Tunneling of two interacting electrons in a quantum wire

A model calculation is reported for the tunneling probability of one as well as two interacting electrons from a quantum well within a narrow channel. We discuss the cases when the two electrons are spin polarized or unpolarized by transforming the system to a noninteracting one with the use of quantal density functional theory to obtain an effective single-particle confining potential. A semiclassical approach is used to obtain the tunneling probability from this effective potential. The calculation is motivated by recent measurements of the conductance of an electron gas in a narrow channel but is not meant to explain the anomalous behavior that has been reported since, for example, we deal with a simplified two-level system. Numerical results for the tunneling probability are presented.     

Fragmentations and reactions of the organophosphate insecticide Diazinon and its oxygen analog Diazoxon studied by electrospray ionization ion trap mass spectrometry

The fragmentations and reactions of Diazinon and related compounds have been studied by electrospray ionization ion trap mass spectrometry. Several novel fragmentation and rearrangements have been observed, including an intramolecular thiono-thiolo rearrangement. The stability, in the gas-phase, of the protomers of 2-isopropyl-4-methyl-6-pyrimidinol has been demonstrated. The complexity of the gas phase ion processes observed suggest that, at present, caution should be exercised in using this approach for the analysis of environmental and other samples until our understanding of these processes increases considerably.     

Energy migration in novel pH-triggered self-assembled beta-sheet ribbons

Energy migration between tryptophan residues has been experimentally demonstrated in self-assembled peptide tapes. Each peptide contains 11 amino acids with a Trp at position 6. The peptide self-assembly is pH-sensitive and forms amphiphilic tapes, which further stack in ribbons (double tapes) and fibrils in water depending on the concentration. Fluorescence spectra, quenching, and anisotropy experiments showed that when the pH is lowered from 9 to 2, the peptide self-assembly buries the tryptophan in a hydrophobic and restricted environment in the interior of stable ribbons as expected on the basis of the peptide design. These fluorescence data support directly and for the first time the presence of such ribbons which are characterized by a highly packed and stable hydrophobic interior. In common with Trp in many proteins, fluorescence lifetimes are nonexponential, but the average lifetime is shorter at low pH, possibly due to quenching with neighboring Phe residues. Unexpectedly, time-resolved fluorescence anisotropy does not change significantly with self-assembly when in water. In highly viscous sucrose-water mixtures, the anisotropy decay at low pH was largely unchanged compared to that in water, whereas at high pH, the anisotropy decay increased significantly. We concluded that depolarization at low pH was not due to rotational diffusion but mainly due to energy migration between adjacent tryptophan residues. This was supported by a master equation kinetic model of Trp-Trp energy migration, which showed that the simulated and experimental results are in good agreement, although on average only three Trp residues were visited before emission.     

Ion-pair formation in aqueous solutions of silver nitrate: A Raman and infrared spectral study

A detailed study of the Raman and infrared spectral line shapes and line parameters of aqueous solutions (both H₂O and D₂O) of AgNO₃ is interpreted in terms of an equilibrium between “free” ions and the ion pair Ag⁺NO ₃ ^? . An association constant of 0.1M ^?1 was obtained from both the 717 cm^?1 and 1047 cm^?1 line intensities. Spectral features suggest a significant degree of covalence in the interaction. A C_s(σ_V) model with Ag⁺ above the plane of NO ₃ ^? is consistent with the data.     

The temperature dependence of the d-electron dipole strengths of cobalt(II) tetrahalides

The dipole strengths of the ⁴A₂ → ⁴T₁(F), ⁴T₁(P), d-electron transitions of the cobalt(II) tetrahalides are found to decrease with an increase in temperature, in agreement with a dynamic ligand-polarisation model for the absorption mechanism. The reduction in dipole strength is accounted for by a decrease in the coulombic coupling between the quadrupole moment of the d-electron transition of the metal ion and the transient electric dipole moment induced in the ligands, due to the anharmonic increase in the mean bond length and the progressively larger mean-square amplitudes of the bending modes of the complex ion in its ground electronic state as the temperature is raised.     

Synthesis and photochemistry of a new class of photocleavable protein cross-linking reagents

A new series of photocleavable protein cross-linking reagents based on bis(maleimide) derivatives of diaryl disulfides have been synthesised. They have been functionalised with cysteine and transient absorption spectra for the photolysis reaction have been recorded by using the pump-probe technique with a time resolution of 100 femtoseconds. Photolysis of the disulfide bond yields the corresponding thiyl radicals in less than a picosecond. There is a significant amount of geminate recombination, but some of the radicals escape the solvent cage and the quantum yield for photocleavage is 30 % in water.     

Multiple polypeptide forms observed in two-dimensional gels of Methylococcus capsulatus (Bath) polypeptides are generated during the separation procedure

We have examined two-dimensional electrophoresis (2-DE) gel maps of polypeptides from the Gram-negative bacterium Methylococcus capsulatus (Bath) and found the same widespread trains of spots as often reported in 2-DE gels of polypeptides of other Gram-negative bacteria. Some of the trains of polypeptides, both from the outer membrane and soluble protein fraction, were shown to be generated during the separation procedure of 2-DE, and not by covalent post-translational modifications. The trains were found to be regenerated when rerunning individual polypeptide spots. The polypeptides analysed giving this type of trains were all found to be classified as stable polypeptides according to the instability index of Guruprasad et al. (Protein Eng. 1990, 4, 155-161). The phenomenon most likely reflects conformational equilibria of polypeptides arising from the experimental conditions used, and is a clear drawback of the standard 2-DE procedure, making the gel picture unnecessarily complex to analyse.