The Solution Structure of [Cu(aq)]²⁺ and Its Implications for Rack-Induced Bonding in Blue Copper Protein Active Sites

Abstract

The structure of [Cu(aq)]2+ has been investigated by using full multiple-scattering theoretical (MXAN) analysis of the copper K-edge X-ray absorption (XAS) spectrum and density functional theory (DFT) to test both ideal Td and square-planar four-coordinate, five-coordinate square-pyramidal, and six-coordinate octahedral [Cu(aq)]2+ models. The best fit was an elongated five-coordinate square pyramid with four Cu-O(eq) bonds (2 x 1.98 +/- 0.03 A and 2 x 1.95 +/- 0.03 A) and a long Cu-O(ax) bond (2.35 +/- 0.05 A). The four equatorial ligands were D2d-distorted from the mean equatorial plane by +/-(17 +/- 4) degrees, so that the overall symmetry of [Cu(H2O)5]2+ is C2v. The four-coordinate MXAN fit was nearly as good, but the water ligands (4 x 1.96 +/- 0.02 A) migrated +/-(13 +/- 4) degrees from the mean equatorial plane, making the [Cu(H2O)4]2+ model again D2d-distorted. Spectroscopically calibrated DFT calculations were carried out on the C2v elongate square-pyramidal and D2d-distorted four-coordinate MXAN copper models, providing comparative electronic structures of the experimentally observed geometries. These calculations showed 0.85e spin on Cu(II) and 0.03e electron spin on each of the four equatorial water oxygens. All covalent bonding was restricted to the equatorial plane. In the square-pyramidal model, the electrostatic Cu-O(ax) bond was worth only 96.8 kJ mol(-1), compared to 304.6 kJ mol(-1) for each Cu-O(eq) bond. Both MXAN and DFT showed the potential well of the axial bond to be broad and flat, allowing large low-energy excursions. The irregular geometry and D2d-distorted equatorial ligand set sustained by unconstrained [Cu(H2O)5]2+ warrants caution in drawing conclusions regarding structural preferences from small molecule crystal structures and raises questions about the site-structural basis of the rack-induced bonding hypothesis of blue copper proteins. Further, previously neglected protein folding thermodynamic consequences of the rack-bonding hypothesis indicate an experimental disconfirmation.

Remote Loading of 64Cu2+ into Liposomes without the Use of Ion Transport Enhancers

“…This latter model shown in Figure 4B was 25 and results in a significantly better agreement with the measured XANES spectrum (R xanes = 0.0217), as compared to the six-coordinate complex (R xanes = 0.0384) and the regular five-coordinate square pyramidal complex (R xanes = 0.0284). A 1.0% reduction of the Cu-O distances due to temperature differences further improved the fit as shown in Figure 4A (R xanes = 0.0179).…”

Hydration of Copper(II): New Insights from Density Functional Theory and the COSMO Solvation Model

“…These results are consistent with the combined EXAFS and XANES studies of aqueous solutions of Cu(II) by Frank and co-workers. 20 Mixed cluster/continuum models that include at least two full solvation shells can be an important tool for determining accurate structural parameters and coordination numbers of metal ions in aqueous solutions, particularly when different experimental and theoretical techniques reach different conclusions regarding the structure of hydrated ions. 97 The hydration free energy of Cu 2+ calculated using the mixed cluster/continuum approach (-509.0 kcal/mol) is in excellent agreement with the experimental value (507.0 ( 4.0 kcal/mol).…”

“…This shows that in bulk water the six-coordinate Cu(II) complex is less stable than the five-coordinate complex, which is consistent with XANES. 18,20 Relative energies are obtained by geometry optimization at each Cu-O distance in the gas phase or in the field of the continuum solvent (COSMO model) at the B3LYP/LACV3P+/6-311++G(d,p) level of theory.…”

The hydration of Cu2+: Can the Jahn-Teller effect be detected in liquid solution?

“…As deduced from the visual examination of this figure, and by checking R 2 values included in Table I, good reproductions were obtained for all structures except for the regular octahedron, as in the case of XANES region where this geometry does not correctly reproduce the shape of the white line. 18 The obvious reason for this improvement is the inclusion of two additional free parameters for the axial water molecules. Astonishingly, an almost equally good fit was achieved for the square-planar structure, SPl ͓see Fig.…”

“…In this study, the authors agree with the suggested fivefold coordination of Cu͑II͒ by Pasquarello et al, 12 although they propose a new distorted square pyramidal configuration, discarding both the regular square pyramid and trigonal bipyramid configurations. 17 Frank et al 18 have recently revisited the problem applying a fitting technique, which employs the full multiple-scattering ͑FMS͒ approach, including geometrically unconstrained models to the XANES data ͑MXAN method͒. They found that the best fit was an elongated square pyramid, although a fourcoordinated nonplanar arrangement of D 2d symmetry was producing a fit almost as good as the five-coordinated arrangement.…”