SYNTHESIS OF ENANTIOMERIC a-PHOSPHATIDIC ACIDS

Chibnall and Channon, investigating the cytoplasma of cabbage and spinach leaves (I), isolated a mixture of substances which resembled monophosphatides in their structure and composition, but mere virtually free of nitrogenous bases. The main constituents were identified as the calcium, magnesium, and potassium salts of phosphoric acid esters of diglycerides. These compounds, constituting a new class of phosphatides, were named phosphatidic acids. Their fatty acid content differed from that of the lecithins and cephalins in that the greater part of the acids consisted of linolic and linolenic acids, whereas palmitic acid and stearic acid were only minor constituents. More recently phosphatidic acids have been found also in wheat germ (2) and in tubercle bacilli (3-6). Doubts have been expressed as to the origin of the phosphatidic acids in the cytoplasm. Whereas Channon and Chibnall were inclined to believe that these substances are intermediate compounds in the metabolism of lecithins and cephalins, Levene and Komatsu expressed the view that they are formed after death of the tissue. The recent discovery by Hanahan and Chaikoff (7) of a phospholipide-splitting enzyme in fresh cabbage leaves, which is capable of attacking the nitrogenous base-phosphoric ester linkage, however, seems to add considerable weight to the contention of Channon and Chibnall. As has been discussed more fully elsewhere (8-11) an asymmetrically substituted triglyceride can be assigned to either one of t.he two optical series. Hence in the specific case of a-phosphatidic acids any particular member can be considered as a derivative of either its diglyceride or of its glycerophosphoric acid (GPA) moiety and thus can be assigned to either the D or L series respectively. Since in every a-phosphatidic acid the glycerophosphoric acid is the same and its stereochemical relationship to glyceraldehyde has already been established (12), it is proposed as the stercochemical reference compound. Thus arbitrarily, but in conformity with the usage in the a-lecithin (8-10) and oc-cephalin (13) series, an cr-phosphatidic acid is assigned the L configuration if it contains L-a(-GPA and vice versa. The synthesis of the racemic forms of fully saturated oc-phosphatidic acids has been reported by a number of investigators (14-21). In all