776 research outputs found
Remote participation during glycosylation reactions of galactose building blocks: Direct evidence from cryogenic vibrational spectroscopy
The stereoselective formation of 1,2âcisâglycosidic bonds is challenging. However, 1,2âcisâselectivity can be induced by remote participation of C4 or C6 ester groups. Reactions involving remote participation are believed to proceed via a key ionic intermediate, the glycosyl cation. Although mechanistic pathways were postulated many years ago, the structure of the reaction intermediates remained elusive owing to their shortâlived nature. Herein, we unravel the structure of glycosyl cations involved in remote participation reactions via cryogenic vibrational spectroscopy and first principles theory. Acetyl groups at C4 ensure αâselective galactosylations by forming a covalent bond to the anomeric carbon in dioxoleniumâtype ions. Unexpectedly, also benzyl ether protecting groups can engage in remote participation and promote the stereoselective formation of 1,2âcisâglycosidic bonds
Atomic clusters of magnetic oxides: Structure and phonons
This work represents a combined experimental and theoretical study of structural and magnetic properties of clusters made of cobalt, chromium, and manganese oxides. The clusters were prepared in a molecular cluster source by oxidation of laser-vaporized metal and studied in a time-of-flight spectrometer. Infrared laser-induced cluster dissociation experiments revealed the spectrum of cluster vibrational states. We also performed ab initio local spin density approximation calculations of the equilibrium geometry, electronic structure, and magnetic properties of these clusters
Tunability of Critical Casimir Interactions by Boundary Conditions
We experimentally demonstrate that critical Casimir forces in colloidal
systems can be continuously tuned by the choice of boundary conditions. The
interaction potential of a colloidal particle in a mixture of water and
2,6-lutidine has been measured above a substrate with a gradient in its
preferential adsorption properties for the mixture's components. We find that
the interaction potentials at constant temperature but different positions
relative to the gradient continuously change from attraction to repulsion. This
demonstrates that critical Casimir forces respond not only to minute
temperature changes but also to small changes in the surface properties.Comment: 4 figures;
http://www.iop.org/EJ/article/0295-5075/88/2/26001/epl_88_2_26001.htm
Plate-height model of ion mobility-mass spectrometry: Part 2-Peak-to-peak resolution and peak capacity
In a previous work, we explored zone broadening and the achievable plate numbers in linear drift tube ion mobility-mass spectrometry through developing a plate-height model [1]. On the basis of these findings, the present theoretical study extends the model by exploring peak-to-peak resolution and peak capacity in ion mobility separations. The first part provides a critical overview of chromatography-influenced resolution equations, including refinement of existing formulae. Furthermore, we present exact resolution equations for drift tube ion mobility spectrometry based on first principles. Upon implementing simple modifications, these exact formulae could be readily extended to traveling wave ion mobility separations and to cases when ion mobility spectrometry is coupled to mass spectrometry. The second part focuses on peak capacity. The well-known assumptions of constant plate number and constant peak width form the basis of existing approximate solutions. To overcome their limitations, an exact peak capacity equation is derived for drift tube ion mobility spectrometry. This exact solution is rooted in a suitable physical model of peak broadening, accounting for the finite injection pulse and subsequent diffusional spreading. By borrowing concepts from the theoretical toolbox of chromatography, we believe that the present study will help in integrating ion mobility spectrometry into the unified language of separation science
Plate-height model of ion mobility-mass spectrometry
In the past decade, ion mobility spectrometry (IMS) in combination with mass spectrometry (IM-MS) became a widely employed technique for the separation and structural characterization of ionic species in the gas phase. Similarly to chromatography, where studies on the mechanism of band broadening and adequate plate-height equations have been aiding method development and promoting advancements in column technology, a suitable resolving power theory of drift tube ion mobility-mass spectrometry (DTIM-MS) is essential to stimulate further progress in this emerging field of separation science. In the present study, therefore, we explore dispersion processes in detail and present a plate-height model of ion mobility-mass spectrometry. We quantify the effects of five major dispersion processes that contribute to zone broadening and determine the resolving power in DTIM-MS: diffusion, Coulomb repulsion, electric field inhomogeneities, the finite initial spread of the ion cloud and dispersion outside the mobility cell. A solution is provided to account for the nonuniform separation field in IM-MS in the presence of multiple compartments. The equations â derived from first principles â serve as the basis for formulating an expression that is similar in nature to van Deemter's plate-height equation for chromatography. A comprehensive set of experiments was performed on a custom-built DTIM-MS instrument to evaluate the accuracy of the plate-height model, resulting in satisfactory agreement between experiment and theory. Building on these findings, the plate-height equation was employed to explore the influence of drift gas pressure, injection pulse-width and the mobility of ions on resolving power from a theoretical point of view. Our findings may aid instrument design and development in the future, as well as the optimization of measurement conditions to improve ion mobility separations. By employing the plate-height concept and the general formalism of differential migration processes to describe zone spreading in IM-MS, we aim to find a common ground between this emerging method and such well-established techniques as HPLC or CZE. We also hope that the work presented here will facilitate a broader acceptance of IMS as a separation method of great potential by the communities of chromatography and electrophoresis, as well as that of mass spectrometry
Generalized Newton-Cartan geometries for particles and strings
We discuss the generalized Newton-Cartan geometries that can serve as gravitational background fields for particles and strings. In order to enable us to define affine connections that are invariant under all the symmetries of the structure group, we describe torsionful geometries with independent torsion tensors. A characteristic feature of the non-Lorentzian geometries we consider is that some of the torsion tensors are so-called âintrinsic torsionâ tensors. Setting some components of these intrinsic torsion tensors to zero leads to constraints on the geometry. For both particles and strings, we discuss various such constraints that can be imposed consistently with the structure group symmetries. In this way, we reproduce several results in the literature.</p
Nazi Punks Folk Off: Leisure, Nationalism, Cultural Identity and the Consumption of Metal and Folk Music
Far-right activists have attempted to infiltrate and use popular music scenes to propagate their racialised ideologies. This paper explores attempts by the far right to co-opt two particular music scenes: black metal and English folk. Discourse tracing is used to explore online debates about boundaries, belonging and exclusion in the two scenes, and to compare such online debates with ethnographic work and previous research. It is argued that both scenes have differently resisted the far right through the policing of boundaries and communicative choices, but both scenes are compromised by their relationship to myths of whiteness and the instrumentality of the pop music industry
Genome Biol.
With genome analysis expanding from the study of genes to the study of gene regulation, 'regulatory genomics' utilizes sequence information, evolution and functional genomics measurements to unravel how regulatory information is encoded in the genome
The impact of leaving group anomericity on the structure of glycosyl cations of protected galactosides
It has been reported that fragments produced by glycosidic bond breakage in mass spectrometryâbased experiments can retain a memory of their anomeric configuration, which has major implications for glycan sequencing. Herein, we use cryogenic vibrational spectroscopy and ion mobilityâmass spectrometry to study the structure of Bâtype fragments of protected galactosides. Cationic fragments were generated from glycosyl donors carrying trichloroacetimidate or thioethyl leaving groups of different anomeric configuration. The obtained infrared signatures indicate that the investigated fragments exhibit an identical structure, which suggests that there is no anomeric memory in Bâtype ions of fully protected monosaccharides
Critical Casimir effect in classical binary liquid mixtures
If a fluctuating medium is confined, the ensuing perturbation of its
fluctuation spectrum generates Casimir-like effective forces acting on its
confining surfaces. Near a continuous phase transition of such a medium the
corresponding order parameter fluctuations occur on all length scales and
therefore close to the critical point this effect acquires a universal
character, i.e., to a large extent it is independent of the microscopic details
of the actual system. Accordingly it can be calculated theoretically by
studying suitable representative model systems.
We report on the direct measurement of critical Casimir forces by total
internal reflection microscopy (TIRM), with femto-Newton resolution. The
corresponding potentials are determined for individual colloidal particles
floating above a substrate under the action of the critical thermal noise in
the solvent medium, constituted by a binary liquid mixture of water and
2,6-lutidine near its lower consolute point. Depending on the relative
adsorption preferences of the colloid and substrate surfaces with respect to
the two components of the binary liquid mixture, we observe that, upon
approaching the critical point of the solvent, attractive or repulsive forces
emerge and supersede those prevailing away from it. Based on the knowledge of
the critical Casimir forces acting in film geometries within the Ising
universality class and with equal or opposing boundary conditions, we provide
the corresponding theoretical predictions for the sphere-planar wall geometry
of the experiment. The experimental data for the effective potential can be
interpreted consistently in terms of these predictions and a remarkable
quantitative agreement is observed.Comment: 30 pages, 17 figure
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