Critical evaluation of buffering solutions for pKa determination by capillary electrophoresis

The performance of the most common and also some other less common CE buffers has been tested for the pKa determination of several types of compounds (pyridine, amines, and phenols). The selected buffers cover a pH ranging from 3.7 to 11.8. Whereas some buffers, like acetic acid/acetate, BisTrisH+/BisTris, TrisH+/Tris, CHES/CHES-, and CAPS/CAPS- can be used with all type of analytes, others like ammonium/ammonia, butylammonium/butylammonia, ethylammonium/ethylammonia, diethylammonium/diethylammonia, and hydrogenphosphate/phosphate are not recommended because they interact with a wide range of compounds. The rest of the tested buffers (dihydrogenphosphate/hydrogenphosphate, MES/MES-, HEPES/HEPES-, and boric acid/borate) can show specific interactions depending on the nature of the analytes, and their use in some applications should be restricted.     

A novel Ni(4) complex exhibiting microsecond quantum tunneling of the magnetization

A highly asymmetric Ni(II) cluster [Ni(4)(OH)(OMe)(3)(Hphpz)(4)(MeOH)(3)](MeOH) (1) (H(2)phpz=3-methyl-5-(2-hydroxyphenyl)pyrazole) has been prepared and its structure determined by means of single-crystal X-ray diffraction by using synchrotron radiation. Variable-temperature bulk-magnetization measurements show that the complex exhibits intramolecular-ferromagnetic interactions leading to a spin ground state S=4 with close-lying excited states. Magnetization and high-frequency EPR measurements suggest the presence of sizable Ising-type magnetic anisotropy, with zero-field splitting parameters D=-0.263 cm(-1) and E=0.04 cm(-1) for the spin ground state, and an isotropic g value of 2.25. The presence of both axial and transverse anisotropy was confirmed through low-temperature specific heat determinations down to 300 mK, but no slow relaxation of the magnetization was observed by AC measurements down to 1.8 K. Interestingly, AC susceptibility measurements down to temperatures as low as 23 mK showed no indication of slow relaxation of the magnetization in 1. Thus, despite the presence of an anisotropy barrier (U approximately 4.21 cm(-1) for the purely axial limit), the magnetization relaxation remains extremely fast down to the lowest temperatures. The estimated quantum tunneling rate, Gamma>0.667 MHz, makes this complex a prime candidate for observation of coherent tunneling of the magnetization.     

Solid–Vapor Reaction Growth of Transition‐Metal Dichalcogenide Monolayers

Two‐dimensional (2D) layered semiconducting transition‐metal dichalcogenides (TMDCs) are promising candidates for next‐generation ultrathin, flexible, and transparent electronics. Chemical vapor deposition (CVD) is a promising method for their controllable, scalable synthesis but the growth mechanism is poorly understood. Herein, we present systematic studies to understand the CVD growth mechanism of monolayer MoSe₂, showing reaction pathways for growth from solid and vapor precursors. Examination of metastable nanoparticles deposited on the substrate during growth shows intermediate growth stages and conversion of non‐stoichiometric nanoparticles into stoichiometric 2D MoSe₂ monolayers. The growth steps involve the evaporation and reduction of MoO₃ solid precursors to sub‐oxides and stepwise reactions with Se vapor to finally form MoSe₂. The experimental results and proposed model were corroborated by ab initio Car–Parrinello molecular dynamics studies.     

Single-walled carbon nanotube binding peptides: probing tryptophan's importance by unnatural amino acid substitution

Peptides selected from phage-displayed libraries have been found to exhibit high-affinity binding to carbon nanotubes including single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes, and single-walled carbon nanohorns. One unique feature of these peptides is that their amino acid sequences are rich in tryptophan and histidine residues. The aim of this study was to investigate the importance of the tryptophan residue in a newly identified SWNT-binding peptide, UW-1, which contains the motif, XTHXXPWTX, where X is any amino acid. Tryptophan was altered in the following ways: mutation to alanine or substitution with three unnatural tryptophan analogues, i.e., 5-fluorotryptophan, 5-hydroxytryptophan, and 7-azatryptophan. Analysis of experimental and computational data suggests that the highest occupied molecular orbital of the tryptophan residue in the peptide interacts with the lowest unoccupied molecular orbital from the SWNT. This information should be important in permitting modulation of peptide affinities to these nanomaterials.     

A fuzzy distance metric for measuring the dissimilarity of planar chromatographic profiles with application to denaturing gradient gel electrophoresis data from human skin microbes: demonstration of an individual and gender-based fingerprint

A newly devised fuzzy metric for measuring the dissimilarity between two planar chromatographic profiles is proposed in this paper. It does not require an accurately assigned sample-feature matrix and can cope with slight imprecision of the positional information. This makes it very suitable for 1-D techniques which do not have a second spectroscopic dimension to aid variable assignment. The usefulness of this metric has been demonstrated on a large data set consisting of nearly 400 samples from Denaturing Gradient Gel Electrophoresis (DGGE) analysis of microbes on human skin. The pattern revealed by this dissimilarity metric was compared with the one represented by a sample-feature matrix and highly consistent results were obtained. Several pattern recognition techniques have been applied on the dissimilarity matrix based on this dissimilarity metric. According to rank analysis, within-individual variation is significantly less than between-individual variation, suggesting a unique individual microbial fingerprint. Principal Coordinates Analysis (PCO) suggests that there is a considerable separation between genders. These results suggest that there are specific microbial colonies characteristic of individuals.     

Analytical artifacts produced by a polycarbonate chamber compared to a Teflon chamber for measuring surface mercury fluxes

We found significant differences in mercury fluxes measured with a dynamic surface mercury flux chamber made of Teflon versus one made of polycarbonate. While both materials responded reasonably well when virgin materials were used, the polycarbonate chamber was found to exhibit significant chamber blanks under light after it was exposed to surface mercury fluxes of >100 ng/m²/h. Most significantly, the polycarbonate chamber blocked all wavelengths of light below ∼320 nm. Given that ultraviolet radiation plays an important role in soil mercury flux, the polycarbonate chamber was found to significantly underestimate observed fluxes from background soil in both high light conditions (by 1–4-fold) and under diffuse, low light conditions (by ∼10-fold). These results suggest that Teflon produces fewer analytical artifacts in the surface emission of mercury measured with a flux chamber than polycarbonate.     

Studies on DMSO-containing carbanilation mixtures: chemistry,oxidations and cellulose integrity

The oxidative effect of carbanilation mixtures containing dimethylsulfoxide (DMSO) was demonstrated by means of alcohol model substances in which competitive carbanilation was prevented due to steric hindrance of the hydroxyl function, rendering those compounds specific probes for oxidation effects. Dimethylsulfonium ions and derived ylide species were shown to be the actually oxidizing species according to trapping methodology using lipophilic olefins which were converted into the corresponding cyclopropane and epoxide derivatives. The experimental data were in good agreement with DFT computations carried out on the B3LYP/6-311+G(d,p) level of theory. The direct interaction of cellulose and sulfoxide solvent was proven by means of methyl-(2-naphthyl)sulfoxide (MNSO) as a model for DMSO, which caused introduction of UV-detectable methylthionaphthyl ether moieties into the cellulose, formed in Pummerer-type side reaction paralleling the chemical behavior of DMSO. A facile color test—responding to sulfoxide-derived oxidizing species—was developed to assess the suitability of carbanilation conditions with regard to cellulose oxidation and degradation. DMSO-based carbanilation systems have to be used with great caution for determination of molecular weight parameters and for similar purposes which require complete maintenance of the cellulose integrity. Cellulose oxidation/degradation by DMSO-derived intermediates upon carbanilation can be minimized but cannot be avoided completely. Thus, if cellulose integrity is an issue as it is in cellulose analytics, it is recommended to replace DMSO by solvent components of similar solution behavior but without the inherent danger of generating oxidants, such as pyridine or DMAc, whenever possible.

Retention of ionisable compounds on high-performance liquid chromatography XVII. Estimation of the pH variation of aqueous buffers with the change of the methanol fraction of the mobile phase

The use of methanol-aqueous buffer mobile phases in HPLC is a common election when performing chromatographic separations of ionisable analytes. The addition of methanol to the aqueous buffer to prepare such a mobile phase changes the buffer capacity and the pH of the solution. In the present work, the variation of these buffer properties is studied for acetic acid-acetate, phosphoric acid-dihydrogenphosphate-hydrogenphosphate, citric acid-dihydrogencitrate-hydrogencitrate-citrate, and ammonium-ammonia buffers. It is well established that the pH change of the buffers depends on the initial concentration and aqueous pH of the buffer, on the percentage of methanol added, and on the particular buffer used. The proposed equations allow the pH estimation of methanol-water buffered mobile phases up to 80% in volume of organic modifier from initial aqueous buffer pH and buffer concentration (before adding methanol) between 0.001 and 0.01 mol L(-1). From both the estimated pH values of the mobile phase and the estimated pKa of the ionisable analytes, it is possible to predict the degree of ionisation of the analytes and therefore, the interpretation of acid-base analytes behaviour in a particular methanol-water buffered mobile phase.