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Preliminary time scans allowedto ascertain that the samplessuffered no damage within1300 seconds exposure to theX-ray beam. For each sample1000 seconds long spectrahave been acquired at differentsample positions in order notto alter the sample. Average of10 scans allowed to obtain agood signal to noise ratio forall the samples.Comparison of pre-edgepeak features with those of CuFig. 2. Structural refinements.model compounds, allowed todetermine the Cu oxidationstate in the three peptides: Cu is purely divalent in GK13, purely trivalent in DK13 andpresent as both Cu 2+ and Cu 3+ in LK13. Also edge energies and EXAFS derived Cu-Ndistances (1.79 Å and 1.90 Å for DK13 and GK13, respectively) are consistent with theXANES determined Cu oxidation states. Theoretical XANES spectra have been calculatedby means of the MXAN code. The initial structural model has been calculated according toa DFT code; the structural refinement with the MXAN code allowed finding optimisedvalues of the overlap between Muffin-Tin spheres; to refine the coordinates of theneighbouring N atoms and refine the coordinates of all other atoms. The good agreementbetween theoretical and experimental XANES data (Figure 1) allows to assume that therefined structure, at least in the first coordination shell around Cu, is a good approximationof the peptide/Cu 3+ complex structure. These results suggest that the CuN 4 unit is not planarbut form a slightly distorted pyramid [2]. Figure 2 shows the experimental XANES curvesfor DK13 and GK13, compared with those obtained after the structural refinements (solidlines). The simulations obtained with the proposed structures are in good agreement withthe experimental ones. Figure 3 depicts the proposed models for the peptide complexesDK13/Cu 3+ and GK13/Cu 2+ . The DK13/Cu 3+ shows a slight variation from square planargeometry, with Cu out of the plane, that justifies the loss of centrosymmetry and causes thehigh absorbance in the pre-peak zone, due to a strong 1 s- 3 d transition.Fig. 3. Proposed models for DK13/Cu 3+ and GK13/Cu 2+ .Even though no clear explanation can be provided yet, available data strongly suggest thatimidazole side chains have an important role in the formation of the final Cu 3+ complex inDK13 peptide.AcknowledgmentsThis work was supported by Ente Cassa di Risparmio di Firenze and MIUR funds (Prin 2005).References1. Pratesi, A., Zanello, P., Giorgi, G., Messori, L., Laschi, F., Casini, A., Corsini, M., Gabbiani, C.,Orfei, M., Rosani, C., Ginanneschi, M. Inorg. Chem. 46, 10038-10040 (2007).2. Pratesi, A., Giuli, G., Cicconi, M.R., Weng, T.C., Pratesi, G., Ginanneschi, M. 14th InternationalConference on X-ray Absorption Fine Structure, p. 239, Camerino (Italy), July 26-31 (2009).611