Dramatic substituent effects on the mechanisms of nucleophilic attack on Se—S bridges

The reactions of XSeSX, XSeSY, and YSeSX (X, Y = CH₃, NH₂, OH, F) with F^? and CN^? nucleophiles have been investigated by means of B3PW91/6‐311+G(2df,p) and G4 calculations. In systems where the two substituents are not identical (XSeSY), the more stable of the two possible isomers corresponds to those in which the most electronegative substituent is attached to Se. Nucleophilic attack takes place at Se, independent of the nature of the nucleophile, with the only exception being XSeSF (X = CH₃, NH₂, OH), in which case the attack occurs at S. In agreement with recent results for disulfide and diselenide linkages, the mechanisms leading to Se—S bond cleavage are not always the more favorable ones because for highly electronegative substituents the most favorable process is fission of the chalcogen‐substituent bond. These dissimilarities in the observed reactivity pattern as a function of the electronegativity of the substituents are due to the fact that the σ‐type Se—S antibonding orbital, which for low‐electronegative substituents is the lowest unnoccupied molecular orbital (LUMO), becomes strongly destabilized when the electronegativity of the substituent increases, and is replaced by an antibonding π‐type Se‐X (or S‐X) orbital. In contrast, however, with what has been found for disulfide and diselenide derivatives, the observed reactivity does not change with the nature of the nucleophile. The activation strain model provides interesting insight into these processes, showing that in most cases the activation barriers are the consequence of subtle differences in the strain or in the interaction energies. © 2013 Wiley Periodicals, Inc.     

Mapping the placement of oligonucleotide molecules using scanning probe microscopy

The successful development of novel bio-inspired devices requires the ability to place specific biomolecules on a substrate with nanometre precision, in such a way so that their bioactivity is retained. A method is required that can verify this bio-modification. Scanning probe microscopy (SPM) can image and probe a surface in a liquid environment with nanometre resolution. Using short chain complementary oligonucleotides as the bioactive molecules we have modified continuous and patterned gold substrates and SPM probes. We demonstrated that the attached oligonucleotides retained their biological activity after surface attachment with a hybridization interaction force that varies between 50 and 400pN as measured by SPM force measurements. Finally, the position of the attached oligonucleotides was determined with nanometre resolution. Thus we have demonstrated the capabilities of SPM in the application of the development of substrates and templates suitable for forming the basis of novel and innovative devices.     

Solid-phase extraction and high-performance liquid chromatography mass spectrometry analysis of nitrosamines in treated drinking water and wastewater

N-Nitrosamines, including N-nitrosodimethylamine (NDMA), were identified as chlorination byproducts in drinking water in 1989. Nitrosamines are known rodent carcinogens and probable human carcinogens, and so they are considered disinfection byproducts (DBPs) of public health concern. Epidemiological studies show a potential association of consumption of chlorinated drinking water with an increased risk of bladder cancer. As small, relatively polar DBPs that often occur at low-ng/L concentrations in water, nitrosamines pose analytical challenges for accurate determination. Sample preparation (e.g., the commonly used solid-phase extraction) plays a critical role in achieving reliable determination of nitrosamines at ng/L concentrations. Historically, gas chromatography (GC)-based techniques have been used for nitrosamine analysis. Recently, newly developed liquid chromatography–tandem mass spectrometry (LC-MS²) methods have shown potential advantages in determining polar DBPs. This review focuses on the sample preconcentration methods and LC-MS² determination of nitrosamines in drinking water and wastewater. It also provides a historical perspective on nitrosamines and their occurrence in drinking water.     

Deflection and pull-in instability of nanoscale beams in liquid electrolytes

An elastic beam suspended horizontally over a substrate in liquid electrolyte was subjected to electric, osmotic, and van der Waals forces. The problem, which is governed by four non-dimensional parameters, was solved using the finite element method. The sum of the electric and osmotic forces, the electrochemical force, is usually attractive. However, the electrochemical force can be repulsive for a narrow range of the ion concentration, the initial separation and surface potentials. Furthermore, the beam deflection is not a monotonic function of the applied surface potentials, the bulk ion concentration, or the initial separation between the beam and the substrate. As these parameters are increased monotonically, the beam bends up and then down. The pull-in voltage increases as the bulk ion concentration increases. The pull-in voltage of a double-wall carbon nanotube suspended over a graphite substrate in liquid can be less than or greater than the pull-in voltage in air, depending on the bulk ion concentration. The critical separation between the DWCNT and the substrate increases with the bulk ion concentration. However, for a given bulk ion concentration, the critical separation is independent of the electric potentials. Furthermore, the critical separation is approximately equal in liquid and air.     

Seasonal changes in the excess energy dissipation from Photosystem II antennae in overwintering evergreen broad-leaved trees Quercus myrsinaefolia and Machilus thunbergii

We monitored chlorophyll (Chl) fluorescence, pigment concentration and the de-epoxidation state of the xanthophyll cycle (DPS(1)) in two warm temperate broad-leaved evergreen species (Quercus myrsinaefolia and Machilus thunbergii). Reduction of the maximal quantum yield of Photosystem II (PSII) (calculated from Fv/Fm, variable to maximal Chl a fluorescence) and retention of a high DPS were observed in both species in the winter, and can be interpreted as acclimation to winter. In particular, the acclimation of PSII in these species can be chiefly attributed to thermal dissipation, which is correlated with the retention of high zeaxanthin. Furthermore, we attempted to divide the fate of the absorbed light energy by the PSII antennae into three components: (i) PSII photochemistry (represented by its quantum yield, ΦPSII), (ii) dissipation by down-regulation via non-photochemical quenching (ΦNPQ) and (iii) other non-photochemical processes (ΦONP). The estimated energy allocation of the absorbed light indicated that the proportion of ΦPSII decreased, whereas that of ΦNPQ+ΦONP increased during winter. This result suggests that the excess energy absorbed in the PSII complexes is safely dissipated from the PSII antennae. Based on these results, we conclude that thermal dissipation from the PSII antennae plays an important role in two overwintering broad-leaved evergreen trees growing in Japan.     

Anomalously Large Antiphase Signals from Hyperpolarized Orthohydrogen Using a MOF-Based SABRE Catalyst

Hyperpolarized orthohydrogen (o-H₂) is a frequent product of parahydrogen-based hyperpolarization approaches like signal amplification by reversible exchange (SABRE), where the hyperpolarized o-H₂ signal is usually absorptive. We describe a novel manifestation of this effect wherein large antiphase o-H₂ signals are observed, with ¹H enhancements up to ≈500-fold (effective polarization P_H≈1.6 %). This anomalous effect is attained only when using an intact heterogeneous catalyst constructed using a metal–organic framework (MOF) and is qualitatively independent of substrate nature. This seemingly paradoxical observation is analogous to the “partial negative line” (PNL) effect recently explained in the context of Parahydrogen Induced Polarization (PHIP) by Ivanov and co-workers. The two-spin order of the o-H₂ resonance is manifested by a two-fold higher Rabi frequency, and the lifetime of the antiphase HP o-H₂ resonance is extended by several-fold.