CONSTITUTION OP CASEIN. 53 



and the percentage of nitrogen divided by its atomic weight 

 will give the number of atoms of nitrogen in the molecule to each 

 atom of sulphur and phosphorus, thus : 



15-65 _ 0-77 + 0-85 _ q 



~lT~ 32 + 31 



On p. 47 the proximate composition of casein is given, and it 

 is seen that there is 3-87 per cent, of di-amino-nitrogen, almost 

 exactly 25 per cent, of the total nitrogen, which renders it pro- 

 bable that the figure 43-5 may be made into 44. Of the di-amino 

 acids that which has been determined with the greatest accuracy 

 is histidine, all observers agreeing in finding 2-5 to 2-6 per cent., 

 which is equivalent to 0-70 per cent, of nitrogen ; the ratio of 

 this to the total nitrogen will give the number of atoms of nitrogen 

 to each atom of nitrogen in histidine in casein, and this multi- 

 plied by 3 (as histidine contains 3 atoms of nitrogen in the mole- 

 cule) will give the atoms of nitrogen for each molecule of histidine. 

 1 F\ f * r*' \/ o - - ' 



' - =67 (= 44 X 1-5 nearly), a number which would 



lead to 1-5 atoms each of phosphorus and sulphur in the molecule ; 

 there must, therefore, be 132 or 134 atoms of nitrogen in the 

 molecule of casein, and 3 each of sulphur and phosphorus, and 

 the molecular weight must be about 11,800 or 12,000. The 

 histidine will amount to 2 molecules ; the ammonia nitrogen 

 is found to be 1-63 per cent., which corresponds to 14 atoms 

 of nitrogen if the total is taken as 134, and the amido-nitrogen 

 (0'93 per cent.) is equal to 8 atoms. There are two other 

 constituents upon which all observers agree tyrosine 4-5 per 

 cent, and lysine 5-95 per cent. 



4-5 X 12,000 nQ 

 181X100 =2-98 or 3 molecules, 



and 



When casein is titrated with alkalies to phenol-phthalein,* 

 it behaves as an acid. Laqueur and Sockur find that the equi- 

 valent is 1,135, Matthaipoulous gives it as 1,131 -5, Pfyl and Turnan 

 as 1,143, and Long as 1,124 in cow's milk and 1,190 in goat's 

 milk, figures which indicate that the molecule containing about 

 134 atoms of nitrogen is 10 or 11 basic. Lehmann found that in 

 "genuine" casein 1-45 to 1-75 parts of lime were combined 

 with 100 parts of casein, Soldner has shown that two lime com- 

 pounds exist containing 1*55 and 2-39 per cent, respectively of 



* Titration of casein with phenol-phthalein does not give an exact measure 

 of acidity, as the results are influenced by temperature and amount of 

 phenol-phthalein, and the equivalents are approximate. 



