X-ray absorption spectroscopy studies of the adducts formed between cytotoxic gold compounds and two major serum proteins.

Gold metallodrugs form a class of promising antiproliferative agents showing a high propensity to react with proteins. We exploit here X-ray absorption spectroscopy (XAS) methods [both X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)] to gain insight into the nature of the adducts formed between three representative gold(I, III) metallodrugs (i.e., auranofin, [Au(2,2'-bipyridine)(OH)(2)](PF(6)), Aubipy, and dinuclear [Au(2)(6,6'-dimethyl-2,2'-bipyridine)(2)(µ-O)(2)](PF(6))(2), Auoxo6) and two major plasma proteins, namely, bovine serum albumin (BSA) and human serum apotransferrin (apoTf). The following metallodrug-protein systems were investigated in depth: auranofin/apoTf, Aubipy/BSA, and Auoxo6/apoTf. XANES spectra revealed that auranofin, upon protein binding, conserves its gold(I) oxidation state. Protein binding most probably takes place through release of the thiosugar ligand and its subsequent replacement by a thiol (or a thioether) from the protein. This hypothesis is independently supported by EXAFS results. In contrast, the reactions of Aubipy with serum albumin and of Auoxo6 with serum apoTf invariantly result in gold(III) to gold(I) reduction. Gold(III) reduction, clearly documented by XANES, is accompanied, in both cases, by release of the bipyridyl ligands; for Auoxo6 cleavage of the gold-gold dioxo bridge is also observed. Gold(III) reduction leads to formation of protein-bound gold(I) species, with deeply modified metal coordination environments, as evidenced by EXAFS. In these adducts, the gold(I) centers are probably anchored to the protein through nitrogen donors. In general, these two XAS methods, i.e., XANES and EXAFS, used here jointly, allowed us to gain independent structural information on metallodrug/protein systems; detailed insight into the gold oxidation state and the local environment of protein-bound metal atoms was achieved in the various cases.

Thermal dependent anharmonicity effects on gold bulk studied by extended x-ray-absorption fine structure.

The structural parameters of the first five coordination shells of an Au bulk obtained from high accuracy L(3)-edge extended x-ray absorption fine structure (EXAFS) spectra in the temperature range 20-300 K are reported. Good agreement with previously reported studies is found. The effective second and third order force constants evaluated using EXAFS data are compatible with those calculated from phonon dispersion curves. A careful comparison of the variations of the EXAFS first shell distance with x-ray diffraction data provided the mean squared relative displacement of the atomic vibrations perpendicular to the first interatomic bond. An alternative new approach that is useful in achieving this parameter when x-ray diffraction data are not available is proposed.

Mapping the intake of different elements in vegetal tissues by dual-energy X-ray imaging at DaPhine synchrotron light source.

This article reports on the first utilization of the soft X-ray beamline at the DaPhine synchrotron light source for mapping the intake of different elements in plant tissues. As a test, the method of dual-energy X-ray microradiography was applied to the investigation of the natural sulfur content in dried leaf and root samples. Our ultimate goal was to monitor the pollutant lead and its intake, which was added in controlled doses to the hydroponic medium of laboratory-controlled samples of vegetal species. The results obtained by the nondestructive X-ray radiographic analysis are compared to the values of concentrations determined by a standard chemical analysis utilizing atomic absorption spectroscopy. From this comparison the validity of the X-ray detection of heavy metals in biological samples has been confirmed. The superposition of the dual energy results on the simple planar radiography shows the representation of the pollutant intake directly on the sample structures. It should be pointed out that this method, developed here for plant root and leaves could be applied to any biological sample of interest, but the preparation and observation conditions necessitate different strategies according to the type of sample under analysis.

Time-resolved X-ray powder diffraction on a three-way catalyst at the GILDA beamline.

Time-resolved X-ray diffraction experiments carried out at the beamline BM08-GILDA of ESRF allowed a study of the structural modifications taking place in a Pt/ceria-zirconia catalyst while the CO oxidation reaction was in progress. The capillary tube in which the sample is stored acts effectively as a chemical microreactor that ensures homogeneity of the sample treatments and minimization of diffusion effects. During the flowing of the reactant CO/He mixture, the investigated catalyst undergoes a fast Ce(IV)-Ce(III) partial reduction that involves the release of one O atom for every two reduced Ce cations. Because Ce(III) has a larger ionic radius than Ce(IV), the structural modification produces an increase of the lattice constant of the ceria-zirconia mixed oxide, and this increase is monitored by the translating imaging-plate device implemented at GILDA. The CO(2) resulting from the oxidation of the fluxed CO is monitored by a quadrupole mass spectrometer during the recording of the time-resolved X-ray diffraction pattern. The chemical and structural information was combined to show that the CO(2) yield is nearly constant until the catalytic system can provide oxygen for the reaction, while the structural rearrangement of the catalyst is delayed with respect to the switching on of the CO/He flux. After this induction time, during which CO(2) is produced with no structural modification of the catalyst, a fast increase of the lattice constant takes place.

Co dimers observed by extended x-ray absorption fine structure spectroscopy.

Cobalt was introduced into a silver matrix by ion implantation and observed by extended x-ray absorption fine structure (EXAFS) spectroscopy. In the range 0.10--0.70 at. % evidence of Co dimers dispersed in the matrix was found. The dimers are somewhat contracted with respect to the bulk Co nearest neighbor distance and distributed in a chainlike configuration with each dimer at 90 degrees from each other along opposite square faces of the Ag fcc lattice.

Anomalous wide-angle X-ray scattering apparatus on the GILDA beamline at the ESRF.

The experimental apparatus for anomalous wide-angle X-ray scattering (AWAXS) on the GILDA beamline at the ESRF is described. The main features are the high beam stability and reproducibility which allow anomalous scattering effects to be resolved also on dilute elements, the large spectral range which allows AWAXS experiments at the K edges of heavy elements, and the use of a high-efficiency detection system. The apparatus has been tested in extreme conditions by performing AWAXS experiments at the Eu K edge in Eu-doped Sr metaphosphate glasses.