Cclxix. the Isoelectric Points of Lecithin and Sphingomyelin

THE latest work on the isoelectric point of lecithin is that of Price and Lewis [1933] investigating the macrocataphoresis oflecithin dispersions. As in previous work it was noted that the electrical properties of lecithin show a gradual change with the time. Thus the (presumably negative) mobility of a freshly prepared dispersion "increased, at given PHX with the age of the lecithin used, reaching a stable value after 3 weeks," when the isoelectric point was found to be at PH 2-7. Price and Lewis attempted to obtain the theoretical value of the isoelectric point from a consideration of the dissociation constants of glycerophosphoric acid and choline. The value of PKa was estimated as 1-4 by analogy with the dissociation constants for glycerophosphoric acid and for phosphoric acid. The value of kb was determined from the degree of hydrolysis of choline chloride: it was obtained by measuring the PH of solutions of the salt, and the resultant value was PKb 506, giving a theoretical isoelectric point ofPH 5-17. Theoretical considerations and the direct measurements described by Fischgold and Chain [1934, 1] make it certain that the dissociation constant of choline is considerably greater than the value obtained by Price and Lewis; the latter is open to the serious objection that smali amounts of free hydrochloric acid in the neutral salt may shift the PH of the salt solution towards the acid side, thus giving a low value for the dissociation constant of the base. This point is further discussed by Fischgold and Chain [1934, 1]. Price and Lewis attribute the discrepancy between the calculated and observed isoelectric points'to the adsorption of hydroxyl ions on the fatty acid portions of the lecithin. This hypothesis however is, in our opinion, unnecessary. Thus it has been demonstrated by Fischgold and Chain [1934, 1] that freshly prepared lecithin rapidly becomes acid on standing, owing to the separation of free fatty acids, and it is therefore virtually impossible to work with a sample of lecithin which is entirely free from acid. The results of previous observers are likely to be in error on this account. Only by using freshly prepared lecithin may some idea be formed of its true isoelectric point.