The solution structure of [Cu(aq)]2+ and its implications for rack-induced bonding in blue copper protein active

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 T-d 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 (2x 1.98 +/- 0.03 angstrom and 2x 1.95 +/- 0.03 angstrom) and a long Cu-O-ax bond (2.35 +/- 0.05 angstrom). The four equatorial ligands were D-2d-distorted from the mean equatorial plane by +/-(17 +/- 4)degrees, so that the overall symmetry of [Cu(H2O)5](2+) is C-2v. The four-coordinate MXAN fit was nearly as good, but the water ligands (4x 1.96 +/- 0.02 angstrom) migrated +/-(13 +/- 4)degrees from the mean equatorial plane, making the [Cu(H2O)4](2+) model again D-2d-distorted. Spectroscopically calibrated DFT calculations were carried out on the C-2v, elongate square-pyramidal and D-2d-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 U 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 D-2d-,r 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.