Copper(II) Complexes of Glycyl-L-histidine, Glycyl-L-histidylglycine, and Glycylglycyl-L-histidine in Aqueous Solution

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The complex formation of glycyl-L-histidine, glycyl-L-histidylglycine, and glycylglycyl-L-histidine with copper (II) ion in aqueous solutions containing equimolar amounts of copper(II) and the respective ligand was investigated by potentiometric and visible spectrophotometric methods. The presence of the following copper (II) complex species was assumed: CuLH2+, CuX, and CunYnn− for glycyl-L-histidine-copper(II) and glycyl-L-histidylglycine–copper(II) systems, and CuLH2+, CuY−, and CuZ2− for glycylglycyl-L-histidine-copper(II) system, where LH is the neutral species of the ligand and LH=XH2=YH3=ZH4. The formation and ionization constants were obtained graphically, and the distribution of copper(II) among the complex species was calculated for each system. The results indicate that copper(II) ions are bound to the imidazole and carboxyl (or carbonyl) groups in CuLH2+ and to the amino, imidazole, and deprotonated amide groups in CuX, CuY−, and CuZ2−. The spectral blue shift accompanying a deprotonation in the region from a=4 to 5 in the glycyl-L-histidine and glycyl-L-histidylglycine copper(II) systems indicates the formation of a polymer complex CunYnn− in which the imidazole ring acts as a bridge to connect copper atoms, where a represents moles of KOH added per completely protonated ligand.
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