Discussion 173 



dealing with what we interpret to be de novo synthesis. I think one 

 could even say that in dealing with bacterial systems, where one does 

 not add a complete complement of amino acids, there may be enough 

 residual amino acids to provide a complete supply of amino acids for a 

 short time, whereas over a longer period of time they would eventually 

 be exhausted. There is a certain amount of proteolysis going on also. 

 One may define "exchange" as a simultaneous opening of two peptide 

 bonds in the interior of a long peptide chain with a substitution of another 

 amino acid. I wonder whether that does in fact happen. 



Work: I feel fairly convinced on purely chemical grounds that it does 

 not. Once the peptide bond is formed it is a completely different order 

 of stability and it is extremely unlikely that it would open again. 



