A pulsed field gradient spin echo NMR spectrometer for diffusion coefficient measurements

A pulsed field gradient spin echo NMR spectrometer has been assembled by interfacing a programmable pulse generator and a data acquisition system designed and fabricated in our laboratory with other imported units. Calibration results of the magnetic field gradients are presented.

NMR study of ammonium halides under high pressure

Wide-line c.w. proton resonance investigations have been carried out on the ammonium halides, namely, ammonium chloride, ammonium bromide and ammonium iodide in the temperature range between 77 and 300 K and in the pressure range between 1 bar and 14 kbar. It has been found that the narrow iodide spectrum at 77 K broadens under the application of hydrostatic pressure. The barrier height for the ammonium ion motion in ammonium iodide under pressure has been estimated by carrying out a temperature variation study. The rotational potential for the motion of ammonium ion in ammonium iodide at 1 bar and 14 kbar has been calculated using earlier theoretical models and compared with values calculated for ammonium chloride and bromide. The barrier height in the case of ammonium iodide under pressure is found to be of the same order of magnitude as the value obtained in the case of ammonium bromide at atmospheric pressure indicating that the high pressure phase of ammonium iodide is likely to have the same structure as the low temperature ordered CsCl phase found in the case of the chloride and the bromide. The increase in the potential barrier height in the case of ammonium iodide under pressure indicates that the reorientational motion executed by the ammonium ions is inhibited by the application of pressure. This is also confirmed by the broadening of the spectral line at 77 K under the application of pressure.

Proton NMR investigations of ferroelectricity in ammonium sulphate

Results of cw wide-line proton magnetic resonance investigations on ammonium sulphate and rubidium ammonium sulphate are presented. The pressure and temperature dependence of some of the properties of ammonium sulphate are explained stressing the importance of the role of the ammonium ions.

The 31p nmr Spectra of Isomeric Diaminohexa-hlorocyclotetraphosphazatetraenes; Examples of a2b2 and aa'bb' spin Systems

The 31P nmr spectra of 2,4- and 2,6-diamino-derivatives of octachlorocyclotetraphosphazatetraene, N4P4Cl6(NR1R2)2 (R1 = H, R2 = But; R1 = H, R2 = CH2Ph; R1 = Me, R2 = Ph), have been measured. The 2,4- and 2,6-isomers were analysed as AA'BB' and A2B2 spin systems respectively. In the 2,4-isomers the spin-spin couplings 2J(PNP) and 4J(PNPNP) were of opposite sign.

Spectral assignments based on the 13C NMR spectra of mixed liquid crystals of opposite diamagnetic anisotropies

The proton-decoupled 13C NMR spectra of mixtures of liquid crystals with opposite diamagnetic anisotropies have been studied in the natural abundance of 13C. A new method to assign the spectral lines to specific carbons in the liquid crystalline phase has been developed. For this purpose, the assignments of lines in the isotropic media are required, and they were obtained from two-dimensional hetero-COSY experiments. From the spectra in the �critical� mixtures where both the orientations of the liquid crystal directors, with the alignments along and perpendicular to the direction of the magnetic field, �coexist,� the 13C chemical-shift anisotropies have been determined, assuming uniaxial symmetry.

Two-dimensional NMR spectroscopy of connected transitions by linear combination of flip angle dependent cosy: An alternative scheme for E.COSY

An alternative pulse scheme which simplifies and improves the recently proposed P.E.COSY experiment is suggested for the retention of connected or unconnected transitions in a coupled spin system. An important feature of the proposed pulse scheme is the improved phase characteristics of the diagonal peaks. A comparison of various experiments designed for this purpose, namely COSY-45, E.COSY, P.E.COSY and the present scheme (A.E.COSY), is also presented. The suppression of unconnected transitions and the measurement of scalar coupling constants and their relative signs are illustrated from A.E.COSY spectra of 2,3-dibromopropionic acid and 2-(2-thienyl)pyridine.

A lock-type cell for NMR studies at high hydrostatic pressures and low temperatures

Details of a simple and convenient high-pressure cell for continuous-wave, wide-line nuclear magnetic resonance investigation at high pressures and low temperatures are described. Experimental results obtained with the cell at 14*108 Pa and 77K for ammonium iodide are presented briefly.

Conformations of dehydrophenylalanine containing peptides. NMR studies on three tripeptides with a central dehydrophenylalanyl residue.

Three tripeptides containing a central Z-dehydrophenylalanine residue (Δz-Phe), Boc-L-Phe-Δz-Phe-X-OMe (X = L-Val 1, L-Leu 2 and X = L-Ala 3) have been synthesized and their solution conformations investigated by 270 MHz 1H NMR spectroscopy. In all three peptides, conformations involving the X residue NH in an intramolecular hydrogen bond were favoured in CDCl3 solutions. Studies of the nuclear Overhauser effect (NOE) provided support for a Type II β turn conformation in these peptides with Phe and Δz-Phe occupying the i + 1 and i + 2 positions, respectively. Significantly different conformations lacking any intramolecular hydrogen bonds were observed for peptide 1 in (CD3)2SO. NOE results were consistent with a significant population of molecules having semi-extended conformations (ø > 100°) at the Δz-Phe residue.

A MAS NMR investigation of aluminosilicate, silicophosphate, and aluminosilicophosphate gels and the evolution of crystalline structures on heating the gels

Gels of various composition containing SiO2, Al2O3, and P2O5 have been investigated by employing high resolution magic-angle-spinning (MAS) 27Al, 29Si, and 31P NMR spectroscopy. Changes occurring in the NMR spectra as the gels are progressively heated have been examined to understand the nature of structural changes occurring during the crystallization of the gels. 27Al resonance is sensitive to changes in the coordination number even when the Al concentration is as low as 1 mol%. As the percentage of Al increases, the hydroxyl groups tend to be located on the Al sites while Si remains as SiO4/2 (Q4). Mullite is the major phase formed at higher temperature in the aluminosilicate gels. In the case of the silicophosphate gels, Si is present in the form of Q4 and Q3 species. There is a change in the coordination of Si from four to six as the gel is heated. The formation of six-coordinated Si is facilitated even at lower temperatures (~673 K) when the P2O5 content is high. The phosphorus atoms present as orthophosphoric acid units in the xerogels change over to metaphosphate-like units as the gel is heated to higher temperatures. In aluminosilicophosphates, Si is present as Q4 and Q3 species while P is present as metaphosphate units; Al in these gels seems to be inducted into the tetrahedral network positions.

High-resolution methyl edited GFT NMR experiments for protein resonance assignments and structure determination

Three-dimensional (3D) structure determination of proteins is benefitted by long-range distance constraints comprising the methyl groups, which constitute the hydrophobic core of proteins. However, in methyl groups (of Ala, Ile, Leu, Met, Thr and Val) there is a significant overlap of C-13 and H-1 chemical shifts. Such overlap can be resolved using the recently proposed (3,2)D HCCH-COSY, a G-matrix Fourier transform (GFT) NMR based experiment, which facilitates editing of methyl groups into distinct spectral regions by combining their C-13 chemical shifts with that of the neighboring, directly attached, C-13 nucleus. Using this principle, we present three GFT experiments: (a) (4,3)D NOESY-HCCH, (b) (4,3)D H-1-TOCSY-HCCH and (c) (4,3)D C-13-TOCSY-HCCH. These experiments provide unique 4D spectral information rapidly with high sensitivity and resolution for side-chain resonance assignments and NOE analysis of methyl groups. This is exemplified by (4,3)D NOESY-HCCH data acquired for 17.9 kDa non-deuterated cytosolic human J-protein co-chaperone, which provided crucial long-range distance constraints for its 3D structure determination.

Preparation and NMR spectra of four isomeric diformyl2.2]paracyclophanes (cyclophanes 66)

Four isomeric dialdehydes 4, readily available from cycloaddition of propiolic aldehyde (2) to 1,2,4,5-hexatetraene (1), were separated by chromatography and recrystallization, and were characterized by their spectroscopic data. The individual isomers can now be easily identified from their H-1 NMR spectra even if only one of them is present.

On the models for deuterium long-range isotope effects in13C NMR spectroscopy

The long-range deuterium isotope effects on13C nuclear shielding are physically not yet completely understood. Two existing models for explaining these effects, vibrational and substituent, are compared here. The vibrational model is based on the Born-Oppenheimer approximation, but it can explain only one-bond deuterium effects. To the contrary, the substituent model may explain many long-range isotope effects, but it is controversial due to the assumption of some distinct electronic properties of isotopes. We explain how long-range deuterium isotope effects may be rationalized by the subtle electronic changes induced by isotope substitution, which does not violate the Born-Oppenheimer approximation.

Optimization of two-dimensional NMR by matched accumulation

It is well known that in the time-domain acquisition of NMR data, signal-to-noise (S/N) improves as the square root of the number of transients accumulated. However, the amplitude of the measured signal varies during the time of detection, having a functional form dependent on the coherence detected. Matching the time spent signal averaging to the expected amplitude of the signal observed should also improve the detected signal-to-noise. Following this reasoning, Barna et al. (J Magn. Reson.75, 384, 1987) demonstrated the utility of exponential sampling in one- and two-dimensional NMR, using maximum-entropy methods to analyze the data. It is proposed here that for two-dimensional experiments the exponential sampling be replaced by exponential averaging. The data thus collected can be analyzed by standard fast-Fourier-transform routines. We demonstrate the utility of exponential averaging in 2D NOESY spectra of the protein ubiquitin, in which an enhanced SIN is observed. It is also shown that the method acquires delayed double-quantum-filtered COSY without phase distortion.

29Si NMR chemical shifts in silicates

29Si chemical shifts in a wide variety of silicates in crystalline, glassy and gel states have been related to a parameter, P, which takes into account the electronegativity and the structural description of the silicate units as well as the ionic potential of the modifier cation. The relation, δ(ppm)=28.4 [1−exp(−P)]−110.5, besides having predictive value, satisfactorily accounts for all the available chemical-shifts data on silicates and shows the right kind of limiting behaviour, with δ approaching the Q0 value at large P.