Gold-catalyzed aerobic oxidation of dibenzylamine: Homogeneous or heterogeneous catalysis?

Au(OAc)₃ is applied as an effective catalyst of the selective oxidation of dibenzylamine to dibenzylimine using molecular oxygen as the only oxidant. When Au(OAc)₃ was preadsorbed onto CeO₂, the supported catalyst was more active than any homogeneous or heterogeneous catalyst known for this reaction. Although, some fascinating color changes in the early stage of the reaction indicated the formation of an amine complex, conventional filtration experiments proved the heterogeneity of the system. The fate of the active gold component was studied by in situ X-ray absorption spectroscopy (XANES) using a specially designed cell. These investigations revealed that in the early stage of the reaction Au(OAc)₃ is dissolved and subsequently reduced by the amine and the in situ formed gold nanoparticles are the real active species of the reaction. Formation of gold nanoparticles during dibenzylamine oxidation was proved independently by transmission electron microscopy. Our findings lead to a simple synthetic procedure using a commercially available gold salt, which upon interaction with the amine forms highly active and selective gold nanoparticles.     

Chemistry or biology: which comes first after the genome is sequenced?

In drug discovery, chemical screening is performed after a protein's function has been determined. By screening for ligands that alter the function of a cell or an organism, new proteins that participate in poorly understood biological processes can be identified. Genomic and expression-cloning technologies can rapidly identify the protein targets of these ligands, enhancing the power of chemical screening as a tool for the initial stages of biological discovery.     

Bridge-bonded formate: active intermediate or spectator species in formic acid oxidation on a Pt film electrode?

We present and discuss the results of an in situ IR study on the mechanism and kinetics of formic acid oxidation on a Pt film/Si electrode, performed in an attenuated total reflection (ATR) flow cell configuration under controlled mass transport conditions, which specifically aimed at elucidating the role of the adsorbed bridge-bonded formates in this reaction. Potentiodynamic measurements show a complex interplay between formation and desorption/oxidation of COad and formate species and the total Faradaic current. The notably faster increase of the Faradaic current compared to the coverage of bridge-bonded formate in transient measurements at constant potential, but with different formic acid concentrations, reveals that adsorbed formate decomposition is not rate-limiting in the dominant reaction pathway. If being reactive intermediate at all, the contribution of formate adsorption/decomposition to the reaction current decreases with increasing formic acid concentration, accounting for at most 15% for 0.2 M DCOOH at 0.7 VRHE. The rapid build-up/removal of the formate adlayer and its similarity with acetate or (bi-)sulfate adsorption/desorption indicate that the formate adlayer coverage is dominated by a fast dynamic adsorption-desorption equilibrium with the electrolyte, and that formate desorption is much faster than its decomposition. The results corroborate the proposal of a triple pathway reaction mechanism including an indirect pathway, a formate pathway, and a dominant direct pathway, as presented previously (Chen, Y. X.; et al. Angew. Chem. Int. Ed. 2006, 45, 981), in which adsorbed formates act as a site-blocking spectator in the dominant pathway rather than as an active intermediate.     

Electrocatalytic activity of palladium–polypyrrole nanocomposite in the formaldehyde oxidation reaction

Palladium–polypyrrole nanocomposite materials with high electrocatalytic activity toward formaldehyde in an alkaline solution (methylene glycolate) have been synthesized via a one-step redox route. Key factors that have an effect on the sensor properties of palladium nanoparticles have been determined. It has been demonstrated for the first time that the emergence of not only the forward but also the reverse wave of formaldehyde oxidation on palladium particles is associated with the oxidation of methylene glycolate rather than intermediate species.     

Two secondary modes in decahydroisoquinoline: which one is the true Johari Goldstein process?

Broadband dielectric measurements were carried out at isobaric and isothermal conditions up to 1.75 GPa for reconsidering the relaxation dynamics of decahydroisoquinoline, previously investigated by Richert et al. [R. Richert, K. Duvvuri, and L.-T. Duong, J. Chem. Phys. 118, 1828 (2003)] at atmospheric pressure. The relaxation time of the intense secondary relaxation tau(beta) seems to be insensitive to applied pressure, contrary to the alpha-relaxation times tau(alpha). Moreover, the separation of the alpha- and beta-relaxation times lacks correlation between shapes of the alpha-process and beta-relaxation times, predicted by the coupling model [see for example, K. L. Ngai, J. Phys.: Condens. Matter 15, S1107 (2003)], suggesting that the beta process is not a true Johari-Goldstein (JG) relaxation. From the other side, by performing measurements under favorable conditions, we are able to reveal a new secondary relaxation process, otherwise suppressed by the intense beta process, and to determine the temperature dependence of its relaxation times, which is in agreement with that of the JG relaxation.     

Prediction of the phase structure in polymer blends: what is needed for success?

Rules for prediction of the phase structure in immiscible polymer blends from the knowledge of their composition, component properties and the flow field in a mixing or processing device are discussed. The reliability of qualitative prediction of the dependence of phase structure on system parameters is used as a criterion of plausibility of the rules. No general reliable rule for prediction of the phase structure type (continuity of phases) is available in the literature. Dependence of the droplet break‐up frequency on its size, contribution of simultaneous collisions of three or more droplets to coalescence and the effect of complex flow field on coalescence must be better described for a reliable qualitative prediction of the dependence of the droplet size on the system parameters.     

Sulfonyl vs. carbonyl group: which is the more electron-withdrawing?

Unexpectedly high reactivity of nitrogenated aromatics protected as amides or carbamates, when compared to sulfonamides, can be explained by a decrease of the aromaticity due to a greater ability of the carbon-centered groups to achieve delocalisation of the nitrogen lone pair, resulting in stronger global withdrawing effects.     

Structure sensitivity of alkynol hydrogenation on shape- and size-controlled palladium nanocrystals: which sites are most active and selective?

The activity and selectivity of structure-sensitive reactions are strongly correlated with the shape and size of the nanocrystals present in a catalyst. This correlation can be exploited for rational catalyst design, especially if each type of surface atom displays a different behavior, to attain the highest activity and selectivity. In this work, uniform Pd nanocrystals with cubic (in two different sizes), octahedral, and cuboctahedral shapes were synthesized through a solution-phase method with poly(vinyl pyrrolidone) (PVP) serving as a stabilizer and then tested in the hydrogenation of 2-methyl-3-butyn-2-ol (MBY). The observed activity and selectivity suggested that two types of active sites were involved in the catalysis--those on the planes and at edges--which differ in their coordination numbers. Specifically, semihydrogenation of MBY to 2-methyl-3-buten-2-ol (MBE) occurred preferentially at the plane sites regardless of their crystallographic orientation, Pd(111) and/or Pd(100), whereas overhydrogenation occurred mainly at the edge sites. The experimental data can be fit with a kinetic modeling based on a two-site Langmuir-Hinshelwood mechanism. By considering surface statistics for nanocrystals with different shapes and sizes, the optimal catalyst in terms of productivity of the target product MBE was predicted to be cubes of roughly 3-5 nm in edge length. This study is an attempt to close the material and pressure gaps between model single-crystal surfaces tested under ultra-high-vacuum conditions and real catalytic systems, providing a powerful tool for rational catalyst design.     

Brønsted base-modulated regioselectivity in the aerobic oxidative amination of styrene catalyzed by palladium

Palladium(II)-catalyzed aerobic oxidative amination of styrene with oxazolidinone proceeds with catalyst-controlled regioselectivity: (CH3CN)2PdCl2 (1) and (Et3N)2PdCl2 (2) catalyze formation of the anti-Markovnikov and Markovnikov enecarbamate products, 3 and 4, respectively. Kinetic studies and deuterium kinetic isotope effects demonstrate that these two reactions possess different rate-limiting steps, and the data indicate that the product regiochemistry arises from the presence or absence of an effective Br?nsted base in the reaction. In the presence of a Br?nsted base such as triethylamine or acetate, the kinetically preferred Markovnikov aminopalladation adduct of styrene is trapped via rapid deprotonation of a zwitterionic intermediate and leads to formation of 4. In the absence of an effective Br?nsted base, however, slow deprotonation of this adduct enables aminopalladation to be reversible, and product formation proceeds through the thermodynamically preferred anti-Markovnikov aminopalladation adduct to yield 3.     

Planar or nonplanar: what is the structure of urea in aqueous solution?

A combined quantum chemical statistical mechanical method has been used to study the solvation of urea in water, with emphasis on the structure of urea. The model system consists of three parts: a Hartree-Fock quantum chemical core, 99 water molecules described with a polarizable force-field, and a dielectric continuum. A free-energy profile along the transition of urea from planar to a nonplanar structure is calculated. This mode in aqueous solution is found to be floppy. That is, the structure of urea in water is not well-defined because the planar to nonplanar transition requires an energy of the order of the thermal energy at room temperature. We discuss the implications of this finding for simulation studies of urea in polar environments like water and proteins.     

Is the bound substrate in nitric oxide synthase protonated or neutral and what is the active oxidant that performs substrate hydroxylation?

We present here results of a series of density functional theory (DFT) studies on enzyme active site models of nitric oxide synthase (NOS) and address the key steps in the catalytic cycle whereby the substrate (L-arginine) is hydroxylated to N(omega)-hydroxo-arginine. It has been proposed that the mechanism follows a cytochrome P450-type catalytic cycle; however, our calculations find an alternative low energy pathway whereby the bound L-arginine substrate has two important functions in the catalytic cycle, namely first as a proton donor and later as the substrate in the reaction mechanism. Thus, the DFT studies show that the oxo-iron active species (compound I) cannot abstract a proton and neither a hydrogen atom from protonated L-arginine due to the strength of the N-H bonds of the substrate. However, the hydroxylation of neutral arginine by compound I and its one electron reduced form (compound II) requires much lower barriers and is highly exothermic. Detailed analysis of proton transfer mechanisms shows that the basicity of the dioxo dianion and the hydroperoxo-iron (compound 0) intermediates in the catalytic cycle are larger than that of arginine, which makes it likely that protonated arginine donates one of the two protons needed during the first catalytic cycle of NOS. Therefore, DFT predicts that in NOS enzymes arginine binds to the active site in its protonated form, but is deprotonated during the oxygen activation process in the catalytic cycle by either the dioxo dianion species or compound 0. As a result of the low ionization potential of neutral arginine, the actual hydroxylation reaction starts with an initial electron transfer from the substrate to compound I to create compound II followed by a concerted hydrogen abstraction/radical rebound from the substrate. These studies indicate that compound II is the actual oxidant in NOS enzymes that performs the hydroxylation reaction of arginine, which is in sharp contrast with the cytochromes P450 where compound II was shown to be a sluggish oxidant. This is the first example of an enzyme where compound II is able to participate in the reaction mechanism. Moreover, arginine hydroxylation by NOS enzymes is catalyzed in a significantly different way from the cytochromes P450 although the active sites of the two enzyme classes are very similar in structure. Detailed studies of environmental effects on the reaction mechanism show that environmental perturbations as appear in the protein have little effect and do not change the energies of the reaction. Finally, a valence bond curve crossing model has been set up to explain the obtained reaction mechanisms for the hydrogen abstraction processes in P450 and NOS enzymes.     

Why is metallic Pt the best catalyst for methoxy decomposition?

The decomposition of methoxy on Cu(111), Ag(111), Au(111), Ni(111), Pt(111), Pd(111), and Rh(111) has been studied in detail by the density functional theory calculations. The calculated activation barriers were successfully correlated with the coupling matrix element V 2 ad and the d-band center (ε d ) for the group IB metals and group VIII metals, respectively. By comparison of the activation energy barriers of the methoxy decomposition on different metals, it was found that Pt is the best catalyst for methoxy decomposition. The possible reason why the metallic Pt is the best catalyst has been analyzed from both the energetic data and the electronic structure information, that is, methoxy decomposition on Pt(111) has the largest exothermic behavior due to the closest p-band center of the CH 3 O among all metals after the adsorption.     

Is allylphosphine a carbon or a phosphorus base in the gas phase?

The gas-phase basicity of allylphosphine (2-propenylphosphine) was measured by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry techniques. A complete survey of the allylphosphine-H(+) potential energy surface, carried out through the use of high-level G2(MP2) and B3LYP/6-311+G(3df,2p) calculations, allows us to conclude that, under low-pressure, low-energy ICR conditions, the interaction between the protonated reference base, B(ref)H(+), and allylphosphine leads to a complex in which B(ref)H(+) attaches to the phosphorus atom of allylphosphine, where the electrostatic potential is strongly attractive. Hence, in the first step only the phosphorus protonated species should be formed. Its isomerization to yield the C(beta)-protonated form, which is the global minimum of the potential energy surface, implies a very high activation barrier that cannot be overtaken under normal experimental ICR conditions. Therefore, the main conclusion of our study is that allylphosphine behaves as a phosphorus base in the gas phase, even though the C(beta)-protonated structure is the most stable protonated species. We have also shown that both C(beta)- and C(gamma)-protonation triggers a cyclization of the system. An analysis of the bonding of the different protonated species as compared with that of the neutral system is presented.     

Sesquiterpene synthases: passive catalysts or active players?

Sesquiterpene synthases catalyse the metal dependent turnover of farnesyl diphosphate to generate a class of natural products characterised by an enormous diversity in structure, stereochemistry, biological function and application. It has been proposed that these enzymes take a passive role in the reactions they catalyse and that they serve mostly as stereochemical templates, within which the reactions take place. Here, recent research into the structure and function of sesquiterpene synthases and the mechanisms of the reactions that they catalyse will be reviewed to suggest that these fascinating enzymes play multifaceted active roles in what are arguably the most complex biosynthetic reactions.     

Site of protonation of nicotine and nornicotine in the gas phase: pyridine or pyrrolidine nitrogen?

The gas-phase basicities (GBs) of nornicotine, nicotine, and model pyrrolidines have been measured by FT-ICR. These experimental GBs are compared with those calculated (for the two sites of protonation in the case of nicotine and nornicotine) at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) level, or those estimated from substituent effects on the GBs of 2-substituted pyrrolidines, 2-substituted N-methylpyrrolidines, and 3-substituted pyridines. It is found that, in contrast to the Nsp(3) protonation in water, in the gas phase nornicotine is protonated on the pyridine nitrogen, because the effects of an intramolecular CH.Nsp(3) hydrogen bond and of the polarizability of the 3-(pyrrolidin-2-yl) substituent add up on the Nsp(2) basicity, while the polarizability effect of the 2-(3-pyridyl) substituent on the Nsp(3) basicity is canceled by its field/inductive electron-withdrawing effect. The same structural effects operate on the Nsp(3) and Nsp(2) basicities of nicotine, but here, the polarizability effect of the methyl group puts the pyrrolidine nitrogen basicity very close to that of pyridine. Consequently, protonated nicotine is a mixture of the Nsp(3) and Nsp(2) protonated forms.     

Synthesis of palladium–polypyrrole nanocomposite and its electrocatalytic properties in the oxidation of formaldehyde

Three alternative methods were developed for the synthesis of modifying palladium–polypyrrole layers on the surface of an inert electrode. Their electrocatalytic activity toward formaldehyde under inert atmosphere was checked. All the suggested methods are one-stage and allow synthesis of a film on the electrode surface from a solution containing a palladium salt and pyrrole in the absence of other active reagents. The electrochemical methods (potentiodynamic and double cathodic and anodic pulses techniques) in an aqueous medium give films with poorly reproducible electrocatalytic properties, while the chemical redox synthesis affords films with reproducibly high electroactivity toward methylene glycolate.     

Native Mass Spectrometry: What is in the Name?

Electrospray ionization mass spectrometry (ESI-MS) is nowadays one of the cornerstones of biomolecular mass spectrometry and proteomics. Advances in sample preparation and mass analyzers have enabled researchers to extract much more information from biological samples than just the molecular weight. In particular, relevant for structural biology, noncovalent protein–protein and protein–ligand complexes can now also be analyzed by MS. For these types of analyses, assemblies need to be retained in their native quaternary state in the gas phase. This initial small niche of biomolecular mass spectrometry, nowadays often referred to as “native MS,” has come to maturation over the last two decades, with dozens of laboratories using it to study mostly protein assemblies, but also DNA and RNA-protein assemblies, with the goal to define structure–function relationships. In this perspective, we describe the origins of and (re)define the term native MS, portraying in detail what we meant by “native MS,” when the term was coined and also describing what it does (according to us) not entail. Additionally, we describe a few examples highlighting what native MS is, showing its successes to date while illustrating the wide scope this technology has in solving complex biological questions.

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Coarsening of the phase structure in immiscible polymer blends. Coalescence or ostwald ripening?

The results of theories of the Ostwald ripening and coalescence applied to molten quiescent polymer blends with dispersed phase structure were analyzed. From a comparison of predictions of the theories with available experimental results follows that coarsening of the phase structure in quiescent polymer blends with medium or high interfacial tension is induced by the coalescence. Both the mechanisms play a role in coarsening of the phase structure in blends with low interfacial tensions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 181–187, 1999