Anomalous Rotational Resonance Spectra in Magic-Angle Spinning NMR

Magic-angle spinning NMR spectra of samples containing dilute spin-1/2 pairs display broadenings or splittings when a rotational resonance condition is satisfied, meaning that a small integer multiple of the spinning frequency matches the difference in the two isotropic shift frequencies. We show experimental rotational resonance NMR spectra of a 13C2-labeled retinal which are in qualitative disagreement with existing theory. We propose an explanation of these anomalous rotational spectra involving residual heteronuclear couplings between the 13C nuclei and the neighboring 1H nuclei. These couplings strongly influence the rotational resonance 13C spectrum, despite the presence of a strong radiofrequency decoupling field at the 1H Larmor frequency. We model the residual heteronuclear couplings by differential transverse relaxation of the 13C single-quantum coherences. We present a superoperator theory of the phenomenon and describe a numerical algorithm for rapid Liouville space simulations in periodic systems. Good agreement with experimental results is obtained by using a biexponential transverse relaxation model for each spin site.     

Mass spectrometric analysis of ceramide composition in mono-, di-, tri-, and tetraglycosylceramides from mouse kidney: an experimental model for uropathogenic Escherichia coli.

Many bacteria have been shown to bind to the carbohydrate part of glycosphingolipids, but also the lipid moieties of receptor-active glycolipids are of importance. To investigate the chemistry of the ceramides of kidney glycolipids to which the uropathogenic Escherichia coli bind, different mass spectrometric techniques were utilized. First, a mixture of glycolipids isolated from man and mice kidney was separated by thin-layer chromatography (TLC) and scanned by direct desorption from the plate by fast atom bombardment mass spectrometry (TLC/FAB-MS). Second, the glycolipids were purified by preparative TLC and analyzed by negative-ion FAB-MS. After methylation, further analyses were made with positive-ion FAB-MS, positive-ion electron ionization (EI)-MS, high-temperature capillary gas chromatography (GC/EI-MS) and positive-ion matrix-assisted laser desorption/ionization (MALDI)-MS. The ceramide compositions of the four glycolipids were determined using all these MS techniques and the reliability of the different methods for this type of analyses is discussed. Comparison of the mouse kidney glycolipids with the corresponding glycolipids from human kidney showed the same degree of hydroxylation of ceramides among mono- and disaccharide glycolipids, but a significantly higher degree of hydroxylation among mouse kidney glycolipids with three and four sugar residues. This result might be of relevance for the binding of P-fimbriated E. coli to the urinary tract tissues.     

Use of immobilized amyloglucosidase as chiral selector in chromatography. Control of enantioselective retention and resolution in liquid chromatography

Summary Several mobile phase parameters were investigated for controlling enantioselective retention and resolution on a chiral stationary phase made in-house. The chiral selector was the enzyme amyloglucosidase, which was immobilized onto a silica support via reductive amination. The influences of the mobile phase pH, concentration and type of uncharged organic modifier, ionic strength and column temperature on enantios-electivity were studied. The analysis time for resolving enantiomers could be adjusted with only a minor decrease in enantioselectivity by using a high ionic strength mobile phase buffer. This indicated a retention mechanism involving ion-exchange interactions. It was further confirmed by the decreasing enantioselectivity of amines when using a mobile phase pH below the isoelectric point of the native protein. Interesting effects were observed when the organic modifier concentration was increased and also when the column temperature was raised. Both retention and enantioselectivity increased with increasing concentration of 2-propanol in the mobile phase. Examples are given where both enantioselectivity and retention increased with increasing column temperature. Thermodynamic studies were performed to calculate the entropy and enthalpy constants. The results showed that, depending on mobile phase composition, the enantioselective retention may be caused by differences in entropy or enthalpy.     

Cryo-TEM Snapshots of Ferritin Adsorbed on Small Zeolite Crystals

The adsorption of macromolecules on zeolite crystal surfaces has been investigated by cryo transmission electron microscopy (TEM) of frozen hydrates. It was determined that ferritin shows different adsorption behavior on NaY (TEM image on the right) and USY zeolites (TEM image on the left).     

Dynamics in prediction of life-time of environmental adaptable polymers

Environmentally adaptable polymers are materials with well-defined life times made by tailoring the synthesis and creating a molecular architecture with precise functionality and morphology. Synthesis of such materials develops initiators/catalysts systems and enzymatic polymerisation while processing need to be adopted to the labile repeating units. The secondary structure of the environmentally adaptable polymers is susceptible to traditional processing introducing e.g. changes in the atomic positions of oxygen of poly(3-hydroxy-co-3-hydroxyvalerate). (Bio-) degradation of these materials is complex and difficult to predict. Modelling test-environments require an understanding of the many biological processes possible in nature. Many biodegradations require e.g. the co-operation of more than one microbial specium, which sometimes render standard biodegradation tests inadequate. The nonabsolute specificity of enzymes are probably responsible for the transformation of many of the novel molecules created in recent times, but it is wishful thinking to expect that every new compound has an enzyme able to catalyse its alteration.     

On MRI turbulence quantification

Turbulent flow, characterized by velocity fluctuations, accompanies many forms of cardiovascular disease and may contribute to their progression and hemodynamic consequences. Several studies have investigated the effects of turbulence on the magnetic resonance imaging (MRI) signal. Quantitative MRI turbulence measurements have recently been shown to have great potential for application both in human cardiovascular flow and in engineering flow. In this article, potential pitfalls and sources of error in MRI turbulence measurements are theoretically and numerically investigated. Data acquisition strategies suitable for turbulence quantification are outlined. The results show that the sensitivity of MRI turbulence measurements to intravoxel mean velocity variations is negligible, but that noise may degrade the estimates if the turbulence encoding parameter is set improperly. Different approaches for utilizing a given amount of scan time were shown to influence the dynamic range and the uncertainty in the turbulence estimates due to noise. The findings reported in this work may be valuable for both in vitro and in vivo studies employing MRI methods for turbulence quantification.     

Thermal analysis as a quality tool for assessing the influence of thermo-mechanical degradation on recycled poly(ethylene terephthalate)

Mechanical recycling of poly(ethylene terephthalate) (PET) was simulated by multiple processing to assess the effects of thermo-mechanical degradation, and characterized using rheological and thermal analysis techniques. Thermo-mechanical degradation under repeated extrusion induces chain scission reactions in PET, which result in a dramatic loss in the deformation capabilities and an increase in the fluidity of the polymer under reprocessing, reducing its recycling possibilities after four extrusion cycles. Multiple reprocessing severely affects the storage modulus and the microstructure of recycled PET, both in the amorphous and crystalline regions. Multimodal melting behavior is observed for reprocessed PET, indicating heterogeneous and segregated crystalline regions. A deconvolution procedure has been applied to individually characterize each crystalline population in terms of lamellar thickness distribution and partial crystallinity. Thermal analysis techniques such as differential scanning calorimetry (DSC) and dynamic-mechanical analysis (DMA) have proved to be suitable techniques for the quality assessment of recycled PET, giving unequivocal information about its degree of degradation compared to the common technological measurements of melt-mass flow rate (MFR) or oxidative stability (T_Ox).     

Oxidation of Ethers,Alcohols, and Unfunctionalized Hydrocarbons by the Methyltrioxorhenium/H2O2 System: A Computational Study on Catalytic C?H Bond Activation

A concerted mechanism that does not involve an ionic intermediate was revealed by a DFT study on oxidation of ethers, alcohols, and unfunctionalized hydrocarbons by methyltrioxorhenium/H₂O₂. Instead, C? H insertion occurs through hydride transfer and then turns into a hydroxide transfer/rebound in a concerted fashion. The picture shows selected frames from an intrinsic reaction coordinate scan from the transition state to the product for the oxidation of cis‐1,2‐dimethylcyclohexane.