1020 



APPENDIX. 



These split products are all amino-acids, some of them belonging to the 

 fatty acid (aliphatic) series of carbon compovmds, some to the aromatic 

 (carbocychc) series, and some to the heterocychc (pyrrol, indol) series. In 

 what may be considered the simplest proteins occurring in nature — namely, 

 the protamins found in the spermatozoa — only from four to six of these groups 

 occur, while in some of the more familiar proteins, such as serum-albumin 

 or casein, a much larger number is found. This fact is illustrated by the 

 following table, taken from Abderhalden, which shows the composition of 

 several proteins belonging to different classes. It will be noted that except 

 for the salmin the known products sum up to less than 100 per cent., showing 

 that there is a large portion of the molecule as yet unknown. 



Serum Serum Casein. Salmin. 

 Albumin. Globulin. 



Glycin 3.5 



Alanin 2.7 2.2 0.9 



Valin present 1.0 4.3 



Leucin 20.0 18.7 10.5 



Prohn 1.0 2.8 3.1 11.0 



Phenylalanin 3.1 3.8 3.2 



Glutamic acid 7.7 8.5 11.0 



Aspartic acid 3.1 2.5 1.2 



Cystin . 2.3 0.7 .065 



Serin 0.6 .... 0.23 7.8 



Tyrosin 2.1 2.5 4.5 



Tryptophan present present 1.5 



Diaminotrioxydodecoic acid .... 0.75 



Oxyprolin 0.25 



Lysin .... 5.80 



Arginin .... 4.84 87.4 



Histidin 2.59 



The a-amino-acids of which these end-products consist all contain the 

 H 



grouping — C — NH2, and Fischer has shown that such bodies possess the 



coon 



property of combining with one another to make complex molecules containing 

 two, three, or more groups of amino-acids. The combination takes place 

 with the elimination of water formed by the union of the OH of the carboxyl 

 (COOH) group in one acid and the H of the amino (NHj) group in another. 

 Thus, two molecules of amino-acetic acid (glycocoU) may be made to unite to 

 form a compound, glycylglycin, as follows: 



NH2CH2COOH + NHjCH^COOH — HjO = NHaCH^CONHCHjCOOH. 

 GlycocoU. GlycocoU. Glycylglycin. 



Compounds of this kind are designated by Fischer as peptids. When formed 

 from the union of two amino-acids they are known as dipeptids; from three, 

 as tripeptids, etc. The more complicated compounds of this sort, the poly- 

 peptids, begin to show reactions similar to those of the proteins. Some of 

 them give the biuret reaction, some are acted upon and split by proteolytic 

 enzymes. It seems justifiable, therefore, to consider proteins as essentially 

 polypeptid compounds of greater or less complexity — that is, they are acid- 

 amids formed by the union of a number of a-amino-acid compounds. More 

 than a hundred of these artificial polypeptids have been thus synthesized, 

 one of the most complex, an octa-deca peptid, consisting of eighteen mon- 

 amino acids, fifteen molecules of glycin, and three of leucin, with a total molec- 

 ular weight of 1213. This conception of the structure of the protein molecule 

 explains a number of their general characteristics — for instance: (1) The 

 fact that they are all decomposed and yield similar products under the inlluence 

 of proteolytic enzymes or boiling dilute acid. (2) The fact that the proteins 



