Figure 13.—Survey of alcoholic fermentation, 1951. The “well-known scheme of alcoholic fermentation” according to Albert Jan Kluyver (1888-1956), presented before the Society of Chemical Industry in the Royal Institution, March 7, 1951. In Chemistry & Industry, 1952, page 136 ff., Kluyver restates that “... the fermentation of one molecule of glucose is indissolubly connected with the formation of two molecules of adenosine triphosphate (ATP) out of two molecules of adenosine diphosphate (ADP).”
Shortly after this experience had been gained, it became valuable for understanding the chemical nature of a new substance extracted from a natural organ. This substance was named lecithin by its discoverer, Nicolas Théodore Gobley[27] (1811-1876), because he obtained it from egg yolk (in Greek, lékidos). He used ether and alcohol for this extraction. Had he used water and mineral acid instead, he would not have found lecithin, but only its components. As Gobley and, slightly later, Oscar Liebreich (1839-1908), subjected lecithin to treatment with boiling water and acid, they separated it into three parts. One of them was the glycerophosphoric acid of Pelouze, the second was the well-known stearic acid of Chevreul, but the third was somewhat mysterious. This third substance was the same as one previously noticed when nerves had been subjected to an extraction by boiling water and acid and, therefore, called nerve-substance or neurine. Adolf Friedrich Strecker (1822-1871) established the identity of this neurine with a product he had extracted from bile and which went under the name of choline. Adolphe Wurtz (1817-1884) succeeded in synthesizing this substance from ethylene oxide, CH2.O.CH2 and trimethylamine N(CH3)3.[28] Thus, all three parts were identified, and Strecker put them together to construct a chemical formula for lecithin, glycerophosphoric acid combined with a fatty acid and with choline (a hydrate of neurine).
| { | OH | } | ||
| N | (CH3)3 | Choline | ||
| C2H4O | ||||
| C18H33O2 | } | HO | } | ||
| PO | |||||
| C16H31O2 | C3H5O | ||||
| Fatty Acids | Glycerophosphate | ||||
| '—————v————' | |||||
| Lecithin | |||||
| according to Strecker | |||||
This formula was not quite correct. Richard Willstätter showed that an internal neutralization takes place between the amino group and the free acidic residue. This is expressed in his lecithin formula of 1918.
Lecithin (1918)
When the aim was to distill elementary phosphorus out of an organic material, it did not matter whether this was fresh or putrified. For obtaining lecithin out of egg yolk and similar materials, it was essential to use it in fresh condition. Otherwise, enzymes would have decomposed it. Through more recent work, four enzymes have been separated, which act specifically in decomposing lecithin. Enzyme A removes one fatty acid and leaves a complex residue, called lysolecithin, intact. Enzyme B attacks this residue and splits off the remaining fatty acid group from it, enzyme C liberates only the choline from lecithin, and enzyme D opens lecithin at the ester bond between glycerol and phosphoric acid. This is shown in the following diagram.
| Enzymatic Splitting of Lecithins | ||
|---|---|---|
| Enzyme | Substrate | Products |
| A | Lecithin | Lysolecithin and fatty acids. |
| B | Lysolecithin | Glycero-phospho-choline and fatty acids. |
| C | Lecithin | Phosphatidic acid and choline. |
| D | Lecithin | Phosphoryl choline and diglyceride. |
Several fatty acids can be present in lecithin from various sources: palmitic and oleic acid, besides the stearic acid which at first had been thought the only one involved. In another group of extracts from brain or nerve tissue, amino-ethanol H2NCH2CH2OH is found instead of the choline of lecithin. The variations include the alcohol, to which the fatty acids and choline phosphate are attached, for example, glycerol can be replaced by the so-called meat-sugar, inositol, which has six hydroxyl groups in its hexagon-shaped molecule C6H6(OH)6.