The isoelectric points of lecithin and sphingomyelin

Abstract

The isoelectric points of carefully purified fresh lecithin, detd. by mierocataph-oresis, was found at pH 6.7 [plus or minus] 0.2; that of sphingomyelin, at pH > 6. These results are in fair agreement with the theoretical value of pH 7.5, calculated from the dissociation constants of the 2 polar groups in the molecule. Freshly prepared lecithin rapidly becomes acid owing to a spontaneous splitting off of fatty acids. It is therefore suggested that the acid isoelectric points of lecithin observed previously by other authors, are due to the use of impure lecithin. The isoelectric point of sphingomyelin, after several purifications, approaches the theoretical value but does not reach it, as it was impossible to remove the last traces of an alkali-titratable impurity by the purification method used.