THE CHEMICAL CONSTITUTION OF THE 

 PROTEINS. 



PART II. 



INTRODUCTION. 

 Earlier Ideas. 



A CONCEPTION of the composition of a complex substance must pre- 

 cede its synthesis. Mulder's supposition that albumin, fibrin, caseino- 

 gen and other similar substances were combinations of a hypothetical 

 radicle, which he termed protein, with sulphur and phosphorus in 

 various proportions, was overthrown by the work of Liebig and his 

 pupils. 



Liebig was the first to recognise that the composition of complex 

 compounds could not be ascertained by elementary analysis but only 

 by the analysis of their decomposition products. He was also the first 

 to regard proteins as combinations of amino acids. At this time only 

 three amino acids, glycine, leucine and tyrosine, were known to Liebig 

 as decomposition products of proteins; before his death in 1873 the 

 number of units known had risen to eight, and since that time the 

 number has increased to seventeen. 



Condensation of Amino Acids by Heating. Anhydrides. 



Before a proper comprehension of the mode of union of the amino 

 acids in proteins was possible, condensation products obtained by 

 heating, or by distilling, amino acids had already been described : 

 leucinimide from leucine, lactimide from alanine and anhydrides of 

 aspartic acid, phenylglycine, phenylalanine and sarcosine ; but only in 

 the case of the anhydride of sarcosine was the constitution determined. 

 Condensation products were also prepared by heating together amino 

 acids and urea. Several of these products resembled proteins in their 

 properties : they were amorphous, easily soluble in water and gave the 



PT. II. I 



