Nature of Metal Binding Sites in Cu(II) Complexes with Histidine and Related N-Coordinating Ligands, As Studied by EXAFS

Abstract

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(NH(3))(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.

Modeling the interactions of phthalocyanines in water: From the Cu(II)-tetrasulphonate to the metal-free phthalocyanine

“…According to this simple analysis, the detection of water molecules in the neighborhood of the copper ion is revealed as a very challenging aim, particularly if one takes into account other relevant contributions arising from multiple scattering phenomena. 55 Our results agrees with the experimental EXAFS analysis as the fit of the experimental signal performed in that case did not improve if axial contributions arising from the solvent molecules were included.…”

“…However, the soluble 3,4 ,4 ,4 -tetrasodiumtetrasulphonate phthalocyanine, i.e., one of the complexes here simulated, has been studied by EXAFS in dilute aqueous solutions analyzing the copper Kedge. 55 That work provides structural information such as distances and Debye-Waller factors for the first nitrogen and carbon atoms around the copper center. On the contrary, such information can be obtained for the solvent molecules by analyzing the waters occupying the axial positions during our simulation.…”

New Insights into the Coordination Chemistry and Molecular Structure of Copper(II) Histidine Complexes in Aqueous Solutions

“…Histidine (his) moieties are known to be of major importance in this process because this type of N-coordinating ligand forms fairly stable complexes with the Cu cation. To unravel the formation processes of these types of enzymes and to find the exact coordination geometry of the Cu/his moieties, a lot of research has been conducted in this area, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] much of which has been summarized in two recent reviews. 18,19 A wide variety of analytical tools have been applied to obtain this type of information directly from the enzyme systems, but the analyses are often hampered by interference of the protein matrix.…”

“…To elucidate the molecular structures of Cu 2+ /his complexes and to obtain insight into the forces that drive anchoring of the metal ion, many studies have been carried out since the late 1960s. In these studies, a wide variety of techniques have been used to obtain the desired information, ranging from spectroscopic techniques, like circular dichroism, 10,25,26 ultraviolet/visible/ near-infrared (UV/vis/NIR), 10,25-29 infrared (IR), 3,14,[26][27][28][29][30][31][32][33] Raman, [3][4][5]10,13,15-17,33-35 1 H NMR, 10,32,36-39 13 C NMR, 36 electron spin resonance (ESR), 10,27,37,40-48 and extended X-ray absorption fine structure (EXAFS), 6,49 to potentiometric 10,26,28,50-54 and calorimetric 55 experiments. Several Cu 2+ /his complexes consisting of one or two central Cu atoms have been proposed in these studies, with one or two his ligands in a mono-, bi-, or tridentate coordination and neutrally, negatively, or positively charged.…”

“…The distribution plot slightly differs from the one proposed by Sarkar and coworkers, 18,19 but as demonstrated, the large number of complementary spectroscopic data produced in our study not only can be used to determine the coordination and ionic states of the his ligands but also admits further refinement in distinguishing the different geometrical orientations within each state of protonation in a quantitative way. Six complexes evidently appear to be present, i.e., Cu(H 2 O) 6 2+ and complexes c and e-h, while three complexes, i.e., a, b, and d, have not been detected. However, this is in line with what was expected.…”