Synthesis of hybrid gold-gold sulfide colloidal particles

The nucleation and growth mechanism of nanometer size gold onto gold sulfide colloidal particles by irradiation-induced reduction is reported. The process is characterized by ultraviolet-visible spectroscopy, electronic diffraction, and high-resolution transmission electron microscopy, allowing for observation of several key intermediates and characteristics of the growth mechanism. The formation mechanism of gold on the surface of the gold sulfide particles is shown to depend strongly on the deposition rate. At low dose rate, gold nucleates preferentially onto specific gold-rich Au2S facets {110}, resulting in epitaxial growth. The gold crystal lattice plastically deforms near the interface to accommodate a substantial lattice mismatch. Upon increasing gold precursor concentration, this low dose rate results in growth of elongated gold island on the gold sulfide surface. At a high dose rate, several randomly oriented gold particles are simultaneously produced on gold sulfide, resulting in a layered structure. The absorption spectra of these particles show a dominant surface plasmon band, whose peak wavelength shifts markedly to the red as layered structure is formed.     

Mangrove trees growing in a very saline condition but not using seawater

Mangrove trees, which develop along tropical coasts, are known to use saline water uptake. In French Guiana, the high salinity condition is the result of seawater evaporation on mud banks formed from the Amazon sediment flumes. In the back mangrove a few kilometres inland, groundwater, soil water and the xylem sap uptake in the trees remain highly salty, and only very tolerant plants like Avicennia germinans can flourish, whereas the less salt-tolerant Rhizophora mangle is more difficult to find. Curiously, the same Avicennia trees propagate on the seafront. However, stable isotope ratio mass spectrometry (IRMS) measurements and ion analysis (high-performance liquid chromatography (HPLC) and inductively coupled plasma atomic emission (ICP-AES) spectroscopy reveal that the origin of the water in the back mangrove is not seawater. It is freshwater percolating into the sand bars from the inland marshes and rainwater during the wet season that redissolves a marine evaporite and gives a saline groundwater. The absence of barren saltine areas ('tanne') in French Guiana could be explained by this freshwater inflow, the aquifer being no longer linked with the ocean. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Quantum mechanical/molecular mechanical study of mechanisms of heme degradation by the enzyme heme oxygenase: the strategic function of the water cluster

Heme degradation by heme oxygenase (HO) enzymes is important in maintaining iron homeostasis and prevention of oxidative stress, etc. In response to mechanistic uncertainties, we performed quantum mechanical/molecular mechanical investigations of the heme hydroxylation by HO, in the native route and with the oxygen surrogate donor H2O2. It is demonstrated that H2O2 cannot be deprotonated to yield Fe(III)OOH, and hence the surrogate reaction starts from the FeHOOH complex. The calculations show that, when starting from either Fe(III)OOH or Fe(III)HOOH, the fully concerted mechanism involving O-O bond breakage and O-C(meso) bond formation is highly disfavored. The low-energy mechanism involves a nonsynchronous, effectively concerted pathway, in which the active species undergoes first O-O bond homolysis followed by a barrier-free (small with Fe(III)HOOH) hydroxyl radical attack on the meso position of the porphyrin. During the reaction of Fe(III)HOOH, formation of the Por+*FeIV=O species, compound I, competes with heme hydroxylation, thereby reducing the efficiency of the surrogate route. All these conclusions are in accord with experimental findings (Chu, G. C.; Katakura, K.; Zhang, X.; Yoshida, T.; Ikeda-Saito, M. J. Biol. Chem. 1999, 274, 21319). The study highlights the role of the water cluster in the distal pocket in creating "function" for the enzyme; this cluster affects the O-O cleavage and the O-Cmeso formation, but more so it is responsible for the orientation of the hydroxyl radical and for the observed alpha-meso regioselectivity of hydroxylation (Ortiz de Montellano, P. R. Acc. Chem. Res. 1998, 31, 543). Differences/similarities with P450 and HRP are discussed.     

Interaction between water-soluble peptidic CdSe/ZnS nanocrystals and membranes: formation of hybrid vesicles and condensed lamellar phases

Due to their tunable optical properties and their well-defined nanometric size, core/shell nanocrystals (quantum dots, QDs) are extensively used for the design of biomarkers as well as for the preparation of nanostructured hybrid materials. It is thus of great interest to understand their interaction with soft lipidic membranes. Here we present the synthesis of water-soluble peptide CdSe/ZnS QDs and their interaction with the fluid lipidic membrane of vesicles. The use of short peptides results in the formation of small QDs presenting both high fluorescence quantum yield and high colloidal stability as well as a mean hydrodynamical diameter of 10 nm. Their interaction with oppositely charged vesicles of various surface charge and size results in the formation of hybrid giant or large unilamellar vesicles covered with a densely packed layer of QDs without any vesicle rupture, as demonstrated by fluorescence resonance energy transfer experiments, zetametry, and optical microscopy. The adhesion of nanocrystals onto the vesicle membrane appears to be sterically limited and induces the reversion of the surface charge of the vesicles. Therefore, their interaction with small unilamellar vesicles induces the formation of a well-defined lamellar hybrid condensed phase in which the QDs are densely packed in the plane of the layers, as shown by freeze-fracture electron microscopy and small-angle X-ray scattering. In this structure, strong undulations of the bilayer maximize the electrostatic interaction between the QDs and the bilayers, as previously observed in the case of DNA polyelectrolytes interacting with small vesicles.     

Structure and magnetic properties of oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) with Fe2O square planar layers representing an antiferromagnetic checkerboard spin lattice

The oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se), which contain Fe2O square planar layers of the anti-CuO2 type, were predicted using a modular assembly of layered secondary building units and subsequently synthesized. The physical properties of these compounds were characterized using magnetic susceptibility, electrical resistivity, specific heat, (57)Fe Mossbauer, and powder neutron diffraction measurements and also by estimating their exchange interactions on the basis of first-principles density functional theory electronic structure calculations. These compounds are magnetic semiconductors that undergo a long-range antiferromagnetic ordering below 83.6-106.2 K, and their magnetic properties are well-described by a two-dimensional Ising model. The dominant antiferromagnetic spin exchange interaction between S = 2 Fe(2+) ions occurs through corner-sharing Fe-O-Fe bridges. Moreover, the calculated spin exchange interactions show that the A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) compounds represent a rare example of a frustrated antiferromagnetic checkerboard lattice.     

alpha-glucosidase inhibitors from Garcinia brevipedicellata (Clusiaceae)

In our continuous search for alpha-glucosidase inhibitors from plants, four new depsidones named brevipsidones A-D (1-4) were isolated from stem bark of Garcinia brevipedicellata together with known damnacanthal, scopoletin and a mixture of stigmasterol and beta-sitosterol. Structural elucidations were made by spectroscopic analyses including 2D-NMR data.     

Aryl Radical Geometry Determines Nanographene Formation on Au(111)

The Ullmann coupling has been used extensively as a synthetic tool for the formation of C?C bonds on surfaces. Thus far, most syntheses made use of aryl bromides or aryl iodides. We investigated the applicability of an aryl chloride in the bottom‐up assembly of graphene nanoribbons. Specifically, the reactions of 10,10′‐dichloro‐9,9′‐bianthryl (DCBA) on Au(111) were studied. Using atomic resolution non‐contact AFM, the structure of various coupling products and intermediates were resolved, allowing us to reveal the important role of the geometry of the intermediate aryl radicals in the formation mechanism. For the aryl chloride, cyclodehydrogenation occurs before dehalogenation and polymerization. Due to their geometry, the planar bisanthene radicals display a different coupling behavior compared to the staggered bianthryl radicals formed when aryl bromides are used. This results in oligo‐ and polybisanthenes with predominantly fluoranthene‐type connections.     

Multifrequency electron paramagnetic resonance study on deproteinized human bone

Irradiated samples of deproteinized powdered human bone (femur) have been examined by electron paramagnetic resonance (EPR) spectroscopy in X, Q and W bands. In the bone powder sample only one type of CO2- radical ion is stabilized in the hydroxyapatite structure in contrast to powdered human tooth enamel, a material also containing hydroxyapatite, widely used for EPR dosimetry and in which a few radicals are stable at room temperature. It is suggested that the use of deproteinized bone for EPR dosimetry could improve the accuracy of dose determination.     

Slow electron velocity-map imaging spectroscopy of the C4H- and C4D- anions

High resolution photodetachment spectra of C4H- and C4D- obtained via slow electron velocity-map imaging (SEVI) are presented. The spectra reveal closely spaced transitions to the neutral 2Sigma+ and 2Pi states which can be distinguished based on the corresponding photoelectron angular distributions. The C4H ground state is confirmed as the X2Sigma+ state, with the excited A2Pi state lying only 213 cm(-1) higher (201 cm(-1) for C4D). The electron affinities (EAs) are slightly revised to EA (C4H)=28,497+/-8 cm(-1) and EA (C4D)=28,478+/-10 cm(-1). Progressions in low frequency bending vibrations are observed in both states, yielding experimental frequencies of nu7=179(169) cm(-1) and nu6=408(392) cm(-1) for the X2Sigma+ state of C4H (C4D), and nu7=220(215)cm(-1) and nu6=446(437) cm(-1) for the A2Pi state.     

On the Landau-Levich transition

We discuss here the nature of the Landau-Levich transition, that is, the dynamical transition that occurs when drawing a solid out of a bath of a liquid that partially wets this solid. Above a threshold velocity, a film is entrained by the solid. We measure the macroscopic contact angle between the liquid and the solid by different methods, and conclude that this angle might be discontinuous at the transition. We also present a model to understand this fact and the shape of the meniscus as drawing the solid.