Because fatty acids can vary greatly, scientists will indicate the number of carbons and double bonds in a fatty acid. For instance a 18:3 fatty acid will be 18 carbons long and have three double bonds. Scientists also use omega system to indicate where double bonds are in a fatty acid. It works like this. If a fatty acid is linked to glycerol, the second carbon closest to the link is referred to as the alpha (α) carbon (see Figure 5.3). Meanwhile, the carbon furthest from the linkage with glycerol is called the omega (ω) carbon.
The omega system is based on the Greek alphabet. Alpha is the first letter of the alphabet and omega is the last. No matter how many carbons are in your fatty acid chain, these carbon atoms will always be addressed in this manner. Looking at a fatty acid not linked to glycerol, the alpha carbon would be the first carbon atom adjacent to the carbon bonded to two atoms of oxygen. Table 5.1 lists common fatty acids and their abbreviations.
To indicate position of the first double bond we count the number of carbons to the first carbon of the first double bond from the omega end. For instance, if the first double bond starts at the third carbon atom in, it is an omega-3 (ω-3) fatty acid (see Figure 5.3). Likewise, if the first double bond appears at the sixth or the ninth carbon atom in, these would be ω-6 and ω-9 fatty acids, respectively. For the most part, when addressing polyunsaturated fatty acids, we indicate only the position of the first double bond because subsequent double bonds seem to occur in series after one saturated carbon atom.
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