X-ray absorption near edge structure (XANES) of Cu(II)–ATP and related compounds in solution: Quantitative determination of the distortion of the Cu site

Abstract

We have studied the Cu binding site in Cu(II)–ATP, Cu(II)–tripolyphosphate, and Cu(II)–D-ribose complexes in aqueous solution at several pH values ranging from 3 to 12 by means of high-resolution x-ray spectroscopy. We have measured the spectra in the x-ray absorption near edge structure (XANES) region to probe the tetragonal distortion of the Cu site in these compounds, as indicated by optical d–d spectroscopy. The spectrophotometric studies proposed that Cu(II) bind to the oxygen atoms of the phosphate groups of ATP with a distorted octahedral symmetry at low pH, while at high pH, the metal atom would be bound to the hydroxyl groups of the ribose moiety with an almost planar symmetry. We have compared the measured XANES spectra with multiple scattering calculations, by using as a structural model a simple tetragonal CuO6 cluster with fixed equatorial distance (1.95 Å) and variable axial distance (1.95–2.7 Å). The multiple scattering calculations fit well the experimental data of Cu–ATP in solution, and allow us to solve the different contribution in the XANES spectrum from axial and equatorial distances. The best agreement is obtained with values of the axial distance of 2.2 Å at low pH, and 2.7 Å at high pH. Our result indicates that an increasing percentage of highly distorted (square-plane) Cu site is present at high pH, and confirm the d–d spectroscopic model.