double bonds.

• First double bond 3 carbons from the methyl terminal group (15 carbon atoms from the carboxyl terminal end).

• omega 3 fatty acid.

Gamma linolenic acid or all-cis-6,9,12-octadecatrienoic acid:

• 18:3(n-6)18 carbons.

• Three double bonds.

• First double bond 6 carbons from the methyl terminal group (12 carbon atoms from the carboxyl terminal end).

• Omega 6 fatty acid.

In natural unsaturated fatty acids, the cis bonds cause kinks and make them bend instead of sitting in a straight line (Figure 10.5). The kinking creates a more fluid membrane because the molecules cannot be packed close together, unlike the saturated fatty acids, which can lie in a line. The kinking also ensures that cis ­unsaturated fatty acids are fluid at room temperature (which is better for areas such as artery walls). Trans fatty acids are problematic because the lipids can lie flat, like a saturated fatty acid, and so cause solid areas of rigid plaque in places like the arteries.

Figure 10.5 The configuration of linoleic and linolenic acids; both have cis configurations.

Saturated Fatty Acids (Figure 10.6)

Because membranes consist largely of lipids, only lipid-soluble substances, which are hydrophobic, can pass through them. This feature is utilized by pharmaceutical companies if they want a drug to be absorbed rapidly into the bloodstream.

There are ways of getting polar substances through a membrane but these usually involve a mechanism that requires energy; it is not a passive process, and will be covered in more detail later (see Chapter 16 ‘How do drugs get into cells?’, p. 123 and Figure 31.1, p. 236).

A great deal of research is going into delivering drugs in liposomes – spherical aggregations of phospholipid bilayers. The drug is encased in the liposome, which has specially engineered qualities to make it favour attachment to a specified tissue or ‘target organ’. In this way, it is hoped that highly toxic drugs, such as those used in chemotherapy, can be delivered more specifically and safely than the more ­inefficient ‘shotgun’ approach of general medication.