Sofía Díaz-Moreno

Solvation Structure and Ion Complexation of La3+ in a 1 Molal Aqueous Solution of Lanthanum Chloride

H/D isotopic substitution neutron scattering and X-ray scattering have been used to investigate the short and intermediate range solution structure in a 1 m aqueous solution of lanthanum chloride. To improve the reliability of the local structural information on the cation environment, information has been incorporated from Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy data into the applied analytical framework. The markedly different structural sensitivities of the experimental probes allow the construction of a detailed three-dimensional atomistic model using the Empirical Potential Structure Refinement (EPSR) technique. The results show that at the investigated concentration La(3+) is hydrated by eight water molecules and one chloride ion, forming an inner-sphere ion complex in which the water molecules maintain angular configurations consistent with a tricapped trigonal prism configuration. This local geometry considerably disrupts the bulk solvent structure.

The 'Invisible' Metal Particles in Catalysis

Abstract An easy, reliable and straightforward method to determine the sizes of small metal particles in supported metal catalyst which are invisible for most techniques (chemisorption, XRD, HRTEM) is presented. The technique we consider more appropriate is EXAFS, because it detects metal-metal bonds even before metal atoms are forming particles. Due to this capability it has become a routine technique in catalysis, although it requires an elaborate data analysis procedure. In the particular case of supported metal catalyst, this procedure can be simplified because nearly everything is known about the investigated structure, the metal particles. With the appropriate fitting strategies, the main contribution to the EXAFS spectrum, the metal-metal bonds, can be emphasized, and within it, most part of the fit parameters are known. The only unknown parameter is the coordination number for each metal shell. Once this value is obtained, the number of metal atoms per particle can be calculated and from that, metal particle diameter can be deduced. An example of this procedure for a Pt Al 2 O 3 catalyst is shown.

Ab initio X-ray absorption spectroscopy study of the solvation structure of Th(IV), U(IV), and Np(IV) in aqueous solution.

The coordination structures of U(IV), Np(IV), and Th(IV) in aqueous solution have been determined by studying the X-ray absorption near edge structure (XANES) of the actinide (An) L(3)-edge absorption spectra. The high sensitivity of XANES to the bonding geometry provides an unambiguous determination of the coordination polyhedron. On the basis of the comparison of ab initio computations with the experimental data we conclude that the hydration sphere of the three An(IV) aqua-ions studied is best modeled by 9 water molecules forming a tricapped trigonal prism.

Application of stopped flow techniques and energy dispersive EXAFS for investigation of the reactions of transition metal complexes in solution: Activation of nickel β-diketonates to form homogeneous catalysts, electron transfer reactions involving iron(III) and oxidative addition to iridium(I)

Stopped-flow techniques of rapid mixing have been combined with energy dispersive X-ray absorption spectroscopy to monitor the reaction of Ni(dpm)2 [dpm = Bu1C(O)CHC(O)Bu1] by aluminium alkyls (AlEt2X, X = OEt and Et) to form the active species for the catalytic di- and tri-merisation of hex-1-ene. Acquisition times down to ca. 30 ms were achieved on Station 9.3 of the SRS using a photodiode array detector. The EXAFS features of the resulting solution complexes are of the form [Ni(O-O)R)(alkene)]. In the presence of PPh3, [Ni(O-O)(R)(PPh3) appears to be the redominant type of species. The reduction of aqueous Fe(III) by hydroquinone was investigated on ID24 at the ESRF by Fe K-edge energy dispersive EXAFS with a CCD camera as detector, spectra were obtained in 1 ms or longer. No intermediate inner sphere complex was detected prior to the formation of aqueous Fe(II). Finally the oxidative addition of CH3SO3CF3 to [IrI2(CO)2]- was monitored on Station 9.3 with a silicon microstrip detector. A single acquisition of 400 micros was feasible, with spectra recorded in multiples of 1.2 ms. In that time, the first stage of the reaction had been completed, with a slower stage thereafter. The results are consistent with the two-stage ionic oxidative addition mechanism.

Solvent structure and the extended range hydration of Cr3+ in aqueous solution.

H/D isotopic substitution neutron scattering has been used to investigate the short and intermediate range solution structure in a 1 m aqueous solution of chromium nitrate. To improve the reliability of the local structural information on the cation environment, information has been incorporated from available extended X-ray absorption fine structure (EXAFS) spectroscopy data into the applied analytical framework. The markedly different structural sensitivities of the experimental probes allow the construction of a detailed three-dimensional atomistic model using the empirical potential structure refinement (EPSR) technique. The method facilitates the construction of a model that is consistent with regards to both the structural details of the immediate Cr(3+) aqua-ion environment and the bulk hydrogen-bonded network of solvent water molecules. The results confirm the suitability of the [Cr(H(2)O)(6)](3+) hydrated ion concept to describe the first hydration shell of this cation and clarify how this pseudomolecular unit is structurally incorporated in the longer range aqueous environment.

The hydration structure of Cu2+: more tetrahedral than octahedral?

A comprehensive multi-technique approach has been used to address the controversial question of the preferred geometric form of the Cu2+ aqua-ion hydration shell. A combination of H/D isotopic substitution neutron scattering and X-ray scattering has been used to refine atomistic models of 0.5 m and 2.0 m solutions of Cu(ClO4)2, that have also been constrained to simultaneously reproduce detailed local structure information about the cation environment obtained by X-ray Absorption spectroscopy. The adoption of the Empirical Potential Structure Refinement (EPSR) technique as a single unified analytical framework minimises the chances for biasing the result in favour of a specific pre-conceived outcome. The results are consistent with an average coordination for each Cu2+ ion of 4.5 ± 0.6 water molecules that matches the more recent picture of five-fold coordination in a 2.0 m solution, but interestingly this combined study highlights that the preferred local geometry of the ion sites is found to have a mixed character of tetrahedral, trigonal bipyramidal and octahedral components. A further point to note is that this new model adds support to a largely ignored result in the literature relating to the linear electric field effect induced g-shifts observed in the electron paramagnetic resonance spectra of glassy Cu2+ complexes (Peisach and Mims, Chem. Phys. Lett., 1976, 37, 307–310) that first highlighted the importance of tetrahedral distortions in the cations hydration shell structure.

Structural basis of mitochondrial dysfunction in response to cytochrome c phosphorylation at tyrosine 48

Significance Cell response to physiological changes and oxidative stress involves the modulation of mitochondrial metabolism. Its dysfunction favors the development of hypoxia-dependent pathologies, including ischemia and cancer. A key modulator of mitochondrial activity is cytochrome c, whose cell function is regulated by tyrosine phosphorylation. However, how such modification affects cytochrome c structure and function is barely known. Here we report that a phosphomimetic mutant of cytochrome c exhibits enhanced dynamics, which could be responsible for the observed differences in cytochrome c functionality in oxidative stress and cell death. Thus, phosphorylation of cytochrome c becomes a target for further development of robust therapeutic approaches. Regulation of mitochondrial activity allows cells to adapt to changing conditions and to control oxidative stress, and its dysfunction can lead to hypoxia-dependent pathologies such as ischemia and cancer. Although cytochrome c phosphorylation—in particular, at tyrosine 48—is a key modulator of mitochondrial signaling, its action and molecular basis remain unknown. Here we mimic phosphorylation of cytochrome c by replacing tyrosine 48 with p-carboxy-methyl-l-phenylalanine (pCMF). The NMR structure of the resulting mutant reveals significant conformational shifts and enhanced dynamics around pCMF that could explain changes observed in its functionality: The phosphomimetic mutation impairs cytochrome c diffusion between respiratory complexes, enhances hemeprotein peroxidase and reactive oxygen species scavenging activities, and hinders caspase-dependent apoptosis. Our findings provide a framework to further investigate the modulation of mitochondrial activity by phosphorylated cytochrome c and to develop novel therapeutic approaches based on its prosurvival effects.

Transient binding of plastocyanin to its physiological redox partners modifies the copper site geometry

The transient complexes of plastocyanin with cytochrome f and photosystem I are herein used as excellent model systems to investigate how the metal sites adapt to the changes in the protein matrix in transient complexes that are involved in redox reactions. Thus, both complexes from the cyanobacterium Nostoc sp. PCC 7119 (former Anabaena sp. PCC 7119) have been analysed by X‐ray absorption spectroscopy. Our data are consistent with a significant distortion of the trigonal pyramidal geometry of the Cu coordination sphere when plastocyanin binds to cytochrome f, no matter their redox states are. The resulting tetrahedral geometry shows a shortening of the distance between Cu and the Sδ atom of its ligand Met‐97, with respect to the crystallographic structure of free plastocyanin. On the other hand, when plastocyanin binds to photosystem I instead of cytochrome f, the geometric changes are not significant but a displacement in charge distribution around the metal centre can be observed. Noteworthy, the electronic density around the Cu atom increases or decreases when oxidised plastocyanin binds to cytochrome f or photosystem I, respectively, thus indicating that the protein matrix affects the electron transfer between the two partners during their transient interaction.

Detecting transient protein–protein interactions by X-ray absorption spectroscopy: The cytochrome c6-photosystem I complex

Reliable analysis of the functionality of metalloproteins demands a highly accurate description of both the redox state and geometry of the metal centre, not only in the isolated metalloprotein but also in the transient complex with its target. Here, we demonstrate that the transient interaction between soluble cytochrome c 6 and membrane‐embedded photosystem I involves subtle changes in the heme iron, as inferred by X‐ray absorption spectroscopy (XAS). A slight shift to lower energies of the absorption edge of Fe2+ in cytochrome c 6 is observed upon interaction with photosystem I. This work constitutes a novel application of XAS to the analysis of weak complexes in solution.

Optimized end station and operating protocols for reflection extended x-ray absorption fine structure "ReflEXAFS… investigations of surface structure at the European Synchrotron Radiation Facility beamline BM29

The development of the capability to engineer the surface properties of materials to match specific requirements demands high quality surface characterization techniques. The ideal tool should provide chemically specific structural characterization as well as surface sensitivity and depth profiling. Ideally the characterization method should also be applicable to systems both with and without long range order. X-ray absorption spectroscopy fine structure, when using the standard transmission detection system, provides all this information with the significant exception of surface sensitivity. In contrast, by detecting the reflected instead of the transmitted beam, it encompasses all these requirements because when the incident beam impinges onto a sample surface at glancing angles, in conditions close to the total reflection, only the outermost regions of the system under study are sampled. Such a technique provides information about the local structure as a function of depth as well as thin layer structure in the case of layered samples. Although it is potentially the ideal tool to study surface modified materials, experimental difficulties have hampered its widespread use in the fields of surface and materials sciences. As a solution to the experimental challenges, we provide a detailed description of an appropriate experimental station, the sample requirements, the measuring protocols, and software routines needed to optimize the collection of the data. To illustrate the capabilities of the technique the results obtained for a model multilayer sample are presented and analyzed under the total external reflection approximation.