Efficiency of random search procedures along the silicon cluster series: Si(n) (n=5-10, 15, and 20)

The efficiency of the simplest isomeric search procedure consisting in random generation of sets of atomic coordinates followed by density functional theory geometry optimization is tested on the silicon cluster series (Si(5-10, 15, 20)). Criteria such as yield, isomer distributions and recurrences are used to clearly establish the performance of the approach with respect to increasing cluster size. The elimination of unphysical candidate structures and the use of distinct box shapes and theoretical levels are also investigated. For the smaller Si(n) (n=5-10) clusters, the generation of random coordinates within a spherical box is found to offer a reasonable alternative to more complex algorithms by allowing straightforward identification of every known low-lying local minima. The simple stochastic search of larger clusters (i.e. Si(15) and Si(20)) is however complicated by the exponentially increasing number of both low- and high-lying minima leading to rather arbitrary and non-comprehensive results.     

Synthesis of New N‐(5‐Oxo‐2,5‐dihydro)pyrrol‐3‐yl Glycines and N‐(5‐Oxo‐2,5‐dihydro)pyrrol‐3‐yl Glycines Esters

Following our efforts towards the synthesis of new potential inhibitors of Xanthine Dehydrogenase (XDH), we describe here a general method for the preparation of N-(5-oxo-2,5-dihydro)pyrrol-3-yl glycines and N-(5-oxo-2,5-dihydro)pyrrol-3-yl glycine esters from glycine ethyl ester hydrochloride and various 4-hydroxy-5-oxo-2,5-dihydro-1H-pyrrole-3-carboxylic acid esters and carbonitriles.     

Heterogeneity of the surface energy on unused C18-chromolith adsorbents in reversed-phase liquid chromatography

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for phenol and caffeine on a new C18-Chromolith column (Merck, Darmstadt, Germany), using a water-rich mobile phase (methanol/water, 15/85, v/v). These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions, based on the expectation-maximization (EM) procedure, were also derived and used for the selection of the best isotherm model. The adsorption energy distributions (AEDs) for phenol and caffeine converged toward a trimodal and a quadrimodal distribution, respectively. Energy distributions with more than two modes had not been reported before for the adsorption of these compounds on packed columns. The third high energy mode observed for both phenol and caffeine seems to be specific of the surface of the monolithic column while the first and second low energy modes have the same physical origin as the two modes detected on packed columns. These results suggest significant differences between the structures of the porous silica in these different materials.     

Systems-based design of bi-ligand inhibitors of oxidoreductases: filling the chemical proteomic toolbox

Genomics-driven growth in the number of enzymes of unknown function has created a need for better strategies to characterize them. Since enzyme inhibitors have traditionally served this purpose, we present here an efficient systems-based inhibitor design strategy, enabled by bioinformatic and NMR structural developments. First, we parse the oxidoreductase gene family into structural subfamilies termed pharmacofamilies, which share pharmacophore features in their cofactor binding sites. Then we identify a ligand for this site and use NMR-based binding site mapping (NMR SOLVE) to determine where to extend a combinatorial library, such that diversity elements are directed into the adjacent substrate site. The cofactor mimic is reused in the library in a manner that parallels the reuse of cofactor domains in the oxidoreductase gene family. A library designed in this manner yielded specific inhibitors for multiple oxidoreductases.     

Photoinduced electron transfer in Zn(II)porphyrin-bridge-Pt(II)acetylide complexes: variation in rate with anchoring group and position of the bridge

The synthesis and photophysical characterization of two sets of zinc porphyrin platinum acetylide complexes are reported. The two sets of molecules differ in the way the bridging phenyl-ethynyl unit is attached to the porphyrin ring. One set is attached via an ethynyl unit on the β position, while the other set is attached via a phenyl unit on the meso position of the porphyrin. These were compared with previously studied complexes where attachment was made via an ethynyl unit on the meso position. Femtosecond transient absorption measurements showed in all systems a rapid quenching of the porphyrin singlet state. Electron transfer is suggested as the quenching mechanism, followed by an even faster recombination to form both the porphyrin ground and triplet excited states. This is supported by the variation in quenching rate and porphyrin triplet yield with solvent polarity, and the observation of an intermediate state in the meso-phenyl linked systems. The different linking motifs between the dyads resulted in significant variations in electron transfer rates.     

Factors affecting the reactivity of thiol-functionalized mesoporous silica adsorbents toward mercury(II)

Numerous mercaptopropyl-functionalized silica spheres have been prepared by either post-synthesis grafting of MCM-41 and MCM-48 or self-assembly co-condensation of mercaptopropyltrimethoxysilane (MPTMS) or mercaptopropyltriethoxysilane (MPTES) and tetraethoxysilane (TEOS) precursors in hydroalcoholic medium in the presence of a cationic surfactant as templating agent and ammonia as catalyst. These materials of approximately the same particle size and morphology featured different functionalization levels, various degrees of structural order, and variable distribution of thiol groups in the mesopores. Their reactivity in solution has been studied using Hg(II) as model analyte. Total accessibility (on a 1:1 S:Hg stoichiometry basis) was demonstrated and quantified for well-ordered materials whereas less open and less organized structures with high degrees of functionalization were subject to less-than-complete loadings. Capacities measured at pH 2 were lower than at pH 4 because of distinct mercury-binding mechanisms. Kinetics associated to the uptake process were studied by in situ electrochemical monitoring of Hg(II) consumption from aqueous suspensions containing the various adsorbents. They indicate only little difference between materials of the MCM-41 and MCM-48 series at similar functionalization levels, fast mass transport in well-ordered mesostructures in comparison to the poorly or non-ordered ones (except at pH 2 where charge formation induced some restriction in materials characterized by long-range structural order), and even faster processes in the wormlike frameworks (characterized by shorter range structural order). Hg(II) binding to thiol-functionalized materials obtained by post-synthesis grafting was found to occur more rapidly in the early beginning of the uptake process as a result of a higher concentration of binding sites at the pore entrance in comparison to the more homogeneous distribution of these groups in the mesochannels of materials obtained by co-condensation.     

A p‐Type NiO‐Based Dye‐Sensitized Solar Cell with an Open‐Circuit Voltage of 0.35 V

In tandem : Employing a molecular dyad and a cobalt‐based electrolyte gives a threefold‐increase in open‐circuit voltage (V_OC) for a p‐type NiO device (V_OC=0.35 V), and a fourfold better energy conversion efficiency. Incorporating these improvements in a TiO₂/NiO tandem dye‐sensitized solar cell (TDSC), results in a TDSC with a V_OC=0.91 V (see figure; CB=conductance band, VB= valence band).

     

Online H/D exchange liquid chromatography as a support for the mass spectrometric identification of the oxidation products of melatonin

The hydrogen–deuterium exchange of protonated melatonin and its in vitro oxidation end‐products have been examined by liquid chromatography coupled with ion‐trap mass spectrometry. Specific H/D scrambling of protons in the C2 and C4 positions of the indole ring during gas‐phase fragmentation process was observed for both melatonin and its oxidation products. Collision‐induced dissociation spectra showed losses of variably deuterated NH₃, H₂O and CH₃CONH₂. In addition, a similar H/D scrambling behaviour was observed for the oxidation products, obtained from the opening of the indole ring by oxidative attack. Fragmentation pathways are proposed and H/D scrambling has been employed as a fingerprint, allowing identification of N¹‐acetyl‐5‐methoxykynurenin (AMK), N¹‐acetyl‐N²‐formyl‐5‐methoxykynurenin (AFMK), dehydro‐AFMK and hydroxymelatonin as the oxidation products of melatonin in vitro. Copyright © 2008 John Wiley & Sons, Ltd.     

Supercritical fluid chromatography hyphenated with twin comprehensive two-dimensional gas chromatography for ultimate analysis of middle distillates

This paper reports the conditions of online hyphenation of supercritical fluid chromatography (SFC) with twin comprehensive two-dimensional gas chromatography (twin-GC × GC) for detailed characterization of middle distillates; this is essential for a better understanding of reactions involved in refining processes. In this configuration, saturated and unsaturated compounds that have been fractionated by SFC are transferred on two different GC × GC columns sets (twin-GC × GC) placed in the same GC oven. Cryogenic focusing is used for transfer of fractions into the first dimension columns before simultaneous GC × GC analysis of both saturated and unsaturated fractions. The benefits of SFC–twin-GC × GC are demonstrated for the extended alkane, iso-alkane, alkene, naphthenes and aromatics analysis (so-called PIONA analysis) of diesel samples which can be achieved in one single injection. For that purpose, saturated and unsaturated compounds have been separated by SFC using a silver loaded silica column prior to GC × GC analysis. Alkenes and naphthenes are quantitatively recovered in the unsaturated and saturated fractions, respectively, allowing their identification in various diesel samples. Thus, resolution between each class of compounds is significantly improved compared to a single GC × GC run, and for the first time, an extended PIONA analysis of diesel samples is presented.