On the strength of hydrogen bonding within water clusters on the coordination limit

Hydrogen bonds (HB) are arguably the most important noncovalent interactions in chemistry. We study herein how differences in connectivity alter the strength of HBs within water clusters of different sizes. We used for this purpose the interacting quantum atoms energy partition, which allows for the quantification of HB formation energies within a molecular cluster. We could expand our previously reported hierarchy of HB strength in these systems (Phys. Chem. Chem. Phys., 2016, 18 , 19557) to include tetracoordinated monomers. Surprisingly, the HBs between tetracoordinated water molecules are not the strongest HBs despite the widespread occurrence of these motifs (e.g., in ice I_h). The strongest HBs within H₂O clusters involve tricoordinated monomers. Nonetheless, HB tetracoordination is preferred in large water clusters because (a) it reduces HB anticooperativity associated with double HB donors and acceptors and (b) it results in a larger number of favorable interactions in the system. Finally, we also discuss (a) the importance of exchange-correlation to discriminate among the different examined types of HBs within H₂O clusters, (b) the use of the above-mentioned scale to quickly assess the relative stability of different isomers of a given water cluster, and (c) how the findings of this research can be exploited to indagate about the formation of polymorphs in crystallography. Overall, we expect that this investigation will provide valuable insights into the subtle interplay of tri- and tetracoordination in HB donors and acceptors as well as the ensuing interaction energies within H₂O clusters.     

CO2 to CO: Photo- and Electrocatalytic Conversion Based on Re(I) Bis-Arene Frameworks: Synergisms Between Catalytic Subunits

Reduction of CO₂ to CO and H₂O is a two electron/two proton process. For this process, multinuclear complexes offer advantages by concentrating reduction equivalents more efficiently than mononuclear systems. We present novel complexes with [Re(η⁶-C₆H₆)₂]⁺ as scaffold conjugated to one or two catalytically active [Ru(dmbpy)(CO)₂Cl₂] subunits (dmbpy=5,5′-dimethyl-2,2′-bipyridine). The [Re(η⁶-C₆H₆)₂]⁺ scaffold was chosen due to its very high photo- and chemical stability, as well as the multiple degrees of freedom it offers for any conjugated functionalities. High efficiency and selectivity for the reduction of CO₂ to CO (over H₂ or HCOOH) is reported. TONs and TOFs were found to be comparable or higher than for the catalyst subunit without the rhenium framework. Cooperativity in photo- and electrocatalysis is observed for the complex comprising two catalytic subunits. The synergistic communication between the two catalytic subunits is responsible for the observed enhancement in both photo- and electrocatalytic performance. Confirmation of electronic communication between the two [Ru(dmbpy)(CO)₂Cl₂] subunits as well as the elucidation of a possible mechanism was supported by electrochemistry, IR-spectroelectrochemistry and DFT studies.     

Discovery of Homobivalent Bitopic Ligands of the Cannabinoid CB2 Receptor**

Single chemical entities with potential to simultaneously interact with two binding sites are emerging strategies in medicinal chemistry. We have designed, synthesized and functionally characterized the first bitopic ligands for the CB₂ receptor. These compounds selectively target CB₂ versus CB₁ receptors. Their binding mode was studied by molecular dynamic simulations and site-directed mutagenesis.     

Distance-Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine-C₆₀ dyads has been investigated. For this, two C₆₀-SiPc-C₆₀ dyads, 1 and 2 , varying in their donor-acceptor distance, have been newly synthesized and characterized. In the case of C₆₀-SiPc-C₆₀ 1 where the SiPc and C₆₀ are separated by a phenyl spacer, faster electron transfer was observed with k_cs equal to 2.7×10⁹ s⁻¹ in benzonitrile. However, in the case of C₆₀-SiPc-C₆₀ 2 , where SiPc and C₆₀ are separated by a biphenyl spacer, a slower electron transfer rate constant, k_cs=9.1×10⁸ s⁻¹, was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ∼4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ∼3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of ³SiPc* as the end product and suggested the final lifetime of the charge separated state to be in the 3–20 ns range. Energy level diagrams established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc^.+-C₆₀^.− charge separated states (1.57 eV) populated the low-lying ³SiPc* (1.26 eV) prior returning to the ground state.     

Toxicity assays applied to wastewater treatment

The utility and validity of toxicity tests for monitoring of wastewater treatment have been assessed. The evaluated acute toxicity tests have been Vibrio fischeri, Selenastrum capricornotum and Daphnia magna tests. The validation studies indicated that the acute toxicity tests can be considered as high sensitivity analytical tools to detect common environmental concentrations of the pollutants at concentration levels as low as ng l⁻¹. The toxicity tests showed to have discriminatory ability to distinguish between different degrees of toxicity, and the toxic specificity of the compounds on target organisms. Synergistic, additive or antagonistic effects were evaluated indicating the capacity of the toxicity test to assess the combined effects of chemicals in wastewaters. The reproducibility of these tests, calculated as relative standard deviation, is acceptable in the range of 5-22.3%. The application of multivariate date analysis proved that toxicity and chemical measures are complementary analytical tools for monitoring of wastewaters quality. The toxicity tests are useful analytical tools for screening of chemical analysis and as an early warning system to monitor the treatment of WWTPs. The use of single toxicity test or battery of tests is the best approach to evaluate the risk because they are reliable indices of the toxic impact of effluents in the aquatic environment. The toxicity tests were applied in the quality control of different European WWTPs.     

Crosslinked penicillin acylase aggregates for synthesis of β-lactam antibiotics in organic medium

Crosslinked enzyme aggregates (CLEAs) of a partially purified penicillin acylase from a recombinant Escherichia coli strain have been produced as a novel type of biocatalysts well endowed to perform in organic media. Different protein precipitants were studied and glutaraldehyde was used as the crosslinking agent. Precipitation curves were obtained for all precipitants to determine the concentrations at which all the protein precipitated out of the solution. The effect of the glutaraldehyde-to-protein ratio was studied with respect to process recovery and the specific activity and stability of the biocatalyst. Recovery of penicillin acylase activity was moderately high, about 50%; major losses of enzyme activity were produced at the precipitation step. Specific activities of all CLEAs were very high, which is one of the advantages of using nonsupported biocatalysts. Ammonium sulfate and tert-butyl alcohol were the best precipitants at a glutaraldehyde-protein mass ratio of 2 and were selected to perform the kinetically controlled synthesis of ampicillin in 60% (v/v) ethylene glycol medium. At comparable conversion yields, volumetric and specific antibiotic productivity were much higher for CLEAs than for carrier-bound penicillin acylases.     

A new light emitting diode-light emitting diode portable carbon dioxide gas sensor based on an interchangeable membrane system for industrial applications

A new system for CO₂ measurement (0–100%) based on a paired emitter–detector diode arrangement as a colorimetric detection system is described. Two different configurations were tested: configuration 1 (an opposite side configuration) where a secondary inner-filter effect accounts for CO₂ sensitivity. This configuration involves the absorption of the phosphorescence emitted from a CO₂-insensitive luminophore by an acid–base indicator and configuration 2 wherein the membrane containing the luminophore is removed, simplifying the sensing membrane that now only contains the acid–base indicator. In addition, two different instrumental configurations have been studied, using a paired emitter–detector diode system, consisting of two LEDs wherein one is used as the light source (emitter) and the other is used in reverse bias mode as the light detector. The first configuration uses a green LED as emitter and a red LED as detector, whereas in the second case two identical red LEDs are used as emitter and detector. The system was characterised in terms of sensitivity, dynamic response, reproducibility, stability and temperature influence. We found that configuration 2 presented a better CO₂ response in terms of sensitivity.     

Mössbauer effect phase determination in iron oxide–polyaniline nanocomposites

Mössbauer effect spectroscopy and thermal analysis techniques were applied to characterize polyaniline composites successfully synthesized by embedding Fe oxide nanoparticles (about 10–13 nm) in a polymeric matrix in the presence of dodecyl benzene sulfonic acid and HCl (dopant). Thermal techniques provided quantitative information on iron oxide content and on polyaniline stability and transformations. Mössbauer results indicated that for the whole studied composition range, 3.4 to 100 iron oxide wt.%, composites hold maghemite particles. A preliminary study of the conductivity of the nanocomposites was performed. The largest conductivity was observed for a 8 wt.% maghemite composite where all particles are magnetically unblocked at room temperature within the Mössbauer time window.     

Double cascade turbulence and Richardson dispersion in a horizontal fluid flow induced by Faraday waves

We report the experimental observation of Richardson dispersion and a double cascade in a thin horizontal fluid flow induced by Faraday waves. The energy spectra and the mean spectral energy flux obtained from particle image velocimetry data suggest an inverse energy cascade with Kolmogorov type scaling E(k) ∝ k(γ), γ ≈ -5/3 and an E(k) ∝ k(γ), γ ≈ -3 enstrophy cascade. Particle transport is studied analyzing absolute and relative dispersion as well as the finite size Lyapunov exponent (FSLE) via the direct tracking of real particles and numerical advection of virtual particles. Richardson dispersion with <ΔR(2)(t)> ∝ t(3) is observed and is also reflected in the slopes of the FSLE (Λ ∝ ΔR(-2/3)) for virtual and real particles.