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At least 361 records · Page 20Linked to original sources
PubMed1992

The MAGIC simulation of surface water acidification at, and first year results from, the Bear Brook Watershed Manipulation, Maine, USA.

The catchments of East and West Bear Brooks, Maine, USA, have been hydrologically and chemically monitored for 3.5 years. Stream chemistries and hydrographs are similar. These clear water streams are low in ANC (0-70 microeq litre(-1)), with variations caused by changing concentrations of base cations, SO4, NO3 and Cl. The latter range between 90-120, 0-40 and 65-75 microeq litre(-1), respectively. The West Bear catchment is being treated with six applications per year of dry (NH4)2SO4 at 1800 eq ha(-1) year(-1). After one year of treatment, the response of the stream chemistry and the response modelled by MAGIC are similar. Retentions of NH4 and SO4 are nearly 100% and greater than 80%, respectively. The additional flux of SO4 is compensated principally by an increased Ca concentration. Episodes of high discharge in the treated catchment are now characterized by lower ANC and pH, and higher Al than prior to the manipulation. Concentrations of NO3 have increased about 10 microeq litre(-1) during the dormant season, presumably due to additional nitrification of N from NH4. Discharge-chemistry relationships indicate that changes in stream chemistry, except for NO3, are dominated by ion exchange reactions in the upper part of the soil profile.

Journal Article↗
PubMed1990

Light-induced membrane protein phosphorylation in the bovine rod outer segment. A magic angle spinning 31P-NMR study.

Magic angle spinning 31P-NMR (MAS 31P-NMR) spectra of bovine rod outer segments, unphosphorylated and phosphorylated, were obtained. In the phosphorylated samples the spectra showed new resonances not assignable to phospholipids. These signals were present only when stimulation of receptor phosphorylation occurred. These resonances were not due to exogenous, soluble phosphorus-containing compounds. Limited proteolysis to remove the carboxyl-terminal region of the photoreceptor that contains the phosphorylation sites removed these resonances. The chemical shifts were in the usual range for serine phosphate and threonine phosphate. The pKa obtained from a pH titration of the 31P chemical shift was typical of serine phosphate. Therefore, these 31P-NMR resonances were assigned to the phosphorylation sites on membrane proteins in the rod outer segment disk membranes. Static 31P-NMR measurements revealed that at least some of these sites gave rise to relatively narrow resonances, indicative of considerable motional freedom of the carboxyl-terminal segment of the photoreceptor when phosphorylated. These data indicate that it is possible to study phosphorylation sites on membrane proteins using MAS 31P-NMR, and that using in vivo 31P 'spin labelling' one can study directly and selectively regions of receptors crucial to receptor function.

Animals↗
PubMed1993

Magic and medicinal plants of the Ayoreos of the Chaco Boreal (Paraguay).

The Ayoreo is a hunter-gatherer tribe of Amerindians which occupy the central-northern part of the Paraguayan Chaco. The whole Ayoreo culture cannot be disassociated from religious beliefs. Disease is considered of supernatural origin and as the result of breaking or disobeying the tabu which regulates existence. A description of the shamanic practices is given to understand better the position of health practices in the Ayoreo culture, particularly the use of medicinal and hallucinogenic plants. Fragments of the Asojna ritual and the methods for becoming shaman; the initiation of the last living shaman, as well as references to the magic powers of the shaman are presented. Diagnosis and treatment included invocations to plant and animal spirits and the use of a few medicinal plants. The plants used as medicine or invoked for healing are presented for the first time. Of particular interest is the identification of two Euphorbiaceae as ritual plants by the Ayoreo.

Female↗
PubMed1993

Quadrupole effects of spin-3/2 nuclei on the solid-state magic-angle spinning nuclear magnetic resonance spectra of spin-1/2 nuclei. Deviations from first-order theory and implications concerning the sign of the indirect coupling constant.

The effect of the interaction between spin-3/2 and spin-1/2 nuclei on solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of the latter is studied in cases where deviations from first-order theory are expected. A comparison is made between the exact and first-order perturbation approaches. Both dipolar and indirect (iso- and anisotropic) coupling interactions are considered. Implications regarding 13C,35,37Cl, 31P,63,65Cu and 119Sn,35,37Cl cases are discussed. It is shown that in the latter two cases the sign of the indirect coupling constant J can be derived.

Magnetic Resonance Spectroscopy↗
PubMed1992

Inhomogeneous interactions in solids under slow off-magic angle spinning.

The central band of an inhomogeneous interaction under slow off-magic angle spinning (off-MAS) is no longer a scaled static spectrum with scaling factor P2(cos upsilon), that is, the spectrum distortion occurs in comparison with that under the rapid rotation condition. The dependence of this distortion in the chemical shift spectrum, for instance, on anisotropy, the asymmetry parameter, spinning rate and the angle between the spinning axis and the applied magnetic field is studied in this paper. It is shown that the distortion can be partly eliminated by using TOSS in certain circumstances. Experimental evidence is presented in this study.

Carbon↗
PubMed1993

Improvements to the magic angle hopping experiment.

Several improvements to the magic angle hopping experiment first introduced by Bax et al. [J. Magn. Reson., 52 (1983) 147] are presented. A dc servo motor driven sample hopping mechanism which requires less than 60 ms to accomplish a 120 degrees sample rotation is described. Modifications to the data acquisition process, including starting the acquisition period immediately after the second hop and acquiring a hypercomplex data set, are also presented. Principal values of the 13C chemical shielding tensor are measured for 1,2,3-trimethoxybenzene and 2,6-dimethoxynaphthalene.

Magnetic Resonance Spectroscopy↗
PubMed1993

A simple theoretical treatment of quadrupolar effects on magic-angle-spinning solid-state NMR spectra of spin-1/2 nuclei in the limit of large quadrupole coupling constants.

A simple approach is discussed for studying the effect of quadrupolar nuclei on the magic-angle-spinning solid-state NMR lines of spin-1/2 nuclei in the limit of large quadrupole coupling constants. Equations are derived both for the isotropic shifts and the Pake-like powder patterns for any quadrupolar spin and for arbitrary orientations of the internuclear vector with respect to the unique axis of an axially symmetric quadrupole tensor. First-order effects due to a small Zeeman perturbation on these lines are explored, as well as deviations from axial symmetry in the electric field gradient when S = 3/2 quadrupolar nuclei are involved. Spectral parameters likely to be observed in the case of coupling between 31P and 201Hg are also discussed.

Magnetic Resonance Spectroscopy↗
PubMed1993

High-field 13C solid-state NMR studies of stream humic and fulvic acids with fast magic-angle spinning.

Standard samples of stream humic and fulvic acids have been studied by high-resolution 13C solid-state NMR. The spectra have been recorded at a high (9.4 T) magnetic field using 1H-13C cross-polarization (CP) and proton high-power decoupling (HPDEC) with fast (up to 15 kHz) magic-angle spinning (MAS). The widths of the spectral lines decrease with increasing spinning rate. This is probably due to the overlap of spinning sidebands with resonances at low speeds and to a more effective averaging of anisotropic broadening at high speeds. However, fast MAS modulates the CP signal intensities, making quantitative analysis very difficult.

Benzopyrans↗
PubMed1995

129Xe magic-angle spinning spectra of xenon in zeolite NaA. Direct observation of mixed clusters of co-adsorbed species.

We present the first demonstration that the individual 129Xe resonances corresponding to Xen (n = 1-8) clusters inside the alpha-cages of zeolite NaA can be narrowed under magic-angle spinning (MAS). Under these high-resolution conditions we also observe upon addition of Kr the individual peaks corresponding to mixed clusters, XenKrm, inside the alpha-cages, which will allow the first direct determination of the distribution of co-adsorbates in a microporous solid. Under MAS the chemical shifts of the Xen clusters are shown to be highly sensitive to "disorder" in the zeolite and provides new, quantitative information about the presence of alpha-cages of several types.

Magnetic Resonance Spectroscopy↗
PubMed1995

Characterization of V2O5-AlPO4 catalysts by 51V and 1H magic-angle spinning solid-state nuclear magnetic resonance spectroscopy.

Amorphous aluminum orthophosphate is used as a carrier for preparing a series of V2O5-AlPO4 catalysts with varying vanadia content. The catalysts were characterized by solid-state 51V and 1H magic-angle spinning nuclear magnetic resonance (MAS NMR), electron spin resonance (ESR) and X-ray diffraction (XRD). The XRD patterns of the catalysts remained amorphous at all loadings studied. 51V Solid-state NMR spectra revealed the presence of V2O5 microcrystallites at higher vanadia loadings. The 1H MAS NMR spectra of the catalysts showed a maximum consumption of support hydroxyl groups at 6 wt.-% V2O5 indicating the completion of a monolayer at this composition.

Aluminum Compounds↗
PubMed1995

Cross-polarization magic-angle spinning nuclear magnetic resonance study of platinum complexes containing the cis-P2PtC2 fragment.

31P and 195Pt cross-polarization magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectra of three platinum complexes of formal oxidation state Pt(0) and Pt(II), respectively, are reported. All three complexes, (Ph3P)2Pt(C2H4) (1), (Et2P-CH2-CH2-PEt2)Pt(C identical to C-H)2 (2) and (Ph2P-CH2-CH2-PPh2)Pt(C identical to C-C(CH3)=CH2)2 (3) contain the square-planar cis-P2PtC2 fragment and show unusual NMR spectroscopic properties insofar that the 195Pt shielding patterns are fairly narrow in relation to what one would generally have to expect for 195Pt in square-planar coordination. Another unexpected NMR property of the cis-P2PtC2 fragment in 1-3 is the absence of spinning frequency-dependent second-order effects in this solid-state ABX spin system.

Magnetic Resonance Spectroscopy↗
PubMed1995

14N population transfers in two-dimensional 13C-14N-1H triple-resonance magic-angle spinning nuclear magnetic resonance spectroscopy.

A two-dimensional (2D) experiment has been used to show that 14N irradiation and magic-angle spinning (MAS) results in population transfers between the 14N Zeeman levels. This experiment was applied to a sample of N-acetyl-D,L-valine, a material where asymmetric doublets resulting from 13C-14N dipolar coupling are clearly resolved in the 13C spectrum at a field of 7 T for Carbon atoms directly bonded to the nitrogen atom. The 13C transverse magnetization was allowed to evolve in the F1 and F2 dimensions, and the 14N spins were irradiated during the mixing period. Cross-peaks were observed in the 2D 13C spectrum between the two peaks of the CH asymmetric doublet. Since one peak of the doublet results primarily from coupling to the [formula: see text] state and the other peak from coupling to [formula: see text] states, population changes between the 14N Zeeman levels have occurred during the mixing period. These population transfers are a consequence of the time dependence of the 14N quadrupole splitting Q under MAS conditions and 14N irradiation. Level anti-crossings of the 14N Zeeman levels occur at the zero-crossings of Q, and a continuous and slow change in Q will result in the transfer of 14N populations between the different Zeeman levels. If these passages are adiabatic, then the system returns to its original state after two zero-crossings. This is consistent with the experimental observation that the intensities of the cross-peaks for 14N irradiation are greater for half a rotor period than a full rotor period.

Magnetic Resonance Spectroscopy↗
PubMed1995

Analytical expression for the magic-angle spinning nuclear magnetic resonance line shape in the presence of the second-order quadrupole interaction for the non-zero asymmetry parameter and large sample rotation frequency.

An analytical expression for the magic-angle spinning nuclear magnetic resonance (MAS NMR) line shape function for the non-zero asymmetry parameter and large sample rotation frequency has been derived using the time-independent part of the quadrupole transition energy based on the stationary perturbation theory. The averaging over Euler angles in the case of a powdered sample is also done analytically, which is so far implemented in the literature by a numerical simulation procedure. While the peak positions obtained from analytical and numerical averaging procedures agree well, the peak heights are significantly different, showing the need for an analytical solution for the averaging over Euler angles, as this would otherwise lead to erroneous values for the quadrupole parameters. The MAS NMR line shape function derived in this paper can be compared directly with the experimental MAS NMR spectrum for the large sample rotation frequency, and the values of the quadrupole parameters can be obtained. We have also described a method of evaluating the quadrupole parameters for the small sample rotation frequency, using the peak heights and peak separation of the central band of the experimental MAS NMR spectrum and the MAS NMR line shape function derived in the present paper. As an example, the quadrupole parameters for vitreous B2O3 are computed.

Magnetic Resonance Spectroscopy↗
PubMed1995

Multifield magic-angle spinning and double-rotation nuclear magnetic resonance studies of a hydrated aluminophosphate molecular sieve: AlPO4-H2.

AlPO4-H2 is a microporous hydrated aluminophosphate, structurally related to VPI-5, whose framework has highly elliptical 10-ring channels (2.9 x 7.6 A) parallel to the c crystallographic axis. To resolve a previously reported discrepancy between nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) data a highly crystalline AlPO4-H2 has been further characterized with 27Al and 31P magic-angle spinning (MAS) NMR at 11.7 T and 27Al double-rotation (DOR) NMR at 4.7 T. These present NMR data definitively show that the true space group symmetry of the AlPO4-H2 framework structure is triclinic rather than the higher orthorhombic symmetry proposed earlier from XRD studies.

Aluminum Compounds↗
PubMed1995

Separating subspectra from cross-polarization magic-angle spinning nuclear magnetic resonance spectra by proton spin relaxation editing.

Differences in proton spin relaxation time constants can be exploited to edit cross-polarization magic-angle spinning nuclear magnetic resonance (CP-MAS NMR) spectra of heterogeneous mixtures of different types of organic matter. This paper describes an extension of the editing procedure from two-component to three-component mixtures. Clean separation of 13C NMR subspectra was achieved for three synthetic polymers mixed as powders. Applying the procedure to both 13C and 31P CP-MAS NMR spectra of solid dairy pond sludge provided clues to the location of the phosphorus relative to different types of organic matter, and provided estimates of the proportions of organic matter in categories labeled "plant fragments", "partly degraded residues" and "recalcitrant structures". The editing procedure increased noise levels by factors between 2 and 11 in these worked examples, depending on the degree of difficulty involved in distinguishing differences in proton spin relaxation time constants.

Artifacts↗
PubMed1994

Multiple-quantum nuclear magnetic resonance spectroscopy of coupled 1/2 spins in solids. Combination with cross-polarization and magic-angle spinning.

Proton multiple-quantum coherence spectroscopy has been combined with magic-angle spinning (MAS) and cross-polarization (CP). This enables the detection of the proton (or any other abundant spin) multiple-quantum coherence spectrum via the high-resolution 13C (any other spin) nuclear magnetic resonance (NMR) spectrum. For this purpose multiple-quantum pulse sequences synchronised to sample rotation have been designed, and the average Hamiltonians of these sequences have been analysed. The analysis allows the design of optimal experimental conditions. As a demonstration of the technique, it has been applied to a mixture of adamantane and hexamethylethane. From earlier 13C spin diffusion experiments it was known that these two molecules form a mixed crystal. With our technique we detected two different phases with different molecular translational self-diffusion coefficients in this system.

Magnetic Resonance Spectroscopy↗
PubMed1994

Effects of magic-angle spinning on spin-lattice relaxations in talc.

The spin-lattice relaxation times T1 of 1H and 29Si spins in talc have been measured at room temperature with and without magic-angle spinning (MAS) of the sample. Paramagnetic impurities work as relaxation centers. 1H T1 depends on the spinning rate, whereas 29Si T1 is independent of the spinning rate. These facts demonstrate that spin diffusion plays an important role in 1H relaxation but not in 29Si relaxation. 29Si spins relax through dipole-dipole interactions with electron spins directly, which mechanism is not affected by spinning. The relaxation rates have been analyzed theoretically.

Magnetic Resonance Spectroscopy↗