418 V. CHEMISTRY OF PHOSPHATIDES AND CEREBROSIDES 



which likewise contained stearic acid as the sole fatty acid. Hydro- 

 lecithin was found to retain its optical activity; the values for [a]^^ in 

 chloroform varied between +5.5° and +6.0°, depending upon the con- 

 centration. 



Table 3 

 Iodine Numbers of Various Lecithin Preparations 



Type of lecithin Iodine No. 



Theoretical Calculations 



Palmityl-oleyl- 32.7 



Palmityl-arachidonyl- 127 . 



Stearyl-oleyl- 31.5 



Stearyl-arachidonyl- 122.6 



Actual Determinations 



Egg-yolk- 47" 



70' 



Brain- 61'' 



Liver- 72.7" 



76" 



90' 



« P. A. Levene and I. P. Rolf, /. Biol. Chem., 67, 659-666 (1926). 

 <• P. A. Levene and L P. Rolf, /. Biol. Chem., 72, 587-590 (1927). 

 " P. A. Levene and T. Ingvaldsen, /. Biol. Chem., 43, 359-378 (1920). 



Lecithin probably exists in the form of a zwitterion which would pre- 

 sumably react with a hydrogen ion as follows : 



H,C-0-CR H,C-0-CR 



I (\ , + ^ 10^/ 



HC-O-CR + H .. HC-O-CR 



I /O- +, . I/O +. . 



In support of this hypothesis, both Jukes^^ and Fischgold and Chain^* 

 have showTi that lecithin cannot combine with alkali, in contrast to ceph- 

 alin, which is able to do so. Cephalin can be titrated with phenolphthal- 

 ein under certain conditions as a monobasic acid, while lecithin behaves as 

 a neutral substance under such conditions. ^'^'^^•^'■^° This is obviously 

 related to the strong basicity of the choline residue in contrast to that of 

 ethanolamine, which causes lecithin to act as an internally neutralized 



" T. H. Jukes, /. Biol. Chem., 107, 783-787 (1934). 



*8 H. Fischgold and E. Chain, Proc. Roy. Soc. London, B117, 239-257 (1935). 



8^ A. Griin and R. Limpacher, Ber., 60, 151-156 (1927). 



9» H. Rudy and I. H. Page, Z. physiol. Chem., 193, 251-268 (1930). 



