150 COMPOUNDS OF THE PROTEINS 



lyzed protein molecules is very small indeed. Thus Van Slyke and 

 Birchard have obtained the following results: 



PERCENTAGE OF TOTAL NITROGEN PRESENT IN FREE AMINO 



GROUPS. 



Hemoglobin 6.0 



Casein 5.5 



Hemocyanin 4.3 



Gelatin 3.1 



Edestin 1.8 



Gliadin . 1.1 



Zein 0.0 



Heteroalbumose 8.1 



Protoalbumose 9.9 



On the other hand, Edestin, after complete hydrolysis by hydrochloric 

 acid, yields a volume of free nitrogen, on treatment with nitrous acid, 

 corresponding to no less than seventy-nine per cent, of its total nitrogen 

 content, and the formol-titration is proportionately increased. A very 

 small proportion of the amino-groups of the amino-acids from which 

 these* proteins are built up are, therefore, present as unneutralized 

 amino-groups in the unhydrolyzed protein. 



It has, in fact, been shown that the free amino-nitrogen in the 

 unaltered protein molecule exactly corresponds in quantity to one-half 

 the Lysine nitrogen. Hence Zein, which contains no lysine radicals, 

 yields no free nitrogen on treatment with nitrous acid. The period 

 required for the complete interaction of proteins with nitrous acid is 

 about thirty minutes (with the technique employed by Van Slyke) which 

 is ten times as long as the period required for complete interaction with 

 a-ami no-groups, but corresponds exactly with that found to be required 

 for the complete interaction of nitrous acid with the co-ammo-group 

 of lysine. From these facts Van Slyke and Birchard infer that lysine 

 is united to the adjacent amino-acid radicals in the protein molecule 

 by means of its carboxyl- and its a-amino-groups, while the w-amino- 

 group remains uncombined and represents, within at most, a fraction 

 of a per cent, of the total protein nitrogen, the entire amount of free 

 amino-nitrogen in the native proteins. The a-amino-groups, which 

 constitute by far the greater part of the free amino-nitrogen formed 

 after complete hydrolysis are, in the intact protein molecule, all con- 

 densed into peptide-linkages. 



In the primary Proteoses, or first split-products of protein hydrolysis, 

 the relations are different. The free amino-groups in hetero- or proto- 

 albumose exceed one-half the content of lysine nitrogen by 3.0 and 4.8 

 per cent, of the total nitrogen respectively, indicating that an appreci- 

 able proportion of the a-amino-groups are uncovered even in the first 

 steps of hydrolysis. 



Notwithstanding the fact, however, that a great majority of the 

 linkages uniting the different amino-acids in the protein molecule have 

 thus been proved to be of the peptide character, the possibility is by 



