12 THE AMINO ACIDS 



protein investigated by this method was found to 

 yield relatively large molecules, such as proteoses and 

 peptones, and on further disintegration a series of 

 comparatively simple nitrogenous substances of low 

 molecular weight which belong to a definite group of 

 chemical compounds namely, the amino acids. An 

 amino acid may be regarded as an organic acid in 

 which one hydrogen is replaced by the amino group 

 (NH2), or viewed from another standpoint, an 

 amino acid may be considered as a substituted am- 

 monia, one hydrogen of ammonia, NHg, being 

 replaced by an organic acid- A description of the 

 amino acids yielded by proteins follows. 



Glycocoll or glycine, amino -acetic acid. CH2- < mow 

 is the simplest of the products obtained from pro- 

 tein by hydrolytic cleavage and it was also the first to be 

 discovered. Its separation dates back to 1820 in which 

 year Braconnot obtained the substance by boiling gelatin with 

 sulphuric acid, and because of its sweet taste called it sugar 

 of gelatin. About twenty-five years later Dessaignes isolated 

 it after a hydrolysis of hippuric acid. It was shown by 

 Strecher in 1848 that glycocholic acid, then called cholic acid, 

 consists of a combination of cholalic acid and glycocoll, and 

 in consequence of its being a constituent of a bile acid, glyco- 

 coll assumed a position of some physiological importance. Its 

 presence in various types of albuminoids, such as elastin, etc., 

 was later demonstrated and finally it was shown to be a 

 decomposition product of globulin. Glycocoll is not present 

 in all proteins for albumin, casein, and hemoglobin fail to 

 yield it, and from the vegetable proteins it is obtained in 

 small quantities only. On the other hand, albuminoids are 

 particularly rich in glycocoll. In an extract of the mollusc 



