A strong positive character of the carbon of the carboxyl results from its bond to two oxygens. This induces an alternating polarity, with the odd carbons positive in the chain. On the other hand, the influence exerted by a double bond in the molecule corresponds to an enhancement of the proper charge of the adjacent carbons. When a carbon is situated in an intermediary position between two double bonds, the influence resulting from the two double bonds is highly increased. These two factors—positive character as an odd carbon and intermediary position between two double bonds—make C11 of linoleic acid a particularly strong positive carbon which appears to be especially able to bond a negative oxygen. We consider this strongly positive methylenic carbon to be the condition which determines whether a fatty acid has the character of an "essential" fatty acid.

Relationship between the positive charge of the methylenic carbon and the character of essential fatty acid

Fig. 247. Relationship between the positive charge of the methylenic carbon and the character of essential fatty acid. The similarity which exists between linoleic and linolenic fatty acids as essential fatty acids, can be explained by the fact that both have only one positive methylenic carbon. Arachidonic acid, with two such positively changed carbons, has this character of essential fatty acids markedly increased.

This explains further a peculiar relationship between the three important essential fatty acids. No differences are seen, from the point of view of activity as essential fatty acid, between linoleic and linolenic acid, although the last has 2 intermediary carbons, one at C11 and another at C14+ This can be explained by the fact that the second intermediary carbon, the C14, as an even carbon, has a negative electrical character. From this point of view of strongly positive methylenic carbon, there is no difference between linoleic and linolenic acids. The relationship between the character of essential fatty acid and intermediary positive carbon is further confirmed by the fact that arachidonic acid, with two positive and one negative intermediary carbons, is also a more active essential fatty acid than linoleic and linolenic acids, each of which has only one positive intermediary carbon. (Fig. 247)