Visible, optical rotatory dispersion, and paramagnetic resonance spectra of the L-histidine–Cu(II)–L-threonine complex

Abstract

The stability constant of L-histidine-Cu(II)-L-threonine relative to Cu(II)-L-histidine and Cu(II)-L-threonine has been determined. The absorption maxima of Cu(II)-L-histidine and Cu(II)-L-threonine were shown to be at 645 mμ and 625 mμ respectively and a solution containing a mixture of L-histidine, Cu(II), and L-threonine in a 1:1:1 molar ratio shows an absorption maximum at 615 mμ. Shifts in absorption spectra and changes in the optical density were observed when the relative concentration of the ligands in the mixture was changed. The optical rotatory dispersion studies reveal that, in the region 640–700 mμ, it is especially apparent that the molar rotation of the mixed solution is by no means an average of the molar rotation of the Cu(II)-L-histidine and Cu(II)-L-threonine complexes. The three complexes show a close overall similarity of the electron paramagnetic resonance (EPR) parameters. The EPR spectrum of cupric complexes in the mixed solution resembles both the L-threonine and L-histidine complexes in its main features but it differs in significant detail from each of them. The hyperfine splitting of the mixed complex in solution is evidently greater than that of either the Cu(II)–L-histidine or Cu(II)–L-threonine. A structure for L-histidine–Cu(II)–L-threonine is proposed.