- Nature of metal binding sites in Cu(II) complexes with histidine and related N-coordinating ligands, as studied by EXAFS
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Knowledge of the complexes formed by N-coordinating ligands and Cu(II) ions is of relevance in understanding the interactions of this ion with biomolecules. Within this framework, we investigated Cu(II) complexation with mono-and polydentate ligands, such as ammonia, ethylenediamine (en), and phthalocyanine (Pc). The obtained Cu-N coordination distances were 2.02 A for [Cu(NH3)4]2+, 2.01 A for [Cu(en) 2]2+, and 1.95 A for CuPc. The shorter bond distance found for CuPc is attributed to the macrocyclic effect. In addition to the structure of the first shell, information on higher coordination shells of the chelate ligands could be extracted by EXAFS, thus allowing discrimination among the different coordination modes. This was possible due to the geometry of the complexes, where the absorbing Cu atoms are coplanar with the four N atoms forming the first coordination shell of the complex. For this reason multiple scattering contributions become relevant, thus allowing determination of higher shells. This knowledge has been used to gain information about the structure of the 1:2 complexes formed by Cu(II) ions with the amino acids histidine and glycine, both showing a high affinity for Cu(II) ions. The in-solution structure of these complexes, particularly that with histidine, is not clear yet, probably due to the various possible coordination modes. In this case the square-planar arrangements glycine-histamine and histamine-histamine as well as tetrahedral coordination modes have been considered. The obtained first-shell Cu-N coordination distance for this complex is 1.99 A. The results of the higher shells EXAFS analysis point to the fact that the predominant coordination mode is the so-called histamine-histamine one in which both histidine molecules coordinate Cu(II) cations through N atoms from the amino group and from the imidazole ring.
- Carrera, Flora,Sanchez Marcos, Enrique,Merkling, Patrick J.,Chaboy, Jesus,Munoz-Paez, Adela
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p. 6674 - 6683
(2008/10/09)
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- Raman Scattering Study on Coordination Structures of Cu(II)-L-Histidine(1:2) in Aqueous Solutions
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The pH and pD dependences of the Raman spectra of Cu(II)-L-histidine (1:2) and Cu(II)-L-histidine-d3-5 (1:2) were analyzed based on the pH titration data.Several ring vibrations of the imidazolyl group with cationic, neutral and anionic forms, the out-of-plane bending vibration of the bound carboxylate, and the Cu(II)-N stretching vibrations of the bound amino groups are used to study the chelation structures of the species taken by the solutions.From the results it was concluded that MHA2+, the major species below pH 3, assumes a glycine-like chelation structure with an unbound imidazolium cation.The results also indicate that the species MHA2+, which is prominent in the pH region 4.0-4.5, takes a mixed-type structure, in which one histidine molecule coordinates to Cu(II) as a terdentate ligand and another binds as a substituted glycine.MA2, the major species around pH 7, is determined to have the chelation structure in which two nitrogen atoms of the amino groups and two nitrogen atoms of the imidazole rings are bound to the Cu(II) ion in the square-planar trans position and at least one carboxyl group in the axial position.
- Itabashi, Masao,Itoh, Koichi
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p. 3131 - 3137
(2007/10/02)
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