How does dielectric solvation affect the size of an ion?

The effect of solvation by a continuum dielectric on the size of an ion is examined using electronic structure calculations. Various measures correlated with size are considered, including the root-mean-square radius of the electronic charge density, the amount of solute charge penetrating outside the cavity, the electronic radial distribution function, the nucleus-electron potential energy, and the electron-electron potential energy. Calculations are made on several representative ionic solutes, and it is found that every measure indicates that the application of a dielectric makes the cations larger and the anions smaller. These counterintuitive trends are examined, and a plausible explanation is offered for the observed behavior.     

How does solvent molecular size affect the microscopic structure in polymer solutions?

Monte Carlo simulation has been used to investigate the effects of linear solvent molecular size on polymer chain conformation in solutions. Increasing the solvent molecular size leads to shrinkage of the polymer chains and increase of the critical overlap concentrations. The root-mean-square radius of gyration of polymer chains (R(g)) is less sensitive to the variation of polymer concentration in solutions of larger solvent molecules. In addition, the dependency of R(g) on polymer concentration under normal solvent conditions and solvent molecular size is in good agreement with scaling laws. When the solvent molecular size approaches the ideal end-to-end distance of the polymer chain, an extra aggregation of polymer chains occurs, and the solvent becomes the so-called medium-sized solvent. When the size of solvent molecules is smaller than the medium size, the polymer chains are swollen or partially swollen. However, when the size of solvent molecules is larger than the medium size, the polymer coils shrink and segregate, enwrapped by the large solvent molecules.     

How much is the accuracy of activation energy affected by ignoring thermal inertia?

This work estimates the magnitude of the effect of thermal inertia on the value of the activation energy determined from heat-flux differential scanning calorimetry (DSC) data. The estimates are obtained via analysis of the literature data on crystallization of copper and thermal degradation of isotactic polystyrene (iPS). The copper crystallization data have been obtained for very large masses (200 mg) and fast heating rates up to 80 K min⁻¹. The iPS degradation data have been collected on small masses (3 mg) and at the heating rates up to 20 K min⁻¹. For crystallization of copper, the Kissinger activation energy obtained from the DSC data corrected for thermal inertia is 34% larger than the value estimated from uncorrected data. This difference drops to 8% and becomes statistically insignificant when the fastest heating rate used is decreased to 10 K min⁻¹. For iPS degradation, the difference in the isoconversional activation energies estimated, respectively, from corrected and uncorrected DSC data is less than 3% and is not statistically significant. Overall, the effect of thermal inertia on the activation energy appears negligible provided that DSC measurements are conducted on smaller samples and at slower heating rates, that is, as advised by the International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommendations. It is suggested that the difference in the activation energies should generally be within the typical 5-10% uncertainty as long as the product of the time constant and the maximum heating rate does not exceed 2-3 K.     

How does cross-linking affect the stability of block copolymer vesicles in the presence of surfactant?

Block copolymer vesicles are conveniently prepared directly in water at relatively high solids by polymerization-induced self-assembly using an aqueous dispersion polymerization formulation based on 2-hydroxypropyl methacrylate. However, dynamic light scattering studies clearly demonstrate that addition of small molecule surfactants to such linear copolymer vesicles disrupts the vesicular membrane. This causes rapid vesicle dissolution in the case of ionic surfactants, with nonionic surfactants proving somewhat less destructive. To address this problem, glycidyl methacrylate can be copolymerized with 2-hydroxypropyl methacrylate and the resulting epoxy-functional block copolymer vesicles are readily cross-linked in aqueous solution using cheap commercially available polymeric diamines. Such epoxy-amine chemistry confers exceptional surfactant tolerance on the cross-linked vesicles and also leads to a distinctive change in their morphology, as judged by transmission electron microscopy. Moreover, pendent unreacted amine groups confer cationic character on these cross-linked vesicles and offer further opportunities for functionalization.     

How does the hydrophobic content of methacrylate ABA triblock copolymers affect polymersome formation?

Polymersomes are exciting self-assembled structures with great potential in pharmaceutical applications. A systematic investigation of a novel series of methacrylate-based polymersomes is reported in this study. Five well-defined ABA triblock copolymers with A being based on tri(ethylene glycol) methyl methacrylate and B being based on 2-(diethylamino)ethyl methacrylate (DMAEMA) were synthesized using a living polymerization method. The effect of the composition of the ABA triblock copolymers on the thickness of the hydrophobic membrane of the polymersomes and the release of a model drug is demonstrated.     

How does a hydrocarbon staple affect peptide hydrophobicity?

Water is essential for the proper folding of proteins and the assembly of protein–protein/ligand complexes. How water regulates complex formation depends on the chemical and topological details of the interface. The dynamics of water in the interdomain region between an E3 ubiquitin ligase (MDM2) and three different peptides derived from the tumor suppressor protein p53 are studied using molecular dynamics. The peptides show bimodal distributions of interdomain water densities across a range of distances. The addition of a hydrocarbon chain to rigidify the peptides (in a process known as stapling) results in an increase in average hydrophobicity of the peptide–protein interface. Additionally, the hydrophobic staple shields a network of water molecules, kinetically stabilizing a water chain hydrogen‐bonded between the peptide and MDM2. These properties could result in a decrease in the energy barrier associated with dehydrating the peptide–protein interface, thereby regulating the kinetics of peptide binding. © 2015 Wiley Periodicals, Inc.     

How do the substituents affect and regulate the relative retention times of polychlorinated biphenyls during gas chromatography?

The effects and regulatory actions of the polychlorinated biphenyls (PCBs) substituent characteristics on their relative retention times (RRTs) during gas chromatography were analyzed based on known experimental RRTs of 209 PCB congeners and biphenyl; the substituent characteristics used for this analysis included the total amount of substituents, the similarity between two phenyl rings in a single PCB congener, the substituents distribution in single phenyl ring, the main/second‐order interactions effects at each position, and the combined effect of two phenyl rings. At last, the universality of regulation was validated on other experimental conditions. Among them, the full factorial experimental design included 10 factors correlated with each substituent position and two levels (0, 1) were initially applied to the domains of the substituent characteristics. The obtained results have revealed that increasing the total amount of substituents can increase the RRTs of PCBs linearly, but similarities between the two rings cannot control the RRTs effectively. Meanwhile, the more compact the substituent distributions on a single phenyl ring are, the bigger the RRTs of PCBs are. Based on a full factorial experimental design, the overall important trend for each position is as follows: para > meta > ortho and the main regulatory substituents for the second‐order interaction effects are distributed in the same phenyl ring in the following sequence: N_o > N_m > N_p. The congener with two perpendicular phenyl rings exhibits a milder combined effect on RRTs and smaller RRT relatively. The regulation has a good universality among different experimental conditions, revealing the dominant effect of substituent characteristics on RRTs of PCBs. Copyright © 2015 John Wiley & Sons, Ltd.     

How much can an intermediate state influence competing reactive pathways?

A molecule undergoing reaction may form a short-lived intermediate. Under certain conditions, the rate at which the reaction proceeds toward the product state via the intermediate may exceed that of a simple, direct path. The competition of two alternative reactive pathways is analyzed here in terms of a stochastic model. The approach allows one to diagnose this competition as a function of the energy of the intermediate relative to the barrier heights of the potential surface and values of the reactive vibrational modes. The result has applications to a variety of problems in chemical physics, ranging from the "lock-and-key" mechanism for the enzymatic activity to control of temporal evolution of complex systems by optimal laser fields.     

How much does the SI,namely the proposed “New SI”, conform to principles of the Metre Treaty?

The International System of Units (SI) was first adopted in 1960, as the more recent implementation of the Metre Treaty signed in 1875. Basic features of the original SI are that (a) seven units are chosen as “base units”, all the others being “derived units”, and (b) the definitions of the base units should not create interdependence. This way, the SI conforms to the basic principle of the Metre Treaty that each signatory country can realise its choice of primary national standards of the very definitions of the units without needing to resort to calibrations obtained from another country, and without obligation to have them realised for all the units. A mismatch already occurs to some extent with respect to the above features in the present definitions of SI base units. This contribution, strictly based on metrological considerations, illustrates how the present proposal concerning new definitions for the base units, called “New SI”, would extend the mismatch. In this frame, also the meaning is discussed of the concepts of hierarchy and traceability in metrology. By outlining some of the consequences, a discussion is stimulated related to the status of base unit, to the meaning of calibration at the level of the standards of the unit definitions, and to the interdependence of countries’ standards.     

How to revise the GUM?

The announcement of a revision of the Guide to the expression of uncertainty in measurement has renewed the debate about the topic of measurement uncertainty. In this paper the author, chairman of Working Group 1 of the Joint Committee for Guides in Metrology, replies to the theses given in two recent papers by Semion Rabinovich. His opinions are personal, and are not necessarily shared by the JCGM/WG1. They are to be intended as a further contribution to the present discussion. Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher.     

A study on the essential oil of Ferulago campestris: How much does extraction method influence the oil composition?

The essential oil of different parts of Ferulago campestris (Bess.) collected in Sicily has been extracted by microwave-assisted hydrodistillation (MAHD) and by classic hydrodistillation (HD). A comparative qualitative-quantitative study on the composition of the oils was carried out. A total of 100 compounds were identified in the oils obtained by MAHD, whereas 88 compounds characterized the HD oils. The most prominent components were, in all different parts of F. campestris and in both extraction methods, 2,4,5-trimethylbenzaldehyde and 2,4,6-trimethylbenzaldehyde isomers; the latter was not previously found. The attempt to evaluate where the oil components are located in all parts of the plant was carried out by means of a kinetic study. Then, electron microscopy observation on the different parts before and after MAHD and HD was performed.     

How much do enzymes really gain by restraining their reacting fragments?

The steric effect, exerted by enzymes on their reacting substrates, has been considered as a major factor in enzyme catalysis. In particular, it has been proposed that enzymes catalyze their reactions by pushing their reacting fragments to a catalytic configuration which is sometimes called near attack configuration (NAC). This work uses computer simulation approaches to determine the relative importance of the steric contribution to enzyme catalysis. The steric proposal is expressed in terms of well defined thermodynamic cycles that compare the reaction in the enzyme to the corresponding reaction in water. The S(N)2 reaction of haloalkane dehalogenase from Xanthobacter autotrophicus GJ10, which was used in previous studies to support the strain concept is chosen as a test case for this proposal. The empirical valence bond (EVB) method provides the reaction potential surfaces in our studies. The reliability and efficiency of this method make it possible to obtain stable results for the steric free energy. Two independent strategies are used to evaluate the actual magnitude of the steric effect. The first applies restraints on the substrate coordinates in water in a way that mimics the steric effect of the protein active site. These restraints are then released and the free energy associated with the release process provides the desired estimate of the steric effect. The second approach eliminates the electrostatic interactions between the substrate and the surrounding in the enzyme and in water, and compares the corresponding reaction profiles. The difference between the resulting profiles provides a direct estimate of the nonelectrostatic contribution to catalysis and the corresponding steric effect. It is found that the nonelectrostatic contribution is about -0.7 kcal/mol while the full "apparent steric contribution" is about -2.2 kcal/mol. The apparent steric effect includes about -1.5 kcal/mol electrostatic contribution. The total electrostatic contribution is found to account for almost all the observed catalytic effect ( approximately -6.1 kcal/mol of the -6.8 calculated total catalytic effect). Thus, it is concluded that the steric effect is not the major source of the catalytic power of haloalkane dehalogenase. Furthermore, it is found that the largest component of the apparent steric effect is associated with the solvent reorganization energy. This solvent-induced effect is quite different from the traditional picture of balance between the repulsive interaction of the reactive fragments and the steric force of the protein.     

Antibacterial cotton fabric prepared by a “grafting to” strategy using a QAC copolymer

Quaternary ammonium compounds (QACs) have outstanding antimicrobial effect, but covalent immobilization of plentiful QAC onto cotton fiber surface to realize a durable function remains a challenge. Herein, a quaternary ammonium monomer, [2-(methacryloyloxy) ethyl] trimethylammonium chloride (DMC) was co-polymerized with methyl acrylate (MA) to prepare an antibacterial copolymer, poly(DMC-co-MA). To graft the copolymer with an improved grafting efficiency, cotton fabric was treated using carboxymethyl chitosan (CMC) to establish an amino-functionalized fiber surface first. This treatment allows the amidation reactions between the amino groups and the pendant ester groups in the poly(DMC-co-MA) to take place, achieving a durable anionic polymer coating onto the fiber surfaces with remarkably antibacterial effect. Characterization results indicated that when DMC/MA monomer ratio was 100:1, the resulting copolymer endows the modified cotton fabric with antibacterial capability that inactivates all Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Even after 50 laundering cycles, more than 98.0% of the antibacterial rate could still be retained. Moreover, the wearing comfort properties such as softness, water absorption and air permeability of the finishing cotton fabrics have been insignificantly changed by comparing to the untreated cotton fabric.

     

How does the solvent guide the hydration process?

The hydration numbers are investigated of the glycine amino acid in solutions of substances with different effects on the structure of water: urea, monomethylurea, and 1,3-dimethylurea. Glycine loses a half of its hydration water in a 20m urea solution and only a quarter of it in a 20m dimethylurea solution. The constancy of the hydration number of glycine in concentrated dimethylurea solutions is due to the compensatory effect of the interactions in the ternary and binary systems.     

Do reaction conditions affect the stereoselectivity in the Staudinger reaction?

The stereochemistry is one of the critical issues in the Staudinger reaction. We have proposed the origin of the stereoselectivity recently. The effects of solvents, additives, and pathways of ketene generation on the stereoselectivity were investigated by using a clean Staudinger reaction, which is a sensitive reaction system to the stereoselectivity. The results indicate that the additives, usually existed and generated in the Staudinger reaction, and the pathways of the ketene generation do not generally affect the stereoselectivity. The solvent affects the stereoselectivity. The polar solvent is favorable to the formation of trans-beta-lactams. The addition orders of the reagents affect the stereoselectivity in the Staudinger reaction between acyl chlorides and imines. The addition of a tertiary amine into a solution of the acyl chloride and the imine generally decreases the stereoselectivity, which is affected by the interval between additions of the acyl chloride and the tertiary amine, and the imine substituents. Our current results provide further understanding on the stereochemistry of the Staudinger reaction between acyl chlorides and imines and on the factors affecting the stereochemistry and also provide a method to prepare beta-lactams with the desired relative configuration via rationally tuning the stereoselectivity-controlling factors in the Staudinger reaction.     

How does column packing microstructure affect column efficiency in liquid chromatography?

Full three-dimensional computer simulations of the fluid flow and dispersion characteristics of model nonporous chromatographic packings are reported. Interstitial porosity and packing defects are varied in an attempt to understand the chromatographic consequences of the packing microstructure. The tracer zone dispersion is calculated in the form of plate height as a function of fluid velocity for seven model particle packs where particles are selectively removed from the packs in clusters of varying size and topology. In an attempt to examine the consequences of loose but random packs, the velocities and zone dispersion of seven defect-free packs are simulated over the range 0.36< or =epsilon< or =0.50, where epsilon is the interstitial porosity. The results indicate that defect-free loose packings can give good chromatographic efficiency but the efficiency can vary depending on subtle details of the pack. When the defect population increases, the zone dispersion increases accordingly. For a particle pack where 6% of the particles are removed from an epsilon=0.36 pack, approximately 33% of the column efficiency is lost. These results show that it is far more important in column packing to prevent defect sites leading to inhomogeneous packing rather than obtaining the highest density pack with the smallest interstitial void volume.