Copper(II) Complexes of L-Histidylglycine and L-Histidylglycylglycine in Aqueous Solution

概要

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Equilibrium constants were determined in the complex formations of L-histidylgylcine (HG) and L-histidylglycylglycine (HGG) with copper(II) ion in aqueous solution by potentiometric titration, and the structures of the complexes were discussed based on the observations in the titration and infrared and electronic absorption spectra. In the acid region, a cationic chelate, in which copper(II) binds to nitrogen atoms of amino and imidazole groups, is formed. In the neutral region, deprotonation of the peptide amide group occurs to form a dimer, in which four coordination positions of copper(II) are occupied by nitrogen atoms of deprotonated amide and amino groups and an oxygen atom of carboxyl or carbonyl group in one molecule and a nitrogen atom of imidazole group of another molecule. In HGG–Cu(II) system, the dimer is decomposed in the alkaline region with further deprotonation of the second peptide amide group to form an anionic chelate as in the case of the glycylglycylglycine–Cu(II) complex.
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