José A. Mejías

Calculation of the absolute hydration enthalpy and free energy of H+ and OH−

The hydration enthalpy and Gibbs free energy of proton and hydroxide are calculated by means of a combination of ab initio density functional theory and a polarizable continuum model within the self-consistent reaction field method. The ion–water cluster models here used include up to 13 water molecules solvating the ions. This allows the first and second solvation shells to be described explicitly from first principles. Vibrational contributions to the enthalpy and entropy have been taken into account. Our best model of the hydrated proton includes three molecules in the first hydration shell and nine molecules in the second shell. The calculated proton hydration enthalpy is ≈−1150 kJ/mol, which is in rather good agreement with the most recent results from cluster–ion solvation data. The hydration free energy of the proton has a larger error of ≈50–80 kJ/mol as compared to recently reported values. The calculated hydroxide hydration enthalpy, ≈−520 kJ/mol, and hydration free energy, ≈−400 kJ/mol, are con...

Strong relaxations at the Cr2O3(0001) surface as determined via low-energy electron diffraction and molecular dynamics simulations

Abstract The surface structure of Cr 2 O 3 (0001) was investigated by quantitative low-energy electron diffraction and molecular dynamic simulations. In qualitative agreement with each other, both methods indicate strong vertical relaxations at and near the surface. These relaxations are concomitant with a charge reduction and depolarization, which stabilize the surface, yielding energies close to those found for non-polar oxide surfaces with non-divergent surface potentials. The lateral arrangement of oxygen atoms is identical to that in the bulk, i.e. there are no lateral distortions to accomodate the strong interlayer relaxations. The latter extend extend deep into the surface, with the experimentally determined changes of the first four interlayer distances being −38%, −21%, −25% and +11% with respect to the unrelaxed bulk values.

The kinetics and mechanism of MgO dissolution

Abstract Reactions and atomic rearrangements at fluid–crystal interfaces play an important role in catalysis and in controlling the kinetics and mechanisms of dissolution. We have studied the attachment and reactions of water molecules at the MgO–water interface by combining measurements of 1 H and 2 D surface penetration and etch pit morphology with ab initio calculations. These studies show that the most common MgO cleavage surface, (001), is thermodynamically unstable when hydrated. Proton rearrangement on such surfaces precedes proton–cation exchange and provides a general mechanism for the detachment of ions during dissolution. The kinetics of dissolution are strongly influenced by the concentration of surface defects and a simple model based on the ab initio results predicts a dissolution rate of 10−10 mol cm−2 s−1 for a typical surface defect concentration of 0.1.

On modelling the interaction of CO on the MgO(100) surface

Abstract In this work we discuss the interaction of an adsorbate on an ionic surface taking as representative example CO on a perfect MgO(100) surface. The main goal is to investigate the different contributions to the interaction and how to model them by using a finite cluster model. To this end we use three different ab initio Hartree-Fock approaches. First, we discuss the convergence properties of the array of point charges used to simulate the Madelung potential. Next, we use large cluster models to show that there is an oscillatory behavior of the interaction energy. We show that rather large clusters are needed to avoid such oscillations. Also, we discuss how to represent these large clusters by using model potentials and compare the results with those obtained from periodic Hartree-Fock calculations. The interaction of CO on MgO is found to be weak and of electrostatic origin, with no noticeable chemical contributions.

Tiopronin monolayer-protected silver nanoparticles modulate IL-6 secretion mediated by Toll-like receptor ligands

AIMS Capped silver nanoparticles that can be coupled to a variety of molecules and biomolecules are of great interest owing to their potential applications in biomedicine. However, there are no data about their toxicity or functional effects on a key innate immune response, such as IL-6 secretion, after the engagement of the main group of pathogen-associated molecular patterns receptors, that is, the Toll-like receptors (TLRs). MATERIALS & METHODS N-(2-mercaptopropionyl)glycine (tiopronin)-capped silver (Ag@tiopronin) nanoparticles of a narrow sized distribution ( approximately 5 nm) were synthesized and characterized by transmission electron microscopy, Fourier transform infrared spectroscopy, Raman, (1)H-NMR and total correlation spectroscopy. Cytotoxicity was determined by lactate deshidrogenase and 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium assays in Raw 264.7 macrophages. IL-6 was measured by ELISA. RESULTS & DISCUSSION Ag@tiopronin nanoparticles have a narrow size distribution ( approximately 5 nm), high solubility and stability in aqueous environment with no cytotoxicity in terms of mitochondrial function or plasma-membrane integrity at concentrations as high as 200 microg/10(6) cells. Ag@tiopronin nanoparticles were not proinflammatory agents, but remarkably they specifically impaired the IL-6 secretion mediated by TLR2, TLR2/6, TLR3 or TLR9 stimulation in co-treatment experiments. However, in pretreatment experiments, nanoparticles enhanced the susceptibility of macrophages to inflammatory stimulation mediated by TLR2/1 and TLR2/6 specific ligands while severely impairing the IL-6 secretion activated by the TLR3 or TLR9 ligands. CONCLUSIONS Contrary to what is found for bare silver nanoparticles, Ag@tiopronin nanoparticles are noncytotoxic to macrophages. Ag@tiopronin nanoparticles showed differential effects on TLR signaling of a high degree of specificity, without proinflammatory effects by themselves. These effects have to be borne in mind when using bioconjugates of Ag@tiopronin nanoparticles for future medical applications.

Chemical stability of Sin+ species in SiOx (x<2) thin films

SiOx thin films have been prepared by evaporation of silicon monoxide powder in an ultrahigh vacuum chamber. The films are characterized by x-ray photoelectron spectroscopy (XPS), synchrotron photoemission, and x-ray absorption spectroscopy at the Si K edge. XPS shows that the films prepared by evaporation in ultrahigh vacuum have a SiO1.3 stoichiometry and are formed by Si3+(∼77%) and Si+(∼23%) species. Based on extended x-ray absorption fine structure analysis, the structure of these films has been described as formed by tetrahedra of the type Si–(Si, O3) and Si–(Si3, O), in agreement with the Si 2p photoelectron spectra. No significant amount of Si2+ species [i.e., Si–(Si2, O2)] tetrahedra) or elemental silicon were detected in these films. When SiOx thin films are prepared by evaporation of silicon monoxide in O2 atmosphere, the oxygen content in the film increases with the partial pressure of this gas. Under these conditions, Si4+ species are formed in detriment of the Si+ and Si3+ oxidation states. ...

Properties of small TiO2, ZrO2 and HfO2 nanoparticles

Ground state structures and energies are predicted for (MO2)n, where n = 1 to 8 and M is one of three isovalent cations, titanium(IV), zirconium(IV) and hafnium(IV), with the minimised binding energies calculated using Density Functional Theory. An increased number of single coordinated oxygen atoms, as opposed to more densely packed configurations, were found for the titania ground state clusters n = 5 and 7. We present, as a function of n, calculated nucleation energies, the energies of the respective highest occupied and lowest unoccupied molecular orbitals, harmonic frequencies of vibration and infrared spectra for all three compounds. Similarities and differences in the data produced for the three oxides are considered. Calculations were performed using the GAMESS-UK software on HPCx (phase 2a); aspects of the computational procedures are discussed.

Erratum to: “Strong relaxations a the Cr2O3(0001) surface as determined via low-energy electron diffraction and molecular dynamics simulations” [Surf. Sci. 372 (1997) L291]

In the LEED intensity analysis, covered by the Erlangen group, the Debye temperature of oxygen was erroneously chosen to be far too low. The fit of all layer Debye temperatures to the experimental data, which has now been carried out in addition and i n parallel to the structural parameters, changes the interlayer relaxation values. This is in contrast to general experience, according to which the consideration of thermal vibrations leaves the structure more or less unchanged but improves the quality of the theory experiment fit. The latter is also true in the present case with the best-fit Pendry R-factor decreasing from 0.32 to 0.26. Using the same procedure of analysis as in the original paper, the best fit develops for O o = 6 0 0 K, OCr--630 K and the interlayer distance relaxations (starting with the top layer) result in a = 6 0 % , b = 3%, c = 2 1 % and d = +6%. In addition to the original work, the next interlayer distance between Oand Cr-layers was also varied and was found to relax by e = +2%. The new values confirm our earlier message that the relax-

Compact model potentials for abinitio embedded cluster calculations. Part I. Basic formulation

In light of the theory of separability of many‐electron systems, a methodology to perform embedded cluster calculations in order to study localized phenomena in a large electronic system is reported. A general equation for the cluster spin orbitals under the assumptions of frozen environment and cluster‐environment strong orthogonality requirements is presented. The interactions between active and frozen electrons are adequately introduced through computationally convenient ab initio compact model potentials (CMPs) built up as a sum of long‐ and short‐range terms. The short‐range term, Coulomb and exchange interactions, are developed as a spectral representation in a nonorthogonal basis set. Cluster wave function, energy, and analytic geometrical derivatives calculations with CMPs have been implemented for several kinds of cluster wave functions. In order to evaluate the accuracy of the results obtained using this methodology, some simple test calculations for different hydrogen molecule‐cation complexes ...

Properties of the in vivo Nitrate Reductase Assay in Maize, Soybean, and Spinach Leaves

Summary The effect of different parameters on the in vivo nitrate reductase activity in leaves of maize, soybean, and spinach has been examined. Leaf sections infiltrated under vacuum with an assay medium containing CO 2 tension and NO - 3 and incubated under air, excreted CO 2 tension and NO - 2 in the dark but not in the light. Nevertheless, addition of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea, a potent inhibitor of the photosynthetic electron transport, promoted an immediate excretion of CO 2 tension and NO - 2 in the light. Under those conditions, subsequent addition of ascorbate-reduced dichlorophenolindophenol or N,N,N′,N′-tetramethylphenylenediamine, which can supply electrons to photosystem I, abolished the excretion of CO 2 tension and NO - 2 by the leaf sections. These results support the view that, in green tissues, the reducing equivalents photogenerated in the chloroplasts are more directly used for NO - 2 reduction than for NO - 2 reduction. Incubation of leaf discs under O 2 -atmosphere severely impaired NO - 2 accumulation. Under strict anaerobic conditions, the rate of NO - 2 production was only slightly enhanced as compared with that observed in the assay under air. Exogenous NO - 3 stimulated the activity, more markedly in soybean than in the other two species. Concentrations of exogenous NO - 3 between 50 and 100 mM were required for maximal activities in all the three species. The external pH optimum for in vivo NO - 3 reduction was 7.5 for maize and spinach and 7.0 for soybean. Inclusion of 1% (v/v) n -propanol in the infiltration-incubation medium enhanced the activity about 15%. However, 7% (v/v) n -propanol caused a drastic decrease in the rate of NO - 2 accumulation which could be greatly overcome by inclusion in the medium of NADH or reduced flavin mononucleotide.