5] STUDIES ON PROTEIN MICROSTRUCTURE 79 



histidine. For practical reasons we initially confined ourselves to the analysis 

 of partial acid hydrolysates in which about 50 per cent, of the peptide bonds 

 had been preserved intact and which consisted of relatively low molecular 

 weight peptides. The isolation of the cysteic acid peptides was ai first car- 

 ried out by means of ion exchange on Amberlite IRA-4B, that of the arginine 

 peptides on Amberlite IRA-400, particularly in those cases where only a 

 statistical analysis of the overall composition of the appropriate peptide 

 fractions was carried out. Later, when we undertook the isolation and 

 identification of the individual peptides contained in each fraction, we 

 developed a fractionation procedure whose essentials are set out schematic- 

 ally in Table 1. 



As an example of the statistical procedure in comparing the microstructure 

 of proteins, some of our results on the serum albumins of a number of 

 species — man, ox, horse, sheep, and duck — may be given. ^ As is shown in 

 Table 2, the amino acid composition of all five proteins is very similar. The 



Table 2 

 AMINO ACID COMPOSITION OF SOME SERUM ALBUMINS 



The figures denote grams of amino acid per 100 g. of protein; they are average values 

 from 8 independent determinations (6 in the case of glutamic acid, threonine, methionine, 

 vahne, and phenylalanine). The probable error is indicated after each value. The values 

 for methionine, tyrosine and tryptophan recorded in the last three lines were obtained 

 by specific colorimetric methods. 



