In a work hypothesis of the biological breakdown of a long chain molecule for caloric metabolism, we consider two factors as being of capital importance: 1) that the result be an even carbon number chain molecule, since the Knoop oxidation leads to complete caloric utilization only for such molecules, and 2) that the length of the chain of carbons bound to the carboxyl in the new molecule be not higher than 11 carbons in order to permit direct beta oxidation. To fulfill the first condition, the breakdown process does not occur between the carbons of the double bonds themselves, since in natural fatty acids the double bonds separate portions of the chains, usually with odd number of carbons. Such a breakdown would lead to molecules resulting in further incomplete Knoop oxidation.

According to the hypothesis the breakdown of the molecule occurs at the carbon nearest the double bond. Through the energetic influence exerted by the double bond, the even numbered carbon nearby appears strongly positive. As a first step, this carbon was seen to fix a molecule of oxygen resulting in a hydroperoxide as shown by Farmer and co workers first for rubber (30) and later for fats in vitro. (31) It is with the passage, in a second step, of this hydroperoxide group into a carboxyl that the breakdown of the molecule occurs at this level, as shown in Figure 242.

The oxidative breaking down of a fatty acid molecule

Fig. 242. The oxidative breaking down of a fatty acid molecule (a) occurs in vivo taking place through the appearance of a hydroperoxide at the carbon adjacent to the double bond (b). It leads ultimately to a carboxyl formation (c) at this adjacent carbon and results thus in chains with even number of carbons.

If the even numbered carbon near the double bond is toward the terminal methyl group, a monocarboxylic acid will result. A similar process taking place at the other carbon adjacent to the double bond toward the carboxyl will lead to a dicarboxylic molecule. The metabolic changes in vitro—and also in vivo—have shown the appearance of these two groups of even carbon mono and dibasic fatty acids. By binding two molecules of water the remaining 2-carbon chain linked by the double bond would result in acetic acid molecule. Such changes occurring in the caloric mono ethenoids permitted us to explain one of the baffling peculiarities seen in the constitution of the monoethenic fatty acids.

In Note 5, we discuss the position of the double bond in the principal naturally occurring monoethenoids as it follows a characteristic pattern. In molecules with 16 or less carbons the double bond is more often placed so as to separate a group with 9 carbons toward the carboxyl end, while in molecules of 18 carbons or more, the double bond separates almost constantly a group of 9 carbons toward the methyl end. Figure 243 shows two characteristic examples.

Fig. 243. Characteristic emplacements are seen for the double bond in two monoethenoids. The double bond separates two groups with odd number of carbons. For the myristoleic acid, it separates a group of 9 carbons toward the carboxylic end, and a group with a short 5 carbon chain toward the methyl end. For hexacosenoic acid (26C) a chain with 17 carbons is separated toward the carboxyl end and one of 9 carbons toward the methyl end.

Characteristic emplacements are seen for the double bond in two monoethenoids

The breakdown of the molecule according to the process mentioned above explains the peculiarity. For the chain with 18 or less carbons, biological fission would result in a diacid with 8 carbons and another mono acid with 8 or less carbons, both subject to Knoop oxidation. In the long chain fatty acid the double bond separating a 9 carbon fraction toward the methyl end of the molecule will result in an 8-carbon chain as monoacid having the methyl group at the other end. The other part of the molecule, with more than 8 carbons and which corresponds to the long fraction having the carboxyl at its end, results in a diacid. (Fig. 244) This also will be subject to Knoop oxidation, even with a long chain. In the diacid, this process, which is related to the intervention of the carboxyl, can take place at both ends where carboxyls are present.

Through the emplacement of the double bond in the molecules of the monoethenoids

Fig. 244. Through the emplacement of the double bond in the molecules of the monoethenoids, the breaking down of the molecules of fatty acids at the carbons adjacent to the double bond leads to the appearance of a dicarboxylic acid for the part of the chain having more than 10 carbons.