Bloch oscillations at room temperature in graphene/h-BN electrostatic superlattices

In this manuscript, we report the fabrication and the measurements of Bloch oscillations at room temperature in electrostatic graphene/h-BN superlattices. The electrostatic superlattice consists of an array of tilted metallic electrodes deposited over graphene monolayer/h-BN monolayer grown on 2 inch wafers of doped Si/SiO₂. We show the formation of minibands at room temperature, negative differential resistance and the evidence of Bloch oscillations. The experimental results are sustained by numerical modelling of these 2D superlattices. The experimental results are sustained by numerical modelling of these 2D superlattices. At room temperature, we have measured a period of Bloch oscillations of 55 meV which corresponds to a frequency of 13 THz.

Wafer-scale graphene-ferroelectric HfO2/Ge-HfO2/HfO2 transistors acting as three-terminal memristors

In this paper we report a set of experiments at the wafer level regarding field-effect transistors with a graphene monolayer channel transferred on the ferroelectric HfO₂/Ge-HfO₂/HfO₂ three-layer structure. This kind of transistor has a switching ratio of 10³ between on and off states due to the bandgap in graphene induced by the ferroelectric structure. Both top and back gates effectively control the carriers' charge flow in graphene. The transistor acts as a three-terminal memristor, termed a memtransistor, with applications in neuromorphic computation.

Aspartate 458 of human glutathione synthetase is important for cooperativity and active site structure

Human glutathione synthetase (hGS) catalyzes the second ATP-dependent step in the biosynthesis of glutathione (GSH) and is negatively cooperative to the γ-glutamyl substrate. The hGS active site is composed of three highly conserved catalytic loops, notably the alanine rich A-loop. Experimental and computational investigations of the impact of mutation of Asp458 are reported, and thus the role of this A-loop residue on hGS structure, activity, negativity cooperativity and stability is defined. Several Asp458 hGS mutants (D458A, D458N and D458R) were constructed using site-directed mutagenesis and their activities determined (10%, 15% and 7% of wild-type hGS, respectively). The Michaelis-Menten constant (K(m)) was determined for all three substrates (glycine, GAB and ATP): glycine K(m) increased by 30-115-fold, GAB K(m) decreased by 8-17-fold, and the ATP K(m) was unchanged. All Asp458 mutants display a change in cooperativity from negative cooperativity to non-cooperative. All mutants show similar stability as compared to wild-type hGS, as determined by differential scanning calorimetry. The findings indicate that Asp458 is essential for hGS catalysis and that it impacts the allostery of hGS.

The correlation-consistent composite approach: application to the G3/99 test set

The correlation-consistent composite approach (ccCA), an ab initio composite technique for computing atomic and molecular energies, recently has been shown to successfully reproduce experimental data for a number of systems. The ccCA is applied to the G3/99 test set, which includes 223 enthalpies of formation, 88 adiabatic ionization potentials, 58 adiabatic electron affinities, and 8 adiabatic proton affinities. Improvements on the original ccCA formalism include replacing the small basis set quadratic configuration interaction computation with a coupled cluster computation, employing a correction for scalar relativistic effects, utilizing the tight-d forms of the second-row correlation-consistent basis sets, and revisiting the basis set chosen for geometry optimization. With two types of complete basis set extrapolation of MP2 energies, ccCA results in an almost zero mean deviation for the G3/99 set (with a best value of -0.10 kcal mol(-1)), and a 0.96 kcal mol(-1) mean absolute deviation, which is equivalent to the accuracy of the G3X model chemistry. There are no optimized or empirical parameters included in the computation of ccCA energies. Except for a few systems to be discussed, ccCA performs as well as or better than Gn methods for most systems containing first-row atoms, while for systems containing second-row atoms, ccCA is an improvement over Gn model chemistries.

A molecular modeling study on the enantioselectivity of aryl alkyl ketone reductions by a NADPH-dependent carbonyl reductase

Automated structural analysis of Sporobolomyces salmonicolor carbonyl reductase (SSCR) indicates that the two largest potential receptor sites are in the vicinity of the nicotinamide reductant. The largest receptor site is a scalene triangle with sides of approximately 8 A by 9 A by 13 A, which is narrow in width; one corner is surrounded by hydrophilic residues that can favorably bond with the ketone oxygen. Docking aryl alkyl ketones shows a distinct preference for binding to the largest receptor site, and for conformations that place the carbonyl oxygen of the substrate in the hydrophilic corner of the largest receptor site. Favorable docking conformations for aryl alkyl ketones fall into two low-energy ensembles. These conformational ensembles are distinguished by the positions of the substituents, presenting either the Si- or Re-face of the ketone to the nicotinamide reductant. For the ketones investigated here, there is a correspondence between the major enantiomer of the alcohol obtained from the reduction of the ketone and the conformer found to have the most stable interaction energy with the receptor site in all cases. The receptor site modeling, docking simulations, molecular dynamics, and enzyme-substrate geometry optimizations lead to a model for understanding the enantioselectivity of this NADPH-dependent carbonyl reductase.

Catalytic loop motion in human glutathione synthetase: A molecular modeling approach

Conformational changes of three flexible loops (G, A, and S) in human glutathione synthetase (hGS) arise to accommodate the substrates inside the active site. The crystal structure of hGS, a member of the ATP-grasp superfamily, has been reported only for the product-enzyme complex. To study the function of the hGS loops, molecular dynamics simulations are performed on three different conformational models: unbound enzyme, reactant-enzyme, and product-enzyme complex of hGS. The conformational changes among the three models are analyzed and the roles of the loops during the catalytic process are described. The modeled structures of hGS show that the central portions of the G- and A-loop have a double role in the reactant complex conformation: they bind the substrates and simultaneously interact with each other through an extensive network of hydrogen bonds. The present study proposes that these favorable loop-ligand and loop-loop interactions are required for opening and closing of the active site of hGS. Additionally, this research identifies important amino acid residues and explains their function within the catalytic loops of hGS.

Oxygen Atom Transfer Energetics: Assessment of the Effect of Method and Solvent

Several density functional methods, the semiempirical methods AM1 and PM3, Hartree-Fock, and Gaussian3 theories were applied to compute the oxygen atom transfer enthalpies for 14 X/XO couples (inorganic and organic systems, charged and neutral species, light and heavy main group element containing molecules). The calculated reaction enthalpies were compared to available experimental data. The G3 method alone was found to perform within the experimental error, while the popular B3LYP and BLYP functionals provided inadequate results. Solvent effects were estimated for 19 neutral and anionic X/XO couples by using the conductor-like polarizable continuum model and several cavity models coupled with the B3LYP/6-31++G(2d,2p) level of theory. Surprisingly, the magnitude of the aqueous solvent correction was found to vary significantly for different solute cavity models, occasionally giving larger errors than the gas-phase calculation.

The Butterfly Dimer [(tBu3SiO)Cr]2(μ-OSitBu3)2 and Its Oxidative Cleavage to (tBu3SiO)2Cr(N−NCPh2)2 and (tBu3SiO)2CrN(2,6-Ph2-C6H3)

Treatment of CrCl2(THF)2 with NaOSitBu3 afforded the butterfly dimer [(tBu3SiO)Cr]2(mu-OSitBu3)2 (1(2)), whose d(CrCr) of 2.658(31) A and magnetism were indicative of strong antiferromagnetic coupling. A Boltzmann distribution of low-energy 1A1, 3B1, 5A1, 7B1, and 9A1 states obtained from calculations on [(HO)2Cr]2(muOH)2 (1'(2)) were used to provide a reasonable fit of the mu(eff) vs T data. Cleavage of 1(2) with various L (L = 4-picoline, p-tolunitrile, tBuCN, tBuNC, Ph2CO, and PMe3) generated (tBu3SiO)2CrL2 (1-L2). The dimer was oxidatively severed by Ph2CN2 to give (tBu3SiO)2Cr(N2CPh2)2 (2) and by RN3 at 23 degrees C to afford (silox)2Cr=NR (3-R) for bulky R (adamantyl (Ad), 2,6-iPr2-C6H3, 2,4,6-Me3-C6H2 = Mes, 2,6-Ph2-C6H3) and (tBu3SiO)2Cr(=NR)2 (4-R) for smaller substituents (R = 1-Naph, 2-Anth). X-ray structural studies were conducted on 1(2), square planar 1-(OCPh2)2, pseudo-Td 2 and pseudo-trigonal 3-(2,6-Ph2-C6H3), whose S = 1 ground state was discussed on the basis of calculations of (H3SiO)2Cr=NPh (3' '-Ph).

A T-Shaped Three-Coordinate Nickel(I) Carbonyl Complex and the Geometric Preferences of Three-Coordinate d9 Complexes

A three-coordinate diketiminate-nickel(I) complex with a carbonyl ligand has been characterized using EPR and IR spectroscopies and X-ray crystallography. The T geometry (bending from the sterically favored C(2)(v)() structure) contrasts with that of isosteric d(9) copper(II) complexes. DFT calculations on a truncated model reproduce experimental geometries, implying that the geometric differences are electronic in nature. Analysis of the charge distribution in the complexes shows that the geometry of the three-coordinate d(9) complexes is affected by differential charge donation of the ligands to the metal center.

Stability Studies of Transition-Metal Linkage Isomers Using Quantum Mechanical Methods. Groups 11 and 12 Transition Metals

Several hypotheses to elucidate the linkage isomer preference of the thiocyanate (SCN(-)) ion have been offered. For complexes with small coordination numbers (i.e., 1 and 2) and groups 11 (Cu-triad) and 12 (Zn-triad) metals, different levels of theory and a variety of basis sets have been employed to study linkage isomerism. Similar results are obtained for all density functionals tested, pure and hybrid. Overall, good agreement, vis-à-vis experimentally identified linkage isomers, is achieved for ab initio techniques, whereas semiempirical quantum mechanical methods show a bias toward S-ligated isomers. Despite the seeming ease for the a priori prediction of the most stable thiocyanate isomers using acid/base principles, this research highlights the sensitivity of quantitative calculations of transition-metal linkage isomerism to the choice of basis set and electron correlation, particularly with post-Hartree-Fock treatments.

Function of conserved residues of human glutathione synthetase: implications for the ATP-grasp enzymes

Glutathione synthetase is an enzyme that belongs to the glutathione synthetase ATP-binding domain-like superfamily. It catalyzes the second step in the biosynthesis of glutathione from gamma-glutamylcysteine and glycine in an ATP-dependent manner. Glutathione synthetase has been purified and sequenced from a variety of biological sources; still, its exact mechanism is not fully understood. A variety of structural alignment methods were applied and four highly conserved residues of human glutathione synthetase (Glu-144, Asn-146, Lys-305, and Lys-364) were identified in the binding site. The function of these was studied by experimental and computational site-directed mutagenesis. The three-dimensional coordinates for several human glutathione synthetase mutant enzymes were obtained using molecular mechanics and molecular dynamics simulation techniques, starting from the reported crystal structure of human glutathione synthetase. Consistent with circular dichroism spectroscopy, our results showed no major changes to overall enzyme structure upon residue mutation. However, semiempirical calculations revealed that ligand binding is affected by these mutations. The key interactions between conserved residues and ligands were detected and found to be essential for enzymatic activity. Particularly, the negatively charged Glu-144 residue plays a major role in catalysis.

[Retroperitoneal appendicular abscess]

This article presents a 28 years old male patient case with a retroperitoneal collection of apendicular origin. After few comments about clinical and paraclinical aspects, the authors emphasise the particular aspects of differential diagnosis and surgical approach which, finally, lead to a full recovery of the patient.