Ping‐Pong Energy Transfer in Covalently Linked Porphyrin‐MoS2 Architectures

Molybdenum disulfide nanosheets covalently modified with porphyrin were prepared and fully characterized. Neither the porphyrin absorption nor its fluorescence was notably affected by covalent linkage to MoS₂. The use of transient absorption spectroscopy showed that a complex ping‐pong energy‐transfer mechanism, namely from the porphyrin to MoS₂ and back to the porphyrin, operated. This study reveals the potential of transition‐metal dichalcogenides in photosensitization processes.     

Amine oxidase amperometric biosensor coupled to liquid chromatography for biogenic amines determination

A selective and sensitive method is presented for biogenic amines (BA) determination. The novelty consists in coupling a highly selective electrochemical biosensor to a weak acid cation-exchange column for online detection of amines. A bienzyme design, based on a recently isolated amine oxidase from grass pea and commercial horseradish peroxidase, was used for the biosensor construction. The enzymes were co-immobilized on the surface of a graphite electrode together with the electrochemical mediator (Os-redox polymer). The electrochemical detection was performed at a low applied potential (?50 mV vs. Ag/AgCl, KCl_0.1 M), where biases from interferences are minimal. The separation and determination of six BA, with relevance in food analysis (tyramine, putrescine, cadaverine, histamine, agmatine and spermidine), were investigated. Irrespective of the BA nature, the amine oxidase-based biosensor showed a linear response up to 5 mM, and its sensitivity decreases in the following order: cadaverine, putrescine, spermidine, agmatine, histamine and tyramine. The approach was used to estimate the BA content in fish samples, after their extraction with methanesulfonic acid.     

Detailed experimental and kinetic modeling study of 3-carene pyrolysis

3-Carene is an important potential biofuel with properties similar to the jet-propellant JP-10. Its thermal decomposition and combustion behavior is to date unknown, which is essential to assess its quality as a fuel. A combined experimental and kinetic modeling study has been conducted to understand the initial decomposition of 3-carene. The pyrolysis of 3-carene was investigated in a jet-stirred quartz reactor at atmospheric pressure, at temperatures varying from 650 to 1050 K, covering the complete conversion range. The decomposition of 3-carene was observed to start around 800 K, and it is almost complete at 970 K. Online gas chromatography shows that primarily aromatics are generated which suggests that 3-carene is not a good fuel candidate. The potential energy surface for the initial decomposition pathways determined by KinBot shows that a hydrogen elimination reaction dominates, giving primarily cara-2,4-diene. Next to this molecular pathway, radical pathways lead to aromatics via ring opening. The kinetic model was automatically generated with Genesys and consists of 2565 species and 9331 reactions. New quantum chemical calculations at the CBS-QB3 level of theory were needed to calculate rate coefficients and thermodynamic properties relevant for the primary decomposition of 3-carene. Both the conversion of 3-carene and the yields of the primary products (ie, benzene and hydrogen gas) are well predicted with this kinetic model. Rate of production analyses shows that the dominant pathways to convert 3-carene are hydrogen elimination reaction and radical chemistry.     

Probing Substrate/Catalyst Effects Using QSPR Analysis on Friedel-Crafts Acylation Reactions over Hierarchical BEA Zeolites

Attempts to optimize heterogeneous catalysis often lack quantitative comparative analysis. The use of kinetic modelling leads to rate (k) and relative sorption equilibrium constants (K), which can be further rationalized using Quantitative Structure-Property Relationships (QSPR) based on Multiple Linear Regressions (MLR). Friedel-Crafts acylation using commercial and hierarchical BEA zeolites as heterogeneous catalysts, acetic anhydride as the acylating agent, and a set of seven substrates with different sizes and chemical functionalities were herein studied. Catalytic results were correlated with the physicochemical properties of substrates and catalysts. From this analysis, a robust set of equations was obtained allowing inferences about the dominant factors governing the processes. Not entirely surprising, the rate and sorption equilibrium constants were found to be explained in part by common factors but of opposite signs: higher and stronger adsorption forces increase reaction rates, but they also make the zeolite active sites less accessible to new reactant molecules. The most relevant parameters are related to the substrates’ molecular size, which can be associated with different reaction steps, namely accessibility to micropores, diffusion capacity, and polarizability of molecules. The relatively large set of substrates used here reinforces previous findings and brings further insights into the factors that hamper/speed up Friedel-Crafts reactions in heterogeneous media.     

Supramolecular Co(II)-[2 x 2] grids: metamagnetic behavior in a single molecule

The magnetic anisotropy of the supramolecular [2 x 2] grid [Co(II)4L4]8+, with a bis(bipyridyl)-pyrimidine-based ligand L, was investigated by single-crystal magnetization measurements at low temperatures. The magnetization curves exhibit metamagnetic-like behavior and are explained by the weak-exchange limit of a minimal spin Hamiltonian including Heisenberg exchange, easy-axis ligand fields, and the Zeeman term. It is also shown that the magnetic coupling strength can be varied by the substituent R1 in the two-position on the central pyrimidine group of the ligand L.     

Effects of peptide density and elution pH on affinity chromatographic purification of human immunoglobulins A and M

A family of linear hexamer peptide ligands HWRGWV, HYFKFD and HFRRHL, initially identified for their affinity to the Fc portion of human immunoglobulin G (hIgG), also have potential for use in the purification of human immunoglobulins A (hIgA) and M (hIgM). HWRGWV demonstrated the strongest binding affinity to hIgM, followed by hIgA and hIgG respectively. The effects of N-terminal acetylation of the peptide, as well as elution buffer pH, on the chromatographic elution of human IgG, IgA and IgM from HWRGWV resins at various peptide densities (0.04-0.55 meq/g) were investigated. Over 80% recovery and 90% purity were achieved for human IgG and IgA isolation from complete minimum essential medium (cMEM) using HWRGWV resin at optimum peptide densities. For human IgM, 75.7% recovery and 86.0% purity were achieved by using HWRGWV at a low peptide density of 0.04 meq/g. Although HYFKFD and HFRRHL exhibited their ability for isolation of human IgG, IgA and IgM from cMEM as well, HWRGWV is the best option among them for large-scale purification of human IgG, IgA and IgM based on conditions tested.     

Analysis of sulfated peptides from the skin secretion of the Pachymedusa dacnicolor frog using IMAC-Ga enrichment and high-resolution mass spectrometry

Immobilized metal ion affinity chromatography (IMAC) has been widely used for the enrichment of phosphopeptides, whereas no report exists describing the use of IMAC columns for the enrichment of sulfopeptides. In this study, we used IMAC-Ga microcolumns for the enrichment of sulfopeptides from a complex mixture of peptides, extracted from skin secretions of the Pachymedusa dacnicolor frog. The enriched fraction obtained by IMAC-Ga was analyzed by liquid chromatograpy/electrospray ionization mass spectrometry (LC/ESI-MS) in an Orbitrap XL and by matrix-assisted laser desorption/ionization time-of-flight time-of-flight (MALDI-TOF/TOF) in an ABI 4800 instrument. From this fraction, different sulfated and non-sulfated peptides belonging to the caerulin and bradykinin families were structurally characterized. Other interesting negatively charged groups, such as phosphate adducts of dermaseptins and pyridoxal phosphate attached to a protease inhibitor, were also characterized. Unexpectedly, some dermaseptin antimicrobial peptides were also enriched by IMAC-Ga and a Sauvatine-like peptide was also fully sequenced. Furthermore, neutral loss of sulfated peptides and their fragmentation patterns in the gas phase were also compared using collision-induced dissociation (CID) and high-energy collision dissociation (HCD). Our present study provides evidence that IMAC-Ga enrichment is a fast, useful and promising method for high-throughput analysis of sulfated-peptides, since high-resolution mass spectrometers can be used for this purpose.     

Nickel‐Catalyzed Carboxylation of Benzylic C−N Bonds with CO2

A user‐friendly Ni‐catalyzed reductive carboxylation of benzylic C?N bonds with CO₂ is described. This procedure outperforms state‐of‐the‐art techniques for the carboxylation of benzyl electrophiles by avoiding commonly observed parasitic pathways, such as homodimerization or β‐hydride elimination, thus leading to new knowledge in cross‐electrophile reactions.