Generalite de l'effet structural d'un motif bitertiaire en rmn 13C: effet gem-6 de 6ème substitution methyle

In di-tertiary groups like (Me₃C)₂¹³C, the sixth methyl substituent $\text{inverts}$ or $\text{attenuates}$ the induced ¹³C chemical shift of sp² or sp³ sites (B_σ and y_π effects).     

Structural, electronic, and bonding properties of zeolite Sn-beta: a periodic density functional theory study

The structural, electronic, and the bonding properties of the zeolite Sn-beta (Sn-BEA) have been investigated by using the periodic density functional theory. Each of the nine different T-sites in BEA were substituted by Sn atoms and all the nine geometries were completely optimized by using the plane-wave basis set in conjunction with the ultra-soft pseudopotential. On the basis of the structural and the electronic properties, it has been demonstrated that the substitution of Sn atoms in the BEA framework is an endothermic process and hence the incorporation of Sn in the BEA is limited. The lowest unoccupied molecular orbitals (LUMO) energies have been used to characterize the Lewis acidity of each T-site. On the basis of the relative cohesive energy and the LUMO energy, the T2 site is shown to be the most favorable site for the substitution Sn atoms in the BEA framework.     

Streptavidin chiral stationary phase for the separation of adenosine enantiomers

In this paper, a microbore column packed with streptavidin particles was used, at various temperatures (0-24 degrees C), to separate the adenosine enantiomers by HPLC. Using an aqueous mobile phase, the apparent enantioseparation was high for a small molecule, varying from 11.5 at 0 degrees C to 6.2 at 24 degrees C. From the experiments carried out with a streptavidin-biotin complex stationary phase, it was demonstrated that the blockage of the biotin sites of the immobilized streptavidin was responsible for a strong decrease in the enantioselectivity via a direct and/or an indirect effect. From the analysis of the concentration dependencies of the solute retention factor, it was also shown that a reduction of the D-adenosine specific binding sites occurred at the lowest temperature. The thermodynamic parameters determined from the van't Hoff plots indicated that the D-adenosine binding to the streptavidin specific sites was enthalpically driven.     

SELECTIVE DNA THYMINE DIMERIZATION DURING UVA IRRADIATION IN THE PRESENCE OF A SATURATED PYRIDOPSORALEN

Abstract— It has been recently shown that UVA (320–400 nm) irradiation of DNA in the presence of pyridopsoralens induces the formation of thymine cyclobutane dimers in addition to monoadducts. In this work, we measured the potency of a saturated pyridopsoralen to photosensitize DNA, despite its inability to covalently attach to DNA. First, from spectroscopic fluorescence measurements, we have shown that both analogs, saturated and unsaturated pyridopsoralens, namely 4',5'-dihydro-7-methyl-pyrido[3,4-clpsoralen (DH-MePyPs) and 7-methylpyrido[3,4-c]psoralen, exhibit a similar global affinity for DNA. Secondly, we demonstrated, by footprinting experiments, that exposure of a DNA sequence to 365 nm UV radiation in the presence of DH-MePyPs results in selective cyclobutane thymine dimerization. Thymines located in the immediate proximity of the 5'-TA-3' step are exclusively affected and the frequency of this photoprocess depends on flanking sequences. We thus probe a selective thymine dimer photosensitizer. Results are discussed in terms of drug affinity and physical properties of the helix at the binding site.     

Least median of squares: a robust method for outlier and model error detection in regression and calibration

The least median of squares method is a robust regression method, which means that it is not sensitive to outliers or other violations of the assumption of the usual normal model. This contrasts with the conventional regression method, which minimizes the sum of squares. It is demonstrated that the proposed method can be used to detect or correct for outliers or model errors in calibration applications and in comparing two procedures.     

A comparative study of structural, acidic and hydrophilic properties of Sn-BEA with Ti-BEA using periodic density functional theory

Periodic density functional theory has been employed to characterize the differences in the structural, Lewis acidic and hydrophilic properties of Sn-BEA and Ti-BEA. We show that the incorporation of Sn increases the Lewis acidity of BEA compared to the incorporation of Ti. Hence, the present work gives insight into the role of Sn in increasing the efficiency of the oxidation reactions. The results also justify that the percentage of Sn substituted in BEA is less than Ti. The structural analysis shows that the first coordination shell of Sn is larger than that of Ti. However, the second coordination of both sites remains the same. The water adsorption properties of these substituted zeolites are quantified. Moreover, we explain the higher Lewis acidity of Sn than the Ti site on the basis of the Fukui functions and charge population analysis.     

Interaction of human plasma fibrinogen with commercially pure titanium as studied with atomic force microscopy and X-ray photoelectron spectroscopy

The surface of a biomaterial interacts with the body fluid upon implantation in the human body. The biocompatibility of a material is strongly influenced by the adsorption of proteins onto the surface. Titanium is frequently used as a biomaterial for implants in orthopedics and cardiovascular devices. Understanding the biocompatibility is very important to improve implants. The surface chemistry of an implant material and its influence on the interaction with body fluid is crucial in that perspective. The main goal of this study was to investigate the conformation of human plasma fibrinogen (HPF) adsorbed on commercially pure titanium (CP Ti) on a molecular level by means of ex situ atomic force microscopy (AFM). With X-ray photoelectron spectroscopy combined with argon ion beam depth profiling, it was shown that the oxide layer present at the surface was mainly composed of TiO2, with a small percentage of Ti2O3. Ex situ AFM imaging showed the conformation of HPF on CP Ti. Single molecules and aggregates of fibrinogen were observed. The trinodular structure of single HPF molecules (two spherical D domains at the distal ends of the extended molecule and the central spherical E domain) adsorbed onto CP Ti was visualized. Aggregate formation through the connection of the D domains of the HPF molecules was observed on CP Ti. The alphaC domains of HPF were not visible on CP Ti. The ex situ AFM images indicated conformational changes of HPF upon adsorption onto CP Ti. The conformation of the adsorbed HPF molecules was different on mica and titanium. The difference in wettability between both substrates caused a larger spread of the protein on the CP Ti surface and thus resulted in a larger perturbation to the native structure of HPF as compared to mica.     

Magnetic effects of disulfide bridges: a density functional and semiempirical study

Density functional chemical shielding calculations are reported for methane and hydrogen disulfide dimers. The calculations show that the contributions of disulfide bridges to the chemical shielding of neighboring protons is sizable at distances that are frequently sampled in protein structures. A semiempirical model of the quantum chemical data is developed. It is shown that magnetic anisotropy effects of disulfide are poorly described by the McConnell equation, both qualitatively and quantitatively. In particular, the ratio of magnetic anisotropy contributions to shielding along and perpendicular to the magnetic anisotropy principal axis do not conform to the predictions of the McConnell equation, and magnetic anisotropy effects are not null along the magic angle axis. A sulfur-based model of the magnetic anisotropy of the disulfide is developed and shown to give much better agreement with the quantum chemical data.     

Comparaison des effets α-methyle sur des sites 13C et 15N a l'aide du modele topologique (α-Me) par difference lineaire (DARC-PULFO)

The extension to ¹⁵N NMR of the model by linear difference previously proposed for (α-Me.) ¹³C shifts gathers in an unifying treatment the different behaviours observed for (α-Me.) shifts in ¹⁵M and ¹³C of, aliphatic amines, hydrochlorides and alkanes.     

Electrochemical study of intermetallic metal hydride as an anode material for Ni–MH batteries

The electrochemical behavior of Cobalt-free LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 alloy electrode in alkaline solution was investigated using electrochemical impedance spectroscopy (EIS) at different number of charge/discharge cycles. A physicochemical model is developed in order to simulate impedance data. Kinetic parameters are obtained by fitting the electrochemical impedance spectrum performed at different number of cycles. The charge-transfer resistance decreases with increasing number of charge/discharge of cycles, whereas exchange current density and hydrogen diffusion coefficient parameters increase with increasing number of cycles. In addition, the specific surface area of LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 alloy electrode increases due to pulverization and the formation of new active sites during charge/discharge cycling. The results of EIS measurements indicate that the performance of the LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 metal hydride electrode was markedly improved with increasing number of cycles which is mainly attributed to the increase in the reaction surface area and the improvement in the electrode surface activation.