Growth and characteristics of ZnO nano-aggregates electrodeposited onto p-Si(1 1 1)

In this paper we study nanocrystalline zinc oxide thin films produced by oxidation of electrodeposited zinc nanolayers on a monocrystalline p-Si(1 1 1) substrate.The electrolyte used is ZnCl₂, an aqueous solution of 4 × 10⁻² mol/l concentration. Several deposits were made for various current densities, ranging from 13 mA/cm² to 44 mA/cm², flowing through the solution at room temperature. A parametric study enabled us to assess the effect of the current density on nucleation potential and time as well as zinc films structure. The grazing incidence X-ray diffraction (GIXD) revealed that both Zn and ZnO films are polycrystalline and nanometric. After 1-h oxidation of zinc films at 450 °C in the open air, the structural analyses showed that the obtained ZnO films remained polycrystalline with an average crystal size of about 47 nm and with (1 0 0), (0 0 2) and (1 0 1) as preferential crystallographic orientations.     

An operational definition of learning

An operational definition offered in this paper posits learning as a multi-dimensional and multi-phase phenomenon occurring when individuals attempt to solve what they view as a problem. To model someone's learning accordingly to the definition, it suffices to characterize a particular sequence of that person's disequilibrium-equilibrium phases in terms of products of a particular mental act, the characteristics of the mental act inferred from the products, and intellectual and psychological needs that instigate or result from these phases. The definition is illustrated by analysis of change occurring in three thinking-aloud interviews with one middle-school teacher. The interviews were about the same task: “Make up a word problem whose solution may be found by computing 4/5 divided by 2/3.”     

Dynamics of vibrational overtone excitations of H2SO4, H2SO4-H2O: hydrogen-hopping and photodissociation processes

Photochemical processes of sulfuric acid (H2SO4) and sulfuric acid monohydrate (H2SO4-H2O) following overtone excitation of the OH stretching mode are studied by classical trajectory simulations using the semiempirical PM3 potential suface in "on the fly" calculations. The main results are the following: (1) In the excitation of H2SO4 to the fifth OH-stretch overtone, hopping of the H atom between oxygen atoms is found to take place in 22% of the trajectories, only once during simulations of 400 ps. (2) All the trajectories for H2SO4 show a rapid cis-trans isomerization. (3) The photolysis of H2SO4 into SO3 + H2O takes place in 5% of the trajectories on a time scale of approximately 9 ps. (4) Only low overtone levels of H2SO4-H2O have sufficiently long lifetimes to be spectroscopically relevant. Excitation to these OH stretching overtones is found to result in the dissociation of the cluster. H hopping or dissociation of H2SO4 does not take place.     

Thermodynamic basis for promiscuity and selectivity in protein-protein interactions: PDZ domains, a case study

Like other protein-protein interaction domains, PDZ domains are involved in many key cellular processes. These processes often require that specific multiprotein complexes be assembled, a task that PDZ domains accomplish by binding to specific peptide motifs in target proteins. However, a growing number of experimental studies show that PDZ domains (like other protein-protein interaction domains) can engage in a variety of interactions and bind distinct peptide motifs. Such promiscuity in ligand recognition raises intriguing questions about the molecular and thermodynamic mechanisms that can sustain it. To identify possible sources of promiscuity and selectivity underlying PDZ domain interactions, we performed molecular dynamics simulations of 20 to 25 ns on a set of 12 different PDZ domain complexes (for the proteins PSD-95, Syntenin, Erbin, GRIP, NHERF, Inad, Dishevelled, and Shank). The electrostatic, nonpolar, and configurational entropy binding contributions were evaluated using the MM/PBSA method combined with a quasi-harmonic analysis. The results revealed that PDZ domain interactions are characterized by overwhelmingly favorable nonpolar contributions and almost negligible electrostatic components, a mix that may readily sustain promiscuity. In addition, despite the structural similarity in fold and in recognition modes, the entropic and other dynamical aspects of binding were remarkably variable not only across PDZ domains but also for the same PDZ domain bound to distinct ligands. This variability suggests that entropic and dynamical components can play a role in determining selectivity either of PDZ domain interactions with peptide ligands or of PDZ domain complexes with downstream effectors.     

Pittcon provides a platform for interesting, important and novel work

The Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, better known as Pittcon, was held this year in New Orleans, LA, USA, 2–7 March. The technical program was broad and impressive, with the world’s experts from industry, government and academia describing their recent work. Our report is not intended to be comprehensive. It is based on what struck us as being interesting, important and novel.     

Simulations of electron transfer in grazing incidence ion–surface collisions

During the interaction of a low energy ion beam with a metallic surface in grazing incidence, electron transfer plays an important role in the final state of the scattered beam. In this work we compare two approaches—rate equations resolution and a Monte Carlo numerical code named ETISC1D—to describe the beam evolution and to compute the final populations of the various atomic and ionic species as well as the corresponding angular distributions. Results of both methods are compared to experimental data obtained by Hecht et al. for 2.3 keV He⁺(1s) and He*(1s2s, ³S₁) beams impinging on an Al(1 1 1) surface under 0.79° of incidence. The limitations of the rate equations method and the advantages of Monte Carlo simulations are pointed out. In particular, we show that although the rate equation approach can give a fast and rather accurate evaluation of the populations, it is unable to provide correct calculations of more sensitive quantities like angular distributions of scattered species as obtained with the ETISC1D code.     

Une méthode semi-paramétrique pour tester un modèle de régression

The purpose is to test the hypothesis H₀ that a regression model is parametric and belongs to a given family versus the alternative H₁ approaches the hypothesis from a specific direction at the rate n^?1/2. For that, we consider an empirical process such that under H₀ this process depends of a parameter θ₀. First, we start by estimating the parameter and we prove that the empirical process converges in distribution to a certain Gaussian process when the parameter is replaced by its estimator $\text{θ}{}_{\mathrm{n}}\text{.}$ However it is important to check the impact of an alternative approaching H₀ from a specific direction ( at the rate n^1/2). For that, we need tests which are consistent on the whole of H₁. Our idea is to use a marked empirical process based on residuals which converges in distribution to a Gaussian process. To cite this article: M. Harel, C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

Testing nonstationary and absolutely regular nonlinear time series models

Statistical Inference for Stochastic Processes - We study some general methods for testing the goodness-of-fit of a general nonstationary and absolutely regular nonlinear time series model. These...     

Assessment of 6'- and 6'-N-acylation of aminoglycosides as a strategy to overcome bacterial resistance

Amongst the many synthetic aminoglycoside analogues that were developed to regain the efficacy of this class of antibiotics against resistant bacterial strains, the 1-N-acylated analogues are the most clinically used. In this study we demonstrate that 6'-N-acylation of the clinically used compound tobramycin and 6'-N-acylation of paromomycin result in derivatives resistant to deactivation by 6'-aminoglycoside acetyltransferase (AAC(6')) which is widely found in aminoglycoside resistant bacteria. When tested against AAC(6')- or AAC(3)-expressing bacteria as well as pathogenic bacterial strains, some of the analogues demonstrated improved antibacterial activity compared to their parent antibiotics. Improvement of the biological performance of the N-acylated analogues was found to be highly dependent on the specific aminoglycoside and acyl group. Our study indicates that as for 1-N-acylation, 6'- and 6'-N-acylation of aminoglycosides offer an additional promising direction in the search for aminoglycosides capable of overcoming infections by resistant bacteria.