1 62 SCIENCE PROGRESS 



the proteid is broken down into the stable and, comparatively, 

 very small molecular groups, which, in the original complex, are 

 associated together by atomic linkages labile to hydrolysis ; 

 when, that is, the proteid is fully digested by a suitable 

 succession of enzymes, or by boiling with water in the presence 

 of strong acids, a crowd of substances, chemically characterised 

 as amino-acids, arises. Of these derivatives some twenty have 

 already been recognised as arising from any one proteid, and we 

 have good evidence for the belief that the tale of them is by no 

 means complete. - 



It is not purposed, nor is it necessary, to enter at all fully into 

 chemical details here ; but it is well to illustrate, by indicating 

 the constitution of some of the typical amino-acids, how hetero- 

 geneous is the molecule from which they are derived. 



The simplest is glycocoll, or amino-acetic acid, CH 2 NH 2 COOH. 

 The aliphatic compound which, from typical proteids, arises 

 in greatest quantity is leucine, an amino-caproic acid, 



ru 



^ 3 >CH.CH 2 .CHNH 2 .COOH. A dibasic acid of the aliphatic 



type is glutaminic acid, COOH.CH 2 .CH 2 .CHNH 2 .COOH. 

 Hydroxy-acids of the type of serine, CH 2 .OH.CH.NH 2 .COOH, 

 also appear, and diamino-acids in which the NH2 (amino) 

 group occurs twice. Of aromatic substances (containing the ring 

 structure in their molecule) we may instance the long familiar 



tyrosine, HO/ ^CH 2 .CHNH 2 .COOH, and, as one containing 



its nitrogen in the ring, prolin, or pyrrolidine carboxylic acid, 



CH 2 — CHCOOH 



I >NH 

 CH 2 — CH 2 



Lastly, we have in tryptophane a double ring compound the 

 structure of which is, in all probability, 







-C — CH 2 = CHNH 2 . COOH 

 -NH 



These substances and their numerous congeners are con- 

 densed together to form the complex proteid molecule. 1 Space 



1 Those unaccustomed to the structural formulas of organic chemistry need 

 do no more than recognise the heterogeneity which these substances exhibit. 

 Other proteid derivatives are alanine, phenyl alanine, aspartic acid, cystine (which 

 contains the sulphur of the proteid), arginine, lysine, pistidine, glucosamine, etc. 



