On Determining the Chemical Structure of Proteins 289 



amino acid (either lysine or arginine) by two amino acid residues and 

 one arginine is separated by three. Of the ten lysine residues, seven 

 are separated from another basic amino acid by six amino acid residues 

 or less. In another part of the molecule we find three alanine residues 

 in a row, and in another peptide seven out of twenty-one amino acid 

 residues are serine. Recently, Davie and Neurath " have isolated the 

 hexapeptide that is split off when trypsinogen is transformed to trypsin 

 and proved it to have four aspartic acid residues joined together in the 

 structure Val.Asp4.Lys. If, in fact, this apparent tendency of amino 

 acids of like structure to group together turns out to be a general 

 occurrence, it means that in the peptide chain there will be found areas 

 in which the positive charge density is high, areas in which the negative 

 charge density is high, and areas in which van der Waals forces pre- 

 dominate. There is suggestive evidence based on studies of enzyme 

 specificity and of the manner in which protein molecules interact with 

 one another that is compatible with this idea. How these areas are 

 disposed on the surface of the protein molecule will, of course, be a 

 function of the manner in which the peptide chain is folded. At this 

 stage of development, speculation is tempting, but the real need is 

 many more facts. It appears that at last the protein chemist is about 

 to possess tools adequate to the difficult task of accumulating these 

 facts. 



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