Zinc complexes of amino acids and peptides .7. Solution behavior and zinc complexation of dipeptides made up solely from histidine and cysteine

The eight possible types of dipeptides made up from histidine and cysteine, namely the unprotected ones H-His-Cys-OH (1) and H-Cys-His-OH (2), the N-protected ones Ac-His-Cys-OH (3) and Ac-Cys-His-OH (4), the C-protected ones H-His-Cys-OEt (5) and H-Cys-His-NH2 (6), and the fully protected ones Ac-His-Cys-OEt (7) and Ac-Cys-His-OEt (8) were prepared analytically pure. Their acid-base behavior and zinc complexation in solution were studied potentiometrically. In all cases 1:1 (zinc:peptide) complexes are the do dominant species near neutral pH. 1:2 complexes could only detected for 4, 6, 7, and 8, and precipitations occurred in basic media. From equimolar mixtures of the peptides and zinc salts (chloride, iodide, perchlorate) solid complexes were obtained upon addition of a base. They always contain coordinating anions (chloride, iodide, or trifluoroacetate which was introduced with the peptide). They have the composition LZnX for L = 5-8 and L(4)Zn(5)X(2) for L = 1-4. It is proposed that all complex species observed contain tetrahedral zinc, that they are monomers in dilute solution and thiolate-bridged oligomers in the solid state, and that the peptides as a rule occupy at least three coordination sites.