On Determining the Chemical Structure of Proteins 283 



gave whole numbers for each of the amino acids in the peptides with 

 an accuracy of ±5 to 10%. The numbers in parenthesis give the yield 

 of each peptide obtained after 3 and 20 hours of hydrolysis, on the 

 assumption that each one of these fragments occurs only once in the 

 molecule.* On the basis of the specificity of trypsin, the basic amino 

 acid, lysine or arginine, is assigned to the C-terminal position. Eventu- 

 ally it would be well to check this point experimentally, but for the 

 present this assignment seems reasonable. It will be noted that peptide 

 10 contains two lysine residues. Inasmuch as DNP end-group analyses 

 placed one lysine at the amino-terminal position, it seems extremely 

 probable that this peptide represents the amino-terminal sequence. 

 The first four amino acids of this sequence were determined by Anfinsen 

 et al. to be Lys.Glu.Thr.Ala-, so that the finding of two more alanine 

 residues and a lysine makes it possible to extend this sequence to seven 

 amino acids without further structural work. 



The mixture of peptides labeled 2, 3, 4, occurring at the beginning 

 of the chromatogram, and those at 9 and at 13 + 14 were rechromato- 

 graphed on Dowex 50-X2 under slightly different conditions to give 

 the results shown in Fig. 4. Three peptides were separated from the 

 front part of the original chromatogram (a 20-hour tryptic hydrol- 

 yzate), two containing lysine and one arginine. Both lysine peptides 

 had the same 21 amino acid residues and probably differed only in 

 their amide content. The fact that the faster moving of the two lysine 

 peptides, the center peak in the top curve of Fig. 4, was present to 

 only a very small extent after 3 hours of tryptic digestion is compatible 

 with this supposition. The arginine peptide is also very large, con- 

 taining nineteen amino acid residues. It is unstable, however, the 

 yield decreasing from 55% in the 3-hour hydrolyzate to 15% in the 

 20-hour hydrolyzate. The reason for this instability is not certain at 

 the moment, but possibly this fragment contains some linkage that is 

 extremely susceptible to the action of chymotrypsin, a trace of which 

 cannot be ruled out as a contaminant of the trypsin. 



Peptide 9 is very large and contains two lysine residues. The pres- 

 ence of proline may explain why one of the lysyl bonds is not split 

 by trypsin to yield two peptides. If the sequence -Lys.Pro- existed, 

 the lysylprolyl bond might be very resistant to tryptic hydrolysis. In 



* The figures with rules under them were computed from the actual amino acid 

 analyses of the peptides. The other yields for different times of hydrolysis were 

 computed by comparing the area of the peak with the area of the corresponding 

 peak for which the amino acid composition was known. 



