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

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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−Oeq bonds (2× 1.98 ± 0.03 Å and 2× 1.95 ± 0.03 Å) and a long Cu−Oax bond (2.35 ± 0.05 Å). The four equatorial ligands were D2d-distorted from the mean equatorial plane by ±(17 ± 4)°, 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× 1.96 ± 0.02 Å) migrated ±(13 ± 4)° 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 CuII 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−Oax bond was worth only 96.8 kJ mol-1, compared to 304.6 kJ mol-1 for each Cu−Oeq 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.