LIPIDES (fats and RELATED SUBSTANCES) ^9 



If this is kept in mind, an understanding of many physical and chemical 

 properties of fats is easily acquired. 



Unsaturated acids have the ability to exist in different isomeric forms, 

 which are called geometric isomers. These are designated by the prefixes 

 CIS- and trans-. This type of isomerism, a consequence of the presence 

 of carbon-to-carbon double bonds, may be illustrated by the formulas of 

 oleic acid and its ^ra/is-isomer, elaidic acid: 



HC(CHj)7CH, H.CCCHOtCH 



HC(CHs)7C00H HC(CH07COOH 



Oleic acid Elaidic acid 



(cis- isomer) (frans- isomer) 



Acids like linoleic with two double bonds can exist in four geometric 

 isomers, corresponding to the cis-trans arrangement about each ; in general, 

 the number of isomers possible is (2)", where n is the number of double 

 bonds present. Generally the natural fatty acids occur in the cis form, 

 although vaccenic appears to be a trans acid. Where trans forms do not 

 occur naturally, they may readily be produced by treating the cis acids 

 with nitrous acid or certain other reagents. This reaction has come 

 to be spoken of as ''elaidinization" from the circumstance that oleic acid 

 is thus partially converted into elaidic. The trans acids are higher 

 melting and less soluble than the corresponding cis forms. 



The most important unsaturated fatty acids, together with their for- 

 mulas and occurrence, are listed in Table 4-4. Many other acids with 

 varying numbers of carbon atoms and different degrees of unsaturation 

 have been reportedly obtained from brain, liver, and other tissues. The 

 chemistry of these acids and their function in the animal organism are 

 not yet clearly defined. Their presence in some of the most important 

 organs of the body leaves little room for doubt that their role is an 

 important one. 



Although it is rather well established that the animal body can de- 

 saturate fats, certain limitations to this process apparently exist in 

 many, if not all, species. Rats kept on diets devoid of unsaturated fats 

 develop a scahness of the skin, lesions in the kidneys, sterility, and 

 loss of weight, and eventually die. This nutritional deficiency can be 

 prevented by including either linoleic or arachidonic acid in the diet. 

 These particular unsaturated fatty acids have therefore come to be 

 called "essential fatty acids." No one has demonstrated a need of the 

 human body for these acids, but even though they may be required, their 

 widespread occurrence in foodstuffs renders any disease in man resulting 

 from their deficiency quite unlikely. 



Quantitative Relations of the Fatty Acids. ISlany of the statements 

 made in the preceding pages regarding the fatty acids become more 



