Chemical physics: The standing of a mature discipline

Background

The use of immobilized enzymes for catalyzing various biotransformations is now a widely used approach. In recent years, cross-linked enzyme aggregates (CLEAs) have emerged as a novel and versatile biocatalyst design. The present work deals with the preparation of a CLEA from a commercial preparation, Pectinex? Ultra SP-L, which contains pectinase, xylanase and cellulase activities. The CLEA obtained could be used for any of the enzyme activities. The CLEA was characterized in terms of kinetic parameters, thermal stability and reusability in the context of all the three enzyme activities.

Results

Complete precipitation of the three enzyme activities was obtained with n-propanol. When resulting precipitates were subjected to cross-linking with 5 mM glutaraldehyde, the three activities initially present (pectinase, xylanase and cellulase) were completely retained after cross-linking. The V_max/K_m values were increased from 11, 75 and 16 to 14, 80 and 19 in case of pectinase, xylanase and cellulase activities respectively. The thermal stability was studied at 50°C, 60°C and 70°C for pectinase, xylanase and cellulase respectively. Half-lives were improved from 17, 22 and 32 minutes to 180, 82 and 91 minutes for pectinase, xylanase and cellulase respectively. All three of the enzymes in CLEA could be reused three times without any loss of activity.

Conclusion

A single multipurpose biocatalyst has been designed which can be used for carrying out three different and independent reactions; 1) hydrolysis of pectin, 2) hydrolysis of xylan and 3) hydrolysis of cellulose. The preparation is more stable at higher temperatures as compared to the free enzymes.     

Evidence for out-of-equilibrium crystal nucleation in suspensions of oppositely charged colloids

We report a numerical study of the rate of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free-energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for subcritical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe should be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation.     

Acoustic and thermal field investigation of low-cost dwellings, a case study in Brazil

The present paper presents acoustic and thermal performance evaluations carried out in low-cost houses located at the Vila Tecnológica de Curitiba (Technological Village of Curitiba), in Curitiba, Brazil. Two dwellings were evaluated, which were built according to different building systems: (1) the building system Batistella, built with pre-fabricated modules and elements, composed of reforestation wood and (2) the building system Constroyer, composed of expanded polystyrene (EPS) plates, framed in a welded iron mesh, with structural plastering. For the thermal performance evaluation, it was verified that the materials used in the envelope were quite significant for controlling heat gains and losses through the building elements as well as for regulating heat storage. In this case, the system with highest thermal inertia presented better results. Results showed that both building systems had poor acoustic performance, characterized by a low weighted apparent sound reduction index of the façade dB. The German DIN 4109 Norm recommends a minimum of dB. The weighted apparent sound reduction index measured for internal walls was dB and dB, for the building systems Battistela and Constroyer, respectively. The minimum required by German regulation VDI 4100 is dB, for internal walls.     

Uncovering seasonal patterns of 56 pesticides in surface coastal waters of the Ria Formosa lagoon (Portugal), using a GC-MS method

This study describes the simultaneous quantification of 56 pesticides in surface coastal water, supported by the development and validation of a gas chromatography (GC)–ion trap (IT) mass spectrometry (MS) method. Samples (500 mL) were pre-concentrated 2500 times by solid phase extraction (OASISTMHLB). The compounds were identified and quantified, within 35 minutes, by GC tandem mass spectrometry (GC-MS/MS) and GC-MS, respectively. The methodology proved to be highly specific for all target pesticides, with an average linearity of 0.99. Detection limits and recovery rates ranged from 0.4 to 1.3 ng L^?1 and 71% to 120%, respectively. The performance of the method was checked using water samples collected from nine sampling sites along the Ria Formosa Lagoon Natural Park (south of Portugal, n = 54) in each season (2010). The total annual concentrations of all pesticides in each category (fungicides, herbicides and insecticides) were 1.4, 0.6 and 9.0 µg L^?1, respectively. Moreover, 89% of the pesticides tested for were detected, 84% could be quantified and 25% had concentrations above the European recommended levels (2013/39/EU). The highest total loads of pesticides were found in the spring, which is in agreement with their seasonal application. Physicochemical parameters such as, nitrites, nitrates, ammonia and phosphates, also indicate poor water quality, supporting the fact that the Ria Formosa lagoon actually needs an effective monitoring programme for effective preservation of its natural reserve status.     

A Tetraferrocenyl‐Resorcinarene Cavitand as a Redox‐Switchable Host of Ammonium Salts

Tetraannulation of a resorcinarene‐octaamino cavitand with ferrocenecarboxaldehyde allows the preparation of a tetrabenzimidazole‐resorcinarene cavitand with four ferrocenyl moieties directly linked to the C2 atom of the imidazole units. Oxidation of the four ferrocenyl moieties produces important structural modifications of the molecule, as indicated by DFT calculations performed for the neutral and tetraoxidized forms of the cavitand. By means of ¹H NMR spectroscopic analysis, the encapsulating properties of the new tetraferrocenyl‐resorcinarene cavitand toward a series of ammonium salts were evaluated, and a clear cutoff point in binding affinity with respect to size was observed. Cyclic voltammetric studies allowed us to estimate the relative association constants for the neutral and oxidized forms of the cavitand, thus indicating that the guest was bound to the neutral (reduced) state of the cavitand and was released from the oxidized form. These redox‐addressable conformational and binding properties of the resorcinarene‐tetraferrocenyl cavitand constitute all the necessary features of a redox‐switchable molecular gripper. By means of mass‐spectrometric analysis, we could unambiguously confirm the molar stoichiometry of the host–guest complex (1:1) and assess the strong guest encapsulation, as indicated by triggering the covalent coupling between host and guest in the gas phase.     

Biomolecular DNP‐Supported NMR Spectroscopy using Site‐Directed Spin Labeling

Dynamic nuclear polarization (DNP) has been shown to greatly enhance spectroscopic sensitivity, creating novel opportunities for NMR studies on complex and large molecular assemblies in life and material sciences. In such applications, however, site‐specificity and spectroscopic resolution become critical factors that are usually difficult to control by current DNP‐based approaches. We have examined in detail the effect of directly attaching mono‐ or biradicals to induce local paramagnetic relaxation effects and, at the same time, to produce sizable DNP enhancements. Using a membrane‐embedded ion channel as an example, we varied the degree of paramagnetic labeling and the location of the DNP probes. Our results show that the creation of local spin clusters can generate sizable DNP enhancements while preserving the intrinsic benefits of paramagnetic relaxation enhancement (PRE)‐based NMR approaches. DNP using chemical labeling may hence provide an attractive route to introduce molecular specificity into DNP studies in life science applications and beyond.     

Unveiling the Importance of π‐Stacking in Borrowing‐Hydrogen Processes Catalysed by Iridium Complexes with Pyrene Tags

This work describes the preparation of a series of pyrene‐tagged N‐heterocyclic carbene complexes of iridium, and their use in two benchmark borrowing hydrogen reactions: the reduction of ketones by transfer hydrogenation and the β‐alkylation of secondary alcohols with primary alcohols. The detailed study of these homogeneously catalysed reactions reveals several important implications regarding the strong influence of the pyrene tags in the catalysts. First, the catalytic activity is partially inhibited by addition of an external amount of pyrene, but only when pyrene‐tagged catalysts and aromatic substrates are used. Second, the rate order of the reaction is highly dependent on the nature of the substrates and the ligand. When pyrene‐tagged catalysts and aromatic substrates are used, the reaction follows a zero‐order dependence on the concentration of the substrate. All other combinations afford a second‐order rate in the substrates. And third, the presence or absence of the pyrene functionality in the catalyst also influences the reaction order with respect to the concentration of the catalyst. Pyrene‐containing catalysts display a fractional rate order of below 1. Finally, two pyrene‐tagged catalysts were supported onto reduced‐graphene oxide (rGO), and used as heterogeneous catalysts. While the dimetallic catalyst was effectively recycled 12 times, the monometallic catalyst maintained its activity for only three runs.     

Membrane-based microextraction techniques in analytical chemistry: A review

The use of membrane-based sample preparation techniques in analytical chemistry has gained growing attention from the scientific community since the development of miniaturized sample preparation procedures in the 1990s. The use of membranes makes the microextraction procedures more stable, allowing the determination of analytes in complex and “dirty” samples. This review describes some characteristics of classical membrane-based microextraction techniques (membrane-protected solid-phase microextraction, hollow-fiber liquid-phase microextraction and hollow-fiber renewal liquid membrane) as well as some alternative configurations (thin film and electromembrane extraction) used successfully for the determination of different analytes in a large variety of matrices, some critical points regarding each technique are highlighted.     

Electrochemical Sensor Based on Gold Nanoparticles Stabilized in Poly(Allylamine hydrochloride) for Determination of Vanillin

Gold nanoparticles stabilized in poly(allylamine hydrochloride) (AuNP‐PAH) were synthesized, characterized and applied in the development of a new sensor for the determination of vanillin by square‐wave voltammetry. Under optimized conditions, the calibration curve showed a linear range for vanillin of 0.90 to 15.0 µmol L^?1, with a limit of detection of 55 nmol L^?1. The sensor demonstrated acceptable selectivity and stability, as well as good intra‐day and inter‐day repeatability and electrode‐to‐electrode repeatability (with relative standard deviations of 3.5, 4.5 and 3.9 %, respectively). The sensor was successfully applied in the determination of vanillin in different commercial samples.     

On‐Site Determination of Carbendazim,Cathecol and Hydroquinone in Tap Water Using a Homemade Batch Injection Analysis Cell for Screen Printed Electrodes

In this work, we present a simple homemade batch‐injection analysis cell for screen‐printed electrodes (BIA‐SPE). The potential of the proposed system for on‐site analysis was demonstrated by the determination of carbendazim, catechol, and hydroquinone in tap water. The system provided reduced injection volume (30 µL), high analytical frequency (≈200 h^?1) and low detection limits (nanomolar level). Moreover, the BIA‐SPE cell presented better stability (RSD≈0.4 %) than a conventional flow injection cell for SPE (RSD≈5.0 %) in organic media. The proposed homemade BIA‐SPE cell is very simple, inexpensive and can be easily constructed in any laboratory.