SECTION II. 



THE CHEMICAL CONSTITUTION OF ITS UNITS, OR THE DISCOVERY 

 AND SYNTHESES OF THE AMINO ACIDS. 



IN Section I. an account was given of how the units of the protein molecule 

 are now isolated and estimated, and the results were embodied in several 

 tables. In general, it may be said, that the amino acids were first dis- 

 covered in that protein in which they occurred in largest amounts. An 

 account will be given in this section of the discovery and of the deter- 

 mination of the constitution of each amino acid. 



A. MONOAMINO-MONOCARBOXYLIC ACIDS. 

 Glycine. 



This, the simplest of the products of hydrolysis of the proteins, was 

 also the first to be discovered ; it was obtained by Braconnot, in 1820, 

 by boiling gelatin with dilute sulphuric acid, and on account of its sweet 

 taste he called it sugar of gelatin. In 1846 Dessaignes obtained it 

 from hippuric acid by hydrolysis, and, in 1848, Strecker showed that 

 cholic acid (now glycocholic acid) consisted of this amino acid and 

 cholalic acid, so that, as a constituent of substances of animal origin, it 

 became of great importance. Its presence in elastin was demonstrated 

 by Jeanneret, in horn by Horbaczewski, in spongin by Krukenberg, in 

 conchiolin by Wetzel, and in silk-fibroin by Cramer ; Faust and Spiro 

 showed that it was present in globulin. It does not occur in albumin, 

 nor in caseinogen, nor in haemoglobin ; it is present only to a small ex- 

 tent in the vegetable proteins, and for this reason it was not isolated 

 until Abderhalden showed its presence in these proteins by Fischer's 

 ester method. 



In the free state, glycine was found by Chittenden in an extract of 

 the American mussel, Pecten irradians, and of recent years it has been 

 recorded as sometimes occurring in the urine. 



Its elementary composition of C 2 H 5 NO 2 was first correctly deter- 

 mined in 1846 by Mulder and by Laurent, and in this year, after Des- 

 saignes had pointed out the unsuitability of the name given to it of 



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