Amino acids and peptides. VIII. The synthesis and some properties of L-cystine diamide

Abstract

L-Cystine diamide has been synthesized by several different routes and isolated as its crystalline di(trifluor0acetate) and dihydrochloride. The compound has a molecular rotation [M]_D²¹ -630° in 1N HCl, compared to [M]_D²⁰ -537° for L-cystine ; the rotation is strongly dependent on pH in the range 5-8, the value of [M]_D²⁰ at pH 8 being -120°. The polarographic behaviour of L-cystine diamide is very similar to that of cystine. The amino groups in cystine diamide are only weakly basic, the pK?_A values being 5.93 and 6.90 for the successive ionizations of the two NH₃⁺ groups. The latter result is in accordance with the known acigenic properties of both amide and disulphide groups. A possible relation between optical rotation of cystine compounds and the pK? values of ionizable groups present in the molecule is discussed.