Both peptides1 and diketopiperazines2 have been suggested as the structural units in proteins.3 A reagent which would behave in a different manner with these structures would be valuable. In the present investigation it has been found that a solution of mercuric acetate in very dilute acetic acid does react differentially. Accordingly, a study was made of the action of mercuric acetate on glycylglycine, 2,5-diketopiperazine, 3,6dimethyl-2,5-diketopiperazine, and three proteins, casein, gelatin, and silk fibroin. The reagent in all the experiments consisted of an aqueous solution of mercuric acetate to which had been added a few drops of acetic acid in order to prevent the precipitation of mercuric oxide or basic mercuric salts. The concentration of the acetic acid was about 0.5 per cent, in most of the experiments.Glycylglycine.-Refluxing glycylglycine with the mercuric acetate solution resulted in the complete reduction of the mercuric acetate to free mercury in thirty minutes. Reduction also occurred when the solution was allowed to stand three days at room temperature. The products of this reaction, other than mercury, have not yet been identified but this result parallels the observations of other workers4 on the action of mercuric acetate with free amino acids. Hydrolysis and oxidation of the peptide evidently took place, resulting in the reduction of the mercuric ion to free mercury.Diketopiperazines.-Treatment of 2,5-diketopiperazine or 3,6-dimethyl-2,5-diketopiperazine with the mercuric acetate solution produced in each case white insoluble precipitates containing mercury. No reduction to free mercury took place.
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