USE OF COPPER (II) ION IN MASKING α-AMINO GROUPS OF AMINO ACIDS

The use of ornithine copper in preparing nn-citrulline suggested (I) the wider application of chelation to synthetic work.Although ions other than copper which are capable of forming chelate rings with ar-amino acids also mask the coordinated groups, the use of such ions ordinarily cannot be expected to be advantageous.The superiority of the cupric ion over others, such as cobalt (III) ion, lies in the ease with which stable complexes may be formed and in the facility with which the copper may be removed subsequently.The stable cupric complexes of amino acids may be formed simply by heating aqueous solutions of the latter with an excess of cupric carbonate, cupric oxide, or cupric hydroxide, but tests with cobalt demonstrated the troublesome necessity of oxidizing the cobalt to the trivalent state.The extremely low solubility of cupric sulfide, even in acidic solutions, permits the ready removal of copper from all of its complexes by treatment with hydrogen sulfide.Three diamino acids have been studied.Ornithine and lysine were selected because of their availability and because the reacting, terminal, amino groups in these compounds are structurally identical with the groups it was desired to mask at the a! position.This structural identity of the ac-and w-amino groups should make tests of the masking ability of the cupric ion the more severe.The amino groups of CY, y-diaminobutyric acid are sufficiently close together to permit, theoretically, the simultaneous coordination of both reactive groups with a single cupric ion.If a chelate ring containing the 2 nitrogen atoms were formed, masking of both might be expected.To determine whether such masking actually occurs, (Y , -r-diaminobutyric acid was also studied. EXPERIMENTALCarbamyl Derivatives-The procedures were analogous to that previously described (1) for the preparation of nn-citrulline except that the amino acid solution, adjusted, when necessary, to pH 7 with sodium hydroxide, was boiled with basic cupric carbonate rather than with cupric oxide.Excess cupric carbonate is more readily removed by filtration than is the oxide.