Cxxv. Note on the Spontaneous Decomposition of Lecithin

THE most recent work on the decomposition of lecithin is due to Fischgold and Chain [1934], who have obtained evidence that lecithin, even in the absence of air, undergoes a spontaneous change. This change was found to be due to separation of free fatty acid from the lecithin molecule. C. W. Price and Lewis [1933, 1] noted a change in the electrophoretic properties of lecithin as it became aged even though the material was kept in the dark and in an atmosphere of dry nitrogen. In the PH region in which the lecithin dispersion was negatively charged, it was found that there was a rapid increase in electrophoretic mobility with the age of the lecithin used for the dispersion. After a period of about 3 weeks the lecithin reached some state of equilibrium, its electrophoretic properties remaining constant (at a given PH) for at least 5 months. Fig. 1 shows the effect of age on the mobility of lecithin at PH 5-02 and Fig. 2 compares the mobility-pH curve of lecithin in its stable condition (A), with that of lecithin aged for 8 days only (B). Owing to the continuous change in the behaviour of the substance, the latter curve is approximate only. However, the points were plotted in such an order that, as nearly as possible, the curve is representative of lecithin aged for the above period. The lecithin was prepared from egg-yolk by the method of Levene and Rolf [1927] and the measurements were carried out by the macro-electrophoretic method described by C. W. Price and Lewis [1933, 1]. From the curves it is evident that there is a very large change in the electrophoretic properties of lecithin with age, including a shift in the isoelectric point towards the acid region. The isoelectric point of lecithin in the "aged" or equilibrium condition has been found by C. W. Price and Lewis to be at PH 287. This corresponds fairly closely to the values obtained by other methods employed by workers in this field. Thus Fugii [1924] observed the isoelectric point of lecithin to be at PH 2-7, H. I. Price and Lewis [1929] at PH 2-6 and Remezov [1930] at PH 20-2-8. It might be concluded therefore, that the material used by these authors was similar to the "aged" material used by us. Recently Chain and Kemp [1934] have reported the isoelectric point of freshly prepared lecithin as PH 6-7; a value more nearly approximating to that calculated, namely pE7H51.