A study of the partial acid hydrolysis of some proteins, with special reference to the mode of linkage of the basic amino-acids.

where R, R', R" are the side-chains of the different amino-acids, of which only 20 odd molecular species have been isolated so far from the whole range of living organisms. It seems clear, therefore, that the distinctive properties of proteins reside in the arrangement rather than the nature of the amino-acid residues. Bergmann & Niemann [1936; 1937; 1938] have put forward a theory of protein structure based on the idea that amino-acid residues are repeated at regular intervals along the peptide chain, and, further, that the 'frequency' of such recurrency must be factorizable entirely by 2 and/or 3, as must the total number of amino-acid residues in the molecule, and therefore the total number of residues of each amino-acid species in the molecule. This hypothesis imposes considerable limitations on the astronomical possibilities of protein isomerism permitted by the Hofmeister-Fischer hypothesis. The main positive chemical evidence presented in its favour by Bergmann and others has been the results of amino-acid analysis of complete hydrolysates of proteins. It is clear that the only direct evidence in favour of a hypothesis concerning the order of amino-acid residues in a peptide chain can come from the isolation and identification of fragments of this chain containing at least two amino-acid residues. Since n molecular species of simple amino-acids can only give rise to n2 molecular species of dipeptides, an investigation of the dipeptides resulting from the partial hydrolysis of proteins should be possible technically, and, if we can be sure that the dipeptides studied do not arise by rearrangement, but are true fragments of the original peptide chain of the protein, the characterization of these compounds should yield information as to the structure of the original protein molecule. A considerable body of data of this sort has been accumulated during the past 40 years. In the course of an examination of this literature undertaken by one of us (R. L. M. S.), special attention was paid to possible rearrangements occurring in the course of preparing and isolating the products of partial hydrolysis. No evidence could be found in the literature suggesting that partial