"Oscillating" metallocene catalysts: what stops the oscillation?

The 150 MHz (13)C NMR microstructural analysis of polypropylene samples produced with two representative "oscillating" metallocene catalysts of largely different steric hindrance, namely [(2-(3,5-di-tert-butyl-4-methoxyphenyl)indenyl)(2)ZrP](+) and [(2-phenylindenyl)(2)ZrP](+) (P = polymeryl), and the implications on the origin of the stereocontrol are presented and discussed in detail. The original mechanistic proposal of an "oscillation" between a rac-like (isotactic-selective) and a meso-like (nonstereoselective) conformation cannot explain the observed polymer configuration. The isotactic-stereoblock nature of the polymers obtained with the former catalyst proves unambiguously that the active cation "oscillates" between the two enantiomorphous rac-like conformations at an average frequency that, even at high propene concentration, is only slightly lower than that of monomer insertion. The less-hindered [(2-phenylindenyl)(2)ZrP](+) gives instead a largely stereoirregular polypropylene, which is the logical consequence of a faster ligand rotation; however, depending on the use conditions (in particular, on the nature of the cocatalyst and the polarity of the solvent), the polymerization products may also contain appreciable amounts of a fairly isotactic fraction. The peculiar microstructure of this fraction, with isotactic blocks of the same relative configuration spanned by very short atactic ones, rules out the possibility that the latter are due to an active species in meso-like conformation and points rather to a conformationally "locked" rac-like species with restricted ring mobility. The hypothesis of a stereorigidity induced by the proximity to a counteranion, which would play the role of the interannular bridge in the rac-bis(indenyl) ansa-metallocenes, was tested by computer modeling on a [rac-(2-phenylindenyl)(2)ZrMe(C(3)H(6))][B(C(6)F(5))(4)] ion couple and found viable.     

Towards thermal equilibrium in the Bose-Einstein condensation of microcavity polaritons

By comparing a kinetic and a thermal-equilibrium theory of polariton Bose-Einstein condensation, we study under what conditions the dynamical condensation under steady-state non-resonant pumping can approach thermal equilibrium. In particular, we study the dependence on two material parameters: the vacuum-field Rabi-splitting and the polariton radiative lifetime. When increasing the Rabi splitting, condensation takes place under strong non-equilibrium conditions, with dominating quantum fluctuations. Increasing the polariton lifetime above 10 ps at moderate Rabi splitting, instead, produces a quasi-equilibrium condensate at low exciton density, consistently with the picture of a weakly interacting Bose gas.     

Sequence and phosphorylation level determination of two donkey β‐caseins by mass spectrometry

Two coeluting components, with experimentally measured M_r values of 25529 and 24606 Da, were identified by reversed‐phase high‐performance liquid chromatography (RP‐HPLC) and mass spectrometric analysis in the dephosphorylated casein fraction of a milk sample collected from an individual donkey belonging to the Ragusano breed of the east of Sicily. By coupling enzymatic digestions, matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) and RP‐HPLC/nano‐electrospray ionization tandem mass spectrometry (nESI‐MS/MS) analysis, the two proteins were identified as donkey β‐CNs and their sequences characterized completely, using the two known β‐CNs from mare as references. The two donkey β‐CNs, showing a mass difference of 923 Da, differ by the presence of the domain E²⁷SITHINK³⁴ in the full‐length component (M_r 25529 Da). In comparison with the mare's β‐CNs used as reference, they present nine amino acid substitutions: L→S³⁷, R→H⁵², S→N⁸¹, P→V⁸⁴, L→V⁹¹, R→Q²⁰³, P→L/I²⁰⁶, L→F²¹⁰ and A→P²¹⁹. Together, these substitutions account for the increase of 18 Da in the M_r of the donkey β‐CNs with respect to the counterparts from the mare. The molecular mass determination by ESI‐MS for the phosphorylated proteins showed that the full‐length component was composed of highly multi‐phosphorylated isoforms with five to seven phosphate groups. By analogy with the homologous mare's β‐CNs, the full‐length (226 amino acids) β‐CN was termed variant A, whereas the shorter (218 amino acids) β‐CN was termed variant A^Δ5. Copyright © 2009 John Wiley & Sons, Ltd.     

The play operator on the rectifiable curves in a Hilbert space

The vector play operator is the solution operator of a class of evolution variational inequalities arising in continuum mechanics. For regular data, the existence of solutions is easily obtained from general results on maximal monotone operators. If the datum is a continuous function of bounded variation, then the existence of a weak solution is usually proved by means of a time discretization procedure. In this paper we give a short proof of the existence of the play operator on rectifiable curves making use of basic facts of measure theory. We also drop the separability assumptions usually made by other authors. Copyright © 2007 John Wiley & Sons, Ltd.     

Enhanced Electrocatalytic Oxygen Evolution in Au–Fe Nanoalloys

Oxygen evolution reaction (OER) is the most critical step in water splitting, still limiting the development of efficient alkaline water electrolyzers. Here we investigate the OER activity of Au–Fe nanoalloys obtained by laser-ablation synthesis in solution. This method allows a high amount of iron (up to 11 at %) to be incorporated into the gold lattice, which is not possible in Au–Fe alloys synthesized by other routes, due to thermodynamic constraints. The Au_0.89Fe_0.11 nanoalloys exhibit strongly enhanced OER in comparison to the individual pure metal nanoparticles, lowering the onset of OER and increasing up to 20 times the current density in alkaline aqueous solutions. Such a remarkable electrocatalytic activity is associated to nanoalloying, as demonstrated by comparative examples with physical mixtures of gold and iron nanoparticles. These results open attractive scenarios to the use of kinetically stable nanoalloys for catalysis and energy conversion.     

Redox and electrocatalytic properties of mimochrome VI, a synthetic heme peptide adsorbed on gold

Mimochrome VI (MC-VI) is a synthetic heme peptide containing a helix-heme-helix sandwich motif designed to reproduce the catalytic activity of heme oxidases. The thermodynamics of Fe(III) to Fe(II) reduction and the kinetics of the electron-transfer process for MC-VI immobilized through hydrophobic interactions on a gold electrode coated with a nonpolar SAM of decane-1-thiol have been determined through cyclic voltammetry. Immobilization slightly affects the reduction potential of MC-VI, which under these conditions electrocatalytically turns over molecular oxygen. This work sets the premise for the exploitation of totally synthetic mimochrome-modified electrode surfaces for clinical and pharmaceutical biosensing.     

Enantioselective synthesis of 3(S)-hydroxy polygodial derivatives and evaluation of their vanilloid activity

The enantioselective synthesis of 3(S)-hydroxy polygodial and its acetyl derivative is described. The construction of the 3-hydroxy drimane skeleton was based on the titanium-catalyzed radical cyclization of (10S)-10, 11-epoxy-farnesyl acetate. Only underivatized 3(S)-hydroxy polygodial showed activity in assays on VR1 vanilloid receptor in HEK cells transfected with the human VR1.     

A microwave-assisted synthesis of triphenodioxazines [TPDOs]

A convenient and efficient method for the synthesis of triphenodioxazines [TPDOs] 1 by oxidative cyclisation of 2,5-bis-(arylamino)-3,6-dichlorocyclohexa-2,5-diene-1,4-diones 6 using potassium persulfate as the oxidising agent in 95?97% sulfuric acid triggered by microwave irradiation is described.     

Identification of naphthoquinonic and anthraquinonic dyes via sequential potential steps applied to the voltammetry of microparticles methodology

An electrochemical method for identifying anthraquinonic, naphthoquinonic, and related dyes in microsamples is reported. This method is based on the sequential application of oxidative and reductive constant-potential polarization steps coupled with the record of square wave voltammograms to solid microsamples of dyes in contact with aqueous electrolytes. As a result, oxidized/reduced products form a layer on the lateral faces of the dye crystals as suggested by attenuated total reflectance-Fourier transform infrared spectroscopy and atomic force microscopy data. This methodology is applied for characterizing alizarin, purpurin, and natural dyes aloe, cochineal red, madder lake, kermes, shellac, and henna attached to paraffin-impregnated graphite electrodes in contact with aqueous potassium phosphate buffer.