HISTIDINE-CONTAINING TERNARY AMINO ACID COPPER(II) COMPLEXES - SYNTHESES AND PROPERTIES

Ternary copper(II) complexes Cu(His)(AA), where His refers to L- or D-histidine and AA to L- or D-asparagine (Lor D-Asn), L-glutamine (L-Gln), L-serine (L-Ser), L-homoserine, or L-citrulline (L-Cit), have been isolated as crystals. The Cu(II)-L.-His-L-AA systems in the visible region almost invariably exhibited absorption peaks at 610-620 nm and positive circular dichroism (CD) extrema at 620-630 nm at pH 7-8, suggesting similar coordinating groups in the copper(II) coordination plane. On the basis of the difference in crystallization between the diastereomers Cu(L-Asn)(L-His) and Cu(D-Asn)(L-His) and Cu(L-Cit)(L-His) and Cu(L-Cit)(D-His), optical resolution of DL-histidine has been performed via formation of the ternary complexes, Cu(L-Asn)(His) and Cu(L-Cit)(His), which were shown to have incorporated L-histidine with optical purities as high as 98 and 78%, respectively, by the calibration curves established by the CD spectral magnitudes. The stability constants (log (β values) for the ternary complexes with L-Asn, L-Gln, L-Ser, and L-Thr have been determined to be in the range 17.0-17.1 by potentiometric titrations at 25°C in 0.1 M KNO3. Facile isolation of the ternary complexes and successful optica] resolution of DL-histidine are interpreted in terms of the solubility differences and the intramolecular hydrogen bond inferred to be formed between the carboxylate oxygen of histidine and the amide or the hydroxyl group present in the side chain of AA. A plausible structure of the ternary complexes in solution has been proposed by assuming terdentate histidine and this intramolecular interaction. © 1979, American Chemical Society. All rights reserved.