On Determining the Chemical Structure of Proteins 271 



moieties that may be present, such as carbohydrates, nucleic acids, or 

 porphyrins. On the basis of this definition, determining the chemical 

 structure of a protein can be broken down into the following sequence 

 of operations: (a) Purification and isolation of a sample of a protein 

 sufficiently pure to warrant detailed structural work. (6) Quantitative 

 amino acid analysis of the purified protein, (c) End-group analysis 

 to determine the number of peptide chains in the molecule, (d) Rup- 

 ture of the disulfide bridges or other cross linkages in the molecule, 

 (e) Separation of the peptide chains from one another, if more than 

 one is present. (/) Partial hydrolysis of a peptide chain, (g) Frac- 

 tionation of the mixture of peptide fragments formed on partial hy- 

 drolysis, (h) Determination of the structure of the isolated fragments. 



It will be apparent that formulation of the problem in this way 

 implicitly assumes that proteins have a definite and determinable 

 chemical structure." It assumes that when a given bovine pancreas 

 synthesizes ribonuclease, for example, the amino acid residues in the 

 chains of the various protein molecules will be laid down in the same 

 order from one molecule to the next. It does not preclude the possi- 

 bility that more than one kind of molecule possessing ribonuclease 

 activity may be synthesized by the pancreas, but it does assume that 

 the number of such kinds of molecules will be small and that the 

 individual species of proteins from a single organism are not popula- 

 tions of molecules the organic chemical structure of which varies more 

 or less continuously around a mean. 



In short, this conception of the proteins ascribes to them the attri- 

 butes of well-defined chemical compounds similar in fundamental char- 

 acter to, though more complicated in structure than, the other types 

 of substances synthesized by living organisms. There is considerable 

 evidence in favor of this conception, the most compelling of which is 

 the fact that Sanger has been able to establish an unambiguous chemi- 

 cal structure for insulin. Were beef insulin to consist of a family of 

 closely related molecular species, it is difficult to see how a unique 

 structure could have been derived. Recent results with ribonuclease 

 point in the same direction. This conception also has the pragmatic 

 advantage that it encourages investigators to attack the problem of 

 protein structure from the point of view of the organic chemist. 

 Organic chemistry has no techniques for handling complex structural 

 problems involving heterogeneous populations of molecules. No matter 



* For a summary, with references, of the contrary point of view, cf. Colvin, 

 Smith, and Cook.- 



