CH. XXVIII.] AMINO- ACIDS 421 



Cystine is a complex diamino-acid in which sulphur is present, 

 and in which the greater part of the sulphur of the protein molecule 

 is contained. 



In addition to all these numerous amino-acids there are other 

 cleavage products, of which it will be sufficient to mention proline 

 and ammonia. In the case of the nucleo-proteins the nuclein 

 component yields in addition what are known as purine and 

 pyrimidine bases. (See further under nucleic acid, p. 431, also 

 under uric acid, Chapter XXXIX.) 



Proline and the purine and pyrimidine bases are all derivatives 

 of rings which remind one of the benzene ring, except that nitrogen 

 is included in the ring formation ; such rings are termed heterocyclic. 

 Thus proline (pyrrolidine-carboxylic acid) is a derivative of the 

 pyrrol ring, and its formula is given below. Cytosine is one of the 

 derivatives of the pyrimidine ring, and hypoxanthine (or oxypurine) 

 is given below as an example of the purine bases. 



H 9 O-CH 2 HN C . NH 2 



"II II 



H 9 C CH.COOH OC CH 



\/ I II 



N HN CH N- 



[Cytosine or amino- [Hypoxanthine.] 



oxypyrimidine.] 



[Proline.] 



Our list now represents the principal groups of chemical nuclei 

 united together in the protein molecule, and its length makes one 

 realise the complicated nature of that molecule and the difficulties 

 which beset its investigation. We may put the problem another 

 way. In the simple sugars, with six atoms of carbon, there are as 

 many as thirty-six different ways in which the atomic groups may 

 be linked up ; the formulas on p. 408 give only three of these which 

 represent the structure of dextrose, Isevulose, and galactose ; but the 

 majority of the remainder have also been prepared by chemists. The 

 molecule of albumin has at least 700 carbon atoms, so the possible 

 combinations and permutations must be reckoned by thousands. 



The workers in Fischer's laboratory are steadily working through 

 the various known proteins, taking them to pieces and identifying 

 and estimating the fragments. I do not intend to burden the 

 readers of this book with anything more than a sample of their 

 results, and will, therefore, only give in a brief table (see page 

 422) the results obtained with some of the cleavage products of a 

 few proteins. The numbers given are percentages. 



Such numbers, of course, are not to be committed to memory, but 

 they are sufficient to convey to the reader the differences between 

 the proteins. There are several blanks left, on account of no accurate 



