Two-compartment radial diffusive exchange analysis of the NMR lineshape of (129)Xe dissolved in a perfluorooctyl bromide emulsion

Hyperpolarized (129)Xe (xenon) gas dissolved in a perfluorooctyl bromide (PFOB) emulsion stabilized with egg yolk phospholipid (EYP) is a possible contrast agent for quantitative blood flow measurements using magnetic resonance imaging. The NMR line shape of xenon dissolved in PFOB emulsion depends strongly on the exchange of spins between PFOB and water. The exchange in this system depends on three factors: the geometrical factors (i.e., droplet size and surrounding water volume), the permeability of the EYP monolayer surrounding the droplet, and the diffusion coefficients of xenon in the two media. A theoretical model which predicts the line shape of xenon in the emulsion based on the Bloch-Torrey equations is presented. Fitting the full width at half maximum (FWHM) of the theoretical line shapes with the FWHM of the experimental spectra obtained from emulsions with different water dilutions allows estimation of the volume-weighted average diameter of the PFOB droplets (3.5+/-0.8) microm and the permeability of the EYP membrane surrounding the droplet (58+/-14) microm / s.     

3He NMR as a sensitive probe of fullerene reactivity: [2 + 2] photocycloaddition of 3-methyl-2-cyclohexenone to C70

The [2 + 2] photoadditions of 3-methyl-2-cyclohexenone to C70 and 3He@C70 have been studied by a combination of HPLC chromatography and FAB-MS, as well as IR and 1H and 3He NMR spectroscopies. The total yield of the mixture of monoadducts was 55% (67% on the basis of the recovered C70). The use of 3He NMR was especially powerful in determining the regioselectivity of the photoaddition reaction of enone to C70. Results of the 3He NMR experiments conducted on the product mixture implicate the two [6,6] bonds closest to the poles of the fullerene (C1-C2 and C5-C6) in the photoaddition process. This reaction mode is analogous to that of most thermal addition reactions to C70. Separation and characterization of the product mixture shows that eight distinct monoadducts are formed in the photoaddition, namely, the four diastereomeric adducts to the C1-C2 and C5-C6 bonds of the C70 cage, each consisting of cis- and trans-fused isomers in a ratio of 2:3. The major mode of photoaddition, accounting for 65% of the product mixture, involves addition to the C1-C2 bond of the ovoid fullerene. Mechanistic implications of these findings are discussed.     

Magnetic Resonance Imaging of Gases: A Single-Point Ramped Imaging with T1 Enhancement (SPRITE) Study

A pure phase-encoding MRI technique, single-point ramped imaging withT₁enhancement, SPRITE, is introduced for the purpose of gas phase imaging. The technique utilizes broadband RF pulses and stepped phase encode gradients to produce images, substantially free of artifacts, which are sensitive to the gasT₁andT^*:₂relaxation times. Images may be acquired from gas phase species with transverse relaxation times substantially less than 1 ms. Methane gas images,¹H, were acquired in a phantom study. Sulfur hexafluoride,¹⁹F, images were acquired from a gas-filled porous coral sample. High porosity regions of the coral are observed in both the MRI image and an X-ray image. Sensitivity and resolution effects due to signal modulation during the time-efficient acquisition are discussed. A method to increase the image sensitivity is discussed, and the predicted improvement is shown through 1D images of the methane gas phantom.     

Aqueous Capillary Electrophoresis of Peptides Part 1: Conformational States, Volumes and Estimates of the Extent of Hydration

Summary A series of eighteen peptides have been modelled. Of the 380 low-energy viable conformers generated, 64 were selected, with 2–5 per peptide representing the range of structures and size. Using the results of previous investigations of this peptide group, it was possible to place an upper limit to the extent of hydration for each peptide. The modelled peptides were then hydrated in stepwise fashion. From the volumetric and dimensional data derived, in conjunction with published mobilities, the relationship between electrophoretic mobility ( _ep ) and the hydrodynamic radius (r) was used to objectively test the fit between the experimental ( _ep ) data and and the modelled size parameters over the range of hydration. Where the fit was tightest it was presumed that the fraction of the maximum reasonable hydration best represented the average degree of hydration of the set. Using ellipsoidal volumes, Connolly surfaces and excluded volumes the average degree of hydration was found to be in the mid range considered and corresponded to the mean of the bimodal distribution of hydration for the individual peptides according to charge-based calculations. For a peptide with two positive charges and a partial negative charge on the carboxy residue (pH=2.5), about 15 waters of hydration are indicated. With 3, 5 and 6 positive charges, the estimated waters are 28, 48 and 85, respectively. Van der Waals volumes were not helpful as the precise summation of the volumes associated with each bonded atom is a poor reflection of the effective volume of the migrating peptide. Conformational changes are examined as a function of the progressive hydration, and, as might be expected, the greatest changes occur in the early stages of hydration.     

Isolation of the 9-fluorenylmethyl chloroformate derivative in the determination of sulphamethazine

Derivatisation of amine-containing analytes with 9-fluorenylmethyl chloroformate (FMOC) to form fluorescent adducts requires a large excess of FMOC. This excess hydrolyses to form FMOC-OH, which is also fluorescent. Solvent extraction has been investigated as a means of isolating the sulphamethazine (SMZ) adduct (FMOC-SMZ) from the hydrolysis product in order to perform rapid spectrophotometric or spectrofluorimetric assays. However, even under the most favourable pH conditions possible, FMOC-OH was not totally removed. Attempts to enhance the separation by reaction of FMOC-OH with 1-ethoxy-4-dichloro-S-triazinylnaphthalene (EDTN) or by acetylation were also unsuccessful. On the other hand, reaction of FMOC with mixed substrates, followed by two pentane extractions to remove the excess FMOC and direct injection into an HPLC provides the desired separations on a reversed phase column (RPLC) with methanol-modified, (pH 3.5) phosphate buffers. FMOC-SMZ is readily separated from FMOC-OH under all elution conditions, from the FMOC-amino acids (under gradient conditions or isocratically up to 75% methanol), and from other FMOC-sulphonamides and FMOC-dihydrofolate reductase inhibitors (isocratically up to 70% methanol). Hence conversion to the FMOC derivatives permits SMZ to be separated from all of the potential interferants tested by isocratic elution with 70% methanol in RPLC. Analysis for the amino acid derivatives of FMOC may be done without interference from SMZ in samples.     

Analysis of reduced glutathione using a reaction with 2,4′-dichloro-1-(naphthyl-4-ethoxy)-s-triazine (EDTN)

A fluorescent adduct was formed between 2,4-dichloro-l-(naphthyl-4-ethoxy)-s-triazine (EDTN) and reduced glutathione in a reaction at 37 °C and pH 9.2. This reaction was used as the basis of an assay for reduced glutathione. The fluorescence was examined at an excitation wavelength of 319 nm and an emission wavelength of 425 nm after extraction of residual unreacted EDTN with methylene dichloride and subsequent dilution of the aqueous phase with ethanol containing 0.01 percent Triton X-100. The reaction rate was low at pH 7 but was accelerated by addition of preparations containing the enzyme glutathione-S-transferase. The adduct gave a discrete peak using isocratic elution with HPLC on a Nova-pak C₁₈ 3 m reverse phase column and a solvent system of methanol: 0.1 M phosphate buffer pH 6.3 (4060). An analytical concentration range of 24 to 240 M reduced glutathione was obtained with an ultraviolet detection system but the concentration range was 7.5 to 75 M when a fluorescence detection system was used. Adducts of other mercapturic acid pathway thiol compounds were not formed at 37 °C under the conditions used and hence did not interfere in the assay. They were formed by heating EDTN and the respective thiol compound at 60 °C for 30 min and they clearly separated from the reduced glutathione compound on HPLC analysis. A strong reaction was observed with digitonin while solutions of tyrosine, at 10 mM concentration, also reacted but these reactants are unlikely to interfere with reduced glutathione analysis in biological systems. When adduct formation was used to estimate reduced glutathione concentrations in some mammalian and plant tissues the reaction using 2,4-dichloro-l-(naphthyl-4-ethoxy)-s-triazine and HPLC separation gave the same results as ano-phthaldialdehyde assay for liver and muscle but the HPLC method gave slightly lower values for other mammalian and plant tissues. The differences were attributed to other material in the tissue extracts which was fluorescing at the same wavelengths as the reduced glutathione adduct.     

Seeking higher resolution and sensitivity for NMR of quadrupolar nuclei at ultrahigh magnetic fields

We report the acquisition of solid-state NMR spectra of quadrupolar nuclei obtained at very high magnetic fields (25 and 40 T), thus improving spectral sensitivity and resolution. For an example compound, the MAS spectrum obtained at 40 T is nearly free from the second-order quadrupolar broadening and can be interpreted quantitatively in a very simple manner.     

(Triphenylarsine)iodinemonobromine: a charge‐transfer adduct in which arsenic selectively bonds to iodine

The title molecule, (iodobromo)triphenylarsenic(III), [As(BrI)(C₆H₅)₃], has a dart shape, with the phenyl rings arranged in a propeller conformation [I—As—C—C 61.3 (4), 43.8 (4) and 54.1 (4)°]. There is no indication that the halogen atoms have mixed site occupancies. Packing forces displace the I atom away from one phenyl ring [I—As—C 117.3 (2)°] towards the other two [I—As—C 109.8 (2) and 108.3 (2)°] and produce an even more pronounced leaning of the terminal bromine [As—I—Br 174.78 (2)°].     

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