Multitechnique surface spectroscopic studies of plasma modified polymers III. H2O and O2/H2O plasma modified poly(methyl methacrylate)s

The surfaces of poly(methyl methacrylate) (PMMA) films modified by O₂H₂O and H₂O radio-frequency glow discharge plasmas were studied using electron spectroscopy for chemical analysis (ESCA or XPS), low energy ion scattering (LEISS or ISS), Fourier transform IR spectroscopy (FTIR) with attenuated total reflectance (ATR) sampling, and critical surface energy from contact angle measurements. The extent and nature of modification with respect to promotion of a hydrophilic surface compared to the hydrophobic surface of the unmodified PMMA has been probed. Results show drastic decreases in C/O ratio at the near surface, which increases to that of the unmodified PMMA as deeper cross sections are analyzed. In addition peak fitting of ESCA data correlated with FTIR functional group information allows for the qualitative and quantitative analysis of the resulting bonding and structure of the modified layer. From these results combined with the polarity and surface energy differences obtained from contact angle measurements, the structural changes are discussed with respect to plasma reaction mechanisms and differences in the structure of the modified polymer films.     

Evaluating the identity and diiron core transformations of a (μ-oxo)diiron(III) complex supported by electron-rich tris(pyridyl-2-methyl)amine ligands

The composition of a (μ-oxo)diiron(III) complex coordinated by tris[(3,5-dimethyl-4-methoxy)pyridyl-2-methyl]amine (R(3)TPA) ligands was investigated. Characterization using a variety of spectroscopic methods and X-ray crystallography indicated that the reaction of iron(III) perchlorate, sodium hydroxide, and R(3)TPA affords [Fe(2)(μ-O)(μ-OH)(R(3)TPA)(2)](ClO(4))(3) (2) rather than the previously reported species [Fe(2)(μ-O)(OH)(H(2)O)(R(3)TPA)(2)](ClO(4))(3) (1). Facile conversion of the (μ-oxo)(μ-hydroxo)diiron(III) core of 2 to the (μ-oxo)(hydroxo)(aqua)diiron(III) core of 1 occurs in the presence of water and at low temperature. When 2 is exposed to wet acetonitrile at room temperature, the CH(3)CN adduct is hydrolyzed to CH(3)COO(-), which forms the compound [Fe(2)(μ-O)(μ-CH(3)COO)(R(3)TPA)(2)](ClO(4))(3) (10). The identity of 10 was confirmed by comparison of its spectroscopic properties with those of an independently prepared sample. To evaluate whether or not 1 and 2 are capable of generating the diiron(IV) species [Fe(2)(μ-O)(OH)(O)(R(3)TPA)(2)](3+) (4), which has previously been generated as a synthetic model for high-valent diiron protein oxygenated intermediates, studies were performed to investigate their reactivity with hydrogen peroxide. Because 2 reacts rapidly with hydrogen peroxide in CH(3)CN but not in CH(3)CN/H(2)O, conditions that favor conversion to 1, complex 1 is not a likely precursor to 4. Compound 4 also forms in the reaction of 2 with H(2)O(2) in solvents lacking a nitrile, suggesting that hydrolysis of CH(3)CN is not involved in the H(2)O(2) activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R(3)TPA ligands and elaborate on conditions required to generate synthetic models of diiron(IV) protein intermediates with this ligand framework.     

Long‐Wavelength Fluorescent Reporters for Monitoring Protein Kinase Activity

In vivo optical imaging must contend with the limitations imposed by the optical window of tissue (600–1000 nm). Although a wide array of fluorophores are available that are visualized in the red and near‐IR region of the spectrum, with the exception of proteases, there are few long wavelength probes for enzymes. This situation poses a particular challenge for studying the intracellular biochemistry of erythrocytes, the high hemoglobin content of which optically obscures subcellular monitoring at wavelengths less than 600 nm. To address this, tunable fluorescent reporters for protein kinase activity were developed. The probing wavelength is preprogrammed by using readily available fluorophores, thereby enabling detection within the optical window of tissue, specifically in the far‐red and near‐IR region. These agents were used to monitor endogenous cAMP‐dependent protein kinase activity in erythrocyte lysates and in intact erythrocytes when using a light‐activatable reporter.     

Improved Molecular Weight Control in Ring‐Opening Metathesis Polymerization (ROMP) Reactions with Ru‐Based Olefin Metathesis Catalysts Using N Donors and Acid: A Kinetic and Mechanistic Investigation

The effect of the addition of H₃PO₄ on the ROMP activity of cyclooctene (COE) with first‐ [Cl₂(PCy₃)₂Ru?CHPh] and second‐generation [(H₂IMes)Cl₂(PCy₃)Ru?CHPh] Grubbs’ catalysts 1 and 4 (Cy=cyclohexyl, Ph=phenyl, Mes=2,4,6‐trimethylphenyl (mesityl)), their inhibited mixtures with 1‐methylimidazole (MIM), as well as their isolated bis‐N,N′‐dimethylaminopyridine (DMAP) derivatives [Cl₂(PCy₃)(DMAP)₂Ru?CHPh)] ( 5 b ) and [Cl₂(H₂IMes)(DMAP)₂Ru?CHPh] ( 7 b ) (DMAP=dimethylaminopyridine), a novel catalyst, has been investigated. The studies include the determination of their initiation rates, as well as a determination of the molecular weights and molecular weight distributions of the polymers obtained with these catalysts and catalyst mixtures from the exo‐7‐oxanorbornene derivative 11 . The structure of catalyst 7 b was confirmed by means of X‐ray diffraction. All N‐donor‐bearing catalysts or N‐donor‐containing catalyst mixtures not only exhibited elevated activity in the presence of acid, but also increased initiation rates. Using the reversible inhibition/activation protocol with MIM and H₃PO₄ enabled us to conduct controlled ROMP with catalyst 4 producing the isolated exo‐7‐oxanorbornene‐based polymer 12 with predetermined molecular weights and narrow molecular weight distributions. This effect was based on fast and efficient catalyst initiation in contrast to the parent catalyst 4 . Hexacoordinate complex 5 b also experienced a dramatic increase in initiation rates upon acid‐addition and the ROMP reactions became well‐controlled in contrast to the acid‐free reaction. In contrast, complex 7 b performs well‐controlled ROMP in the absence of acid, whereas the polymerization of the same monomer becomes less controlled in the presence of H₃PO₄. The closer evaluation of catalysts 5 b and 7 b demonstrated that their initiation rates exhibit a linear dependency on the substrate concentration in contrast to catalysts 1 and 4 . As a consequence, their initiation rates are determined by an associative step, not a dissociative step as seen for catalysts 1 and 4 . A feasible associative metathesis initiation mechanism is proposed.     

Investigation of steric and electronic effects in the copper-catalysed asymmetric oxidation of sulfides

Steric and electronic effects in the copper-catalysed asymmetric oxidation of aryl benzyl, aryl alkyl and alkyl benzyl sulfides have been investigated. The presence of an aryl group directly attached to the sulfur is essential to afford sulfoxides with high enantioselectivities, with up to 97% ee for 2-naphthyl benzyl sulfoxide, the highest enantioselectivity achieved to date for copper-catalysed asymmetric sulfoxidation. In contrast, the benzyl substituent can be replaced by sterically comparable groups with no effect on enantioselectivity. Copper-mediated oxidation of substituted aryl benzyl sulfides display modest steric and electronic effects resulting in comparable or lower enantioselectivities to those obtained with the unsubstituted benzyl phenyl sulfide.     

A scalable synthesis of (1R,3S,5R)-2-(tert-butoxycarbonyl)-2-azabicyclo[3.1.0]hexane-3-carboxylic acid

A stereoselective and scalable synthesis of (1R,3S,5R)-2-(tert-butoxycarbonyl)-2-azabicyclo[3.1.0]hexane-3-carboxylic acid (3a) is described. Key to the success of the devised route was the realization that the stereoselectivity of a cyclopropanation step could be controlled by the composition of the functional group at C-α.     

High–quality protein backbone reconstruction from alpha carbons using Gaussian mixture models

Coarse‐grained protein structure models offer increased efficiency in structural modeling, but these must be coupled with fast and accurate methods to revert to a full‐atom structure. Here, we present a novel algorithm to reconstruct mainchain models from C traces. This has been parameterized by fitting Gaussian mixture models (GMMs) to short backbone fragments centered on idealized peptide bonds. The method we have developed is statistically significantly more accurate than several competing methods, both in terms of RMSD values and dihedral angle differences. The method produced Ramachandran dihedral angle distributions that are closer to that observed in real proteins and better Phaser molecular replacement log‐likelihood gains. Amino acid residue sidechain reconstruction accuracy using SCWRL4 was found to be statistically significantly correlated to backbone reconstruction accuracy. Finally, the PD2 method was found to produce significantly lower energy full‐atom models using Rosetta which has implications for multiscale protein modeling using coarse‐grained models. A webserver and C++ source code is freely available for noncommercial use from: http://www.sbg.bio.ic.ac.uk/phyre2/PD2_ca2main/ . © 2013 Wiley Periodicals, Inc.     

Intra- and Inter-molecular Sulf- hydryl Hydrogen Bonding: Facilitating Proton Transfer Events for Determination of pH in Sea Water

This work presents the electrochemical response of a 2-(methylthio)phenol glassy carbon based electrode for a promising voltammetric pH sensor in both buffered and low-buffered solutions. Electropolymerization of the redox species was performed with the resulting polymer presenting a Nernstian response in buffered media, with a sensitivity of 51 mV/pH unit. The effectiveness of the sulfhydryl bond to facilitate proton transfer from the bulk solution to the phenol molecules has been confirmed, providing an accurate pH measurement of 8.28 in sea water media, compared to that measured with a calibrated glass pH probe of 8.30.     

Excitation dependent quenching of luminescence in LED phosphors

Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu²⁺ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm². The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu²⁺ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Investigating microcrystalline cellulose crystallinity using Raman spectroscopy

Cellulose - Microcrystalline cellulose (MCC) is a semi-crystalline material with inherent variable crystallinity due to raw material source and variable manufacturing conditions. MCC crystallinity...