Ligand Structural Effects on the Electrochemistry of Chromium(III) Amino Carboxylate Complexes

The aqueous electrochemical behavior of 10 Cr(III) complexes with potentially tri- and hexadentate amino carboxylate ligands is reported and is shown to depend on the composition and spatial arrangement of the donor atom set. Complexes with two amine and four carboxylate donors (N(2)O(4)) and two amine, one aquo, and three carboxylate donors (N(2)O(3)O') in which the N atoms are coordinated cis to one another undergo chemically and electrochemically reversible reduction at ca. -1.4 and ca. -1.2 V vs SCE, respectively. However, complexes with a trans-N(2)O(4) donor atom set, as exemplified by Cr(MIDA)(2)(-) (MIDA(2)(-) = N-methyliminodiacetate), undergo quasi-reversible Cr(III/II) reduction at ca. -1.4 V that is followed by a sequence of reactions which establishes an electrochemical square scheme. The chemical reactions in the scheme involve displacement of a bound carboxylate group following reduction to Cr(II) and its reattachment after reoxidation to Cr(III). This mechanistic sequence is analyzed by digital simulation, and values of formal potentials, transfer coefficients, and chemical and electrochemical rate constants are reported for Cr(MIDA)(2)(-) and its N-ethyl homolog. The difference in electrochemical behavior between cis- and trans-N(2)O(4) complexes is attributed to differences in the Jahn-Teller distortions experienced by these structures upon reduction to Cr(II). It is proposed that simultaneous N-Cr-N bond elongation, which is possible only for trans species, leads to greater strain in the facially coordinated N-alkyliminodiacetate ligand and thus increases the barrier to electron transfer and facilitates Cr-carboxylate bond cleavage after reduction.     

Prediction of the bed-load transport by gas–liquid stratified flows in horizontal ducts

Solid particles can be transported as a mobile granular bed, known as bed-load, by pressure-driven flows. A common case in industry is the presence of bed-load in stratified gas–liquid flows in horizontal ducts. In this case, an initially flat granular bed may be unstable, generating ripples and dunes. This three-phase flow, although complex, can be modeled under some simplifying assumptions. This paper presents a model for the estimation of some bed-load characteristics. Based on parameters easily measurable in industry, the model can predict the local bed-load flow rates and the celerity and the wavelength of instabilities appearing on the granular bed.     

Periodic Director Patterns in Twist and Splay Frederiks Transitions in Tobacco Mosaic Virus Liouid Crystals

When a magnetic field is applied in the plane and perpendicular to the unperturbed director of a uniform planar-aligned liquid crystal, a twist Frederiks transition occurs. For fields greater than a critical value, periodic bend-modulated twist distortions are dynamically favored over the uniform twist distortion. This is because the periodic flow pattern associated with the bend twist distortion lowers the effective viscosity more than it raises the elastic restoring torque. Under crossed polarizers the sample has a striped texture with the stripes perpendicular to the initial director. The wavelength of the stripe texture is field dependent.

When the magnetic field is applied perpendicular to the uniform planar sample, a similar striped distortion occurs above a critical field value, dominated by splay rather than twist. The most striking difference from the twist transition is that there is a thickness-dependent angle between the stripes and the original director whereas in thinner samples, the stripes tend to be more parallel to the original director. The initial development of the structure appears as a lattice of diffuse spots on a diamond-shaped grid, a precursor that is not seen in the twist instability, which nucleates as stripes at the boundaries of the cell. The splay Frederiks transition also exhibits a field-dependent wavelength distortion.     

Sulfinimine-derived 2,3-diamino esters in the asymmetric synthesis of piperidine (2S,3S)-(+)-CP-99,994

Sulfinimine-derived, differentially protected, 2,3-diamino esters are useful building blocks for the asymmetric synthesis of heterocycles and is illustrated by an efficient synthesis of amino piperidine (+)-CP-99,994.     

The Motion of a Pair of Charged Particles

We revisit the problem of two (oppositely) charged particles interacting electromagnetically in one dimension with retarded potentials and no radiation reaction. The specific quantitative result of interest is the time it takes for the particles to fall in towards one another. Starting with the nonrelativistic form, we answer this question while adding layers of complexity until we arrive at the full relativistic delay differential equation that governs this problem. That case can be solved using the Synge method, which we describe and discuss.     

Probing dynamic protein ensembles with atomic proximity measures

The emerging role of internal dynamics in protein fold and function requires new avenues of structure analysis. We analyzed the dynamically restrained conformational ensemble of ubiquitin generated from residual dipolar coupling data, in terms of protruding and buried atoms as well as interatomic distances, using four proximity-based algorithms, CX, DPX, PRIDE and PRIDE-NMR (http://hydra.icgeb.trieste.it/protein/). We found that Ubiquitin, this relatively rigid molecule has a highly diverse dynamic ensemble. The environment of protruding atoms is highly variable across conformers, on the other hand, only a part of buried atoms tends to fluctuate. The variability of the ensemble cautions against the use of single conformers when explaining functional phenomena. We also give a detailed evaluation of PRIDE-NMR on a wide dataset and discuss its usage in the light of the features of available NMR distance restraint sets in public databases.     

Comparison of particle-in-cell simulations with experimentally observed frequency shifts between ions of the same mass-to-charge in fourier transform ion cyclotron resonance mass spectrometry

It has been previously observed that the measured frequency of ions in a Fourier transform mass spectrometry experiment depend upon the number of trapped ions, even for populations consisting exclusively of a single mass-to-charge. Since ions of the same mass-to-charge are thought not to exert a space-charge effect among themselves, the experimental observation of such frequency shifts raises questions about their origin. To determine the source of such experimentally observed frequency shifts, multiparticle ion trajectory simulations have been conducted on monoisotopic populations of Cs⁺ ranging from 10² ions to 10⁶ ions. A close match to experimental behavior is observed. By probing the effect of ion number and orbital radius on the shift in the cyclotron frequency, it is shown that for a monoisotopic population of ions, the frequency shift is caused by the interaction of ions with their image-charge. The addition of ions of a second mass-to-charge to the simulation allows the comparison of the magnitude of the frequency shift resulting from space-charge (ion-ion) effects versus ion interactions with their image charge.     

Is BTZ a separate superselection sector of CTMG?

We exhibit exact solutions of (positive) matter coupled to original “wrong G-sign” cosmological TMG. They all evolve to conical singularity, rather than to black hole – here negative mass – BTZ. This provides evidence that the latter constitute a separate “superselection” sector, one that unlike in GR, is not reachable by physical sources.