344 Mr. R. H. A. Plimmer [April 8, 



The methods hardly give exactness as far as the decimal figure, and 

 it would have been better if the data had been returned to the 

 nearest whole number. Many workers still give their data to two 

 places of decimals, so that an entirely wrong impression is given of 

 the accuracy of the method. Fischer pointed out that his method 

 was not quantitative, but others have neglected this important 

 statement. 



The figures for the hexone bases are more accurate ; it is still not 

 sufficient to express results to two decimal places. Kossel considers 

 that the hexone bases form a special nucleus on account of their 

 presence in all proteins. We might value a protein by its content of 

 hexone bases, but it is not sufficient, because their total only tells us 

 about a third or less of the whole molecule. 



Tryptophan, discovered by Hopkins and Cole, is perhaps the most 

 important unit in the protein molecule. It is not estimated except 

 by direct isolation, a method which is laborious and requires con- 

 siderable skill. Its amount is not known except in casein and a few 

 other proteins. By its distinctive colour reaction with glyoxylic and 

 sulphuric acids it can readily be proved to be a constituent of most 

 proteins. 



The amount of cystine in proteins is only known in a few cases, 

 but its amount can be gauged by the sulphur content of the protein. 

 It is the one unit known which contains sulphur, but there are 

 indications that there is another sulphur-containing unit. 



The differences in proteins are not confined to such quantitative 

 data ; they are still more involved. Fischer's synthetical work with 

 the amino acids has proved that the amino acids are combined 

 together in a polypeptide form, i.e. the amino group of one amino 

 acid is combined with the carboxyl group of another, the amino 

 group of this acid being united with the carboxyl group of still 

 another. We therefore consider a protein molecule to be a chain 

 of amino acids, thus — 



HN.CH 2 .CO-NH.CH(CH3).CO-HN.CH(C 4 H 7 ).CO-HN.CH(C 3 H 5 ).CO- 



This method of combination allows theoretically of endless variation. 



If we take three amino acids, we can arrange them in six different 



ways — 



glycyl-alanyl-tyrosine alanyl-tyrosyl-glycine 



glycyl-tyrosyl-alanine tyrosyl-glycyl-alanine 



alanyl-glycyl-tyrosine tyrosyl-alanyl-glycine 



With 18 or 20 amino acids the number of arrangements is almost 

 infinite. 



Differences in arrangement may be the cause of differences in 

 proteins. Two proteins may perhaps have exactly similar amounts 

 of amino acids and yet be different. The interchange of one amino 



