Improved synthesis of oligomeric phthalonitriles and studies designed for low temperature cure

An improved synthetic method has been developed for oligomeric aromatic ether ketone‐based phthalonitrile (PN) resins. A new curing additive was studied that lowers the cure temperature of the PN resin to around 150 °C and compared to the traditional high‐temperature aromatic diamine. Mechanical and thermo‐oxidative analyses of polymeric samples from both systems were determined and compared under various curing conditions. The PN polymer exhibited low water absorption regardless of the chosen cure system. Published 2014. This article is a U.S. Government work and is in the public domain in the USA. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1662–1668     

PCR protocol- and inulin catabolism-based differentiation of inulinolytic soil bacteria

Applied Biochemistry and Biotechnology - Bacteria collected from rotting dahlia tubers, instead of degrading inulin to D-fructose, preferentially formed the known DFA III (Β-2.1′:...     

Cyclohexane-based low molecular weight hydrogelators: a chirality investigation

Seven new 1,3,5-cyclohexyltricarboxamide-phenylalanine derivatives were synthesized in order to investigate the effect of the amino acid chirality on the gelating properties of these small molecules in water. Gelation tests have shown that enantiomerically pure homochiral 1,3,5-cyclohexyltricarboxamide-L-phenylalanine is a non-hydrogelator as it crystallizes from water, whereas the heterochiral derivatives with either two L-phenylalanine moieties and one D-phenylalanine (LLD), or vice versa (DDL), are very good hydrogelators. Concentration-dependent gel-to-sol transition-temperature (T(gs)) curves for LLD or DDL gels show a sigmoidal behaviour, which is in contrast to the logarithmic curves generally observed for gels derived from low molecular weight gelators (LMWGs). Such sigmoidal behaviour can be related to interactions between fibre bundles, which give rise to intertwined bundles of fibres. Transmission electron microscopy (TEM) images of LLD and DDL gels show a network of thin, unbranched, fibre bundles with diameters of 20 nm. Right-handed twisted fibre bundles are present in the LLD gel, whereas left-handed structures can be found in the DDL gel. Each bundle of fibres consists of a finite number of primary fibres. Gels consisting of mixtures of gelators, LLD and DDL, and nongelators (LLL or DDD) were investigated by means of T(gs) measurements, CD spectroscopy and TEM. Results show that the incorporation of nongelator molecules into gel fibres occurs; this leads to higher T(gs) values and to changes in the helicity of the fibre bundles. Furthermore, it was found that peripheral functionalization of the homochiral derivatives LLL or DDD by means of a second amino acid or a hydrophilic moiety can overcome the effect of chirality; this process in turn leads to good hydrogelators.     

Evidence for a nucleophilic anti-attack on the cleaved C(2)-oxygen bond in Cl2AlH-catalyzed ring-opening of 2-substituted 1,3-dioxolanes

The Cl₂AlH-mediated ring-opening of 2-substituted cis-4-methyl-5-trifluoromethyl-1,3-dioxolanes was found to occur with regioselective cleavage of the O1-C2 bond by attack of the aluminum hydride from the direction anti to the departing oxygen. This stereochemical outcome, which appears to be unprecedented in the reductive cleavage of chiral acetals by aluminum reagents, is interpreted on the basis of theoretical calculations.     

Hydroxylamines as oxidation catalysts: thermochemical and kinetic studies

Bond dissociation enthalpies (BDE) of hydroxylamines containing alkyl, aryl, vinyl, and carbonyl substituents at the nitrogen atom have been determined by using the EPR radical equilibration technique in order to study the effect of the substituents on the O-H bond strength of these compounds. It has been found that substitution of an alkyl group directly bonded to the nitrogen atom with vinyl or aryl groups has a small effect, while substitution with acyl groups induces a large increase of the O-H BDE value. Thus, dialkyl hydroxylamines have O-H bond strengths of only ca. 70 kcal/mol, while acylhydroxylamines and N-hydroxyphthalimide (NHPI), containing two acyl substituents at nitrogen, are characterized by BDE values of ca. 80 and 88 kcal/mol, respectively. Since the phthalimide N-oxyl radical (PINO) has been recently proposed as an efficient oxidation catalyst of hydrocarbons or other substrates, the large BDE value found for the parent hydroxylamine (NHPI) justifies this proposal. Kinetic studies, carried out in order to better understand the mechanism of the NHPI-catalyzed aerobic oxidation of cumene, are consistent with a simple kinetic model where the rate-determining step is the hydrogen atom abstraction from the hydroxylamine by cumylperoxyl radicals.     

A hybrid density functional study of O-O bond cleavage and phenyl ring hydroxylation for a biomimetic non-heme iron complex

Density functional calculations using the B3LYP functional have been used to study the reaction mechanism of [Fe(Tp(Ph2))BF] (Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate; BF = benzoylformate) with dioxygen. This mononuclear non-heme iron(II) complex was recently synthesized, and it proved to be the first biomimetic complex reproducing the dioxygenase activity of alpha-ketoglutarate-dependent enzymes. Moreover, the enthalpy and entropy of activation for this biologically interesting process were derived from kinetic experiments offering a unique possibility for direct comparison of theoretical and experimental data. The results reported here support a mechanism in which oxidative decarboxylation of the keto acid is the rate-limiting step. This oxygen activation process proceeds on the septet potential energy surface through a transition state for a concerted O-O and C-C bond cleavage. In the next step, a high-valent iron-oxo species performs electrophilic attack on the phenyl ring of the Tp(Ph2) ligand leading to an iron(III)-radical sigma-complex. Subsequent proton-coupled electron-transfer yields an iron(II)-phenol intermediate, which can bind dioxygen and reduce it to a superoxide radical. Finally, the protonated superoxide radical leaves the first coordination sphere of the iron(III)-phenolate complex and dismutates to dioxygen and hydrogen peroxide. The calculated activation barrier (enthalpy and entropy) and the overall reaction energy profile agree well with experimental data. A comparison to the enzymatic process, which is suggested to occur on the quintet surface, has been made.     

Design of a Novel Composite H2S‐Releasing Hydrogel for Cardiac Tissue Repair

The design of 3D scaffolds is a crucial step in the field of regenerative medicine. Scaffolds should be degradable and bioresorbable as well as display good porosity, interconnecting pores, and topographic features; these properties favour tissue integration and vascularization. These requirements could be fulfilled by hybrid hydrogels using a combination of natural and synthetic components. Here, the mechanical and biological properties of a polyethylene glycol‐fibrinogen hydrogel (PFHy) are improved in order to favour the proliferation and differentiation of human Sca‐1^pos cardiac progenitor cells (hCPCs). PFHys are modified by embedding air‐ or perfluorohexane‐filled bovine serum albumin microbubbles (MBs) and characterized. Changes in cell morphology are observed in MBs–PFHys, suggesting that MBs could enhance the formation of bundles of cells and influence the direction of the spindle growth. The properties of MBs as carriers of active macromolecules are also exploited. For the first time, enzyme‐coated MBs have been used as systems for the production of hydrogen sulfide (H₂S)‐releasing scaffolds. Novel H₂S‐releasing PFHys are produced, which are able to improve the growth of hCPCs. This novel 3D cell–scaffold system will allow the assessment of the effects of H₂S on the cardiac muscle regeneration with its potential applications in tissue repair.

     

A mechanistic design principle for protein tyrosine kinase sensors: application to a validated cancer target

A new mechanistic principle for reporting the phosphorylation of tyrosine is described, which should prove applicable to even the most fastidious of protein tyrosine kinases, as demonstrated by the acquisition of a fluorescent sensor for the extraordinarily demanding anaplastic lymphoma kinase.