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Over the past decade several pulsed field gradient stimulated-echo methods have been presented for diffusion measurements in heterogeneous media. These methods have reduced or eliminated the coupling between the applied magnetic field gradient and a constant internal magnetic field gradient caused by susceptibility changes throughout the sample. For many research purposes the z-storage delay between the second and third pi/2 RF pulse has been included in order to increase the decay of the echo attenuation to an appropriate level and to increase the signal-to-noise ratio by avoiding T2 relaxation of the magnetization in parts of the pulse sequence. For these reasons a stimulated-echo method has been applied instead of a spin-echo method. When studying systems where it is necessary to keep the duration of the pulse sequence at a minimum, and one is not dependent on using z-storage time to increase the echo attenuation or to study diffusion as a function of observation time, a spin-echo method should be chosen. Here we propose a bipolar pulsed field gradient spin-echo method which is well suited to this purpose, and preliminary diffusion measurements are presented as illustration. Copyright 1999 Academic Press.

NMR studies have been used to examine conformational effects in thyrotropin-releasing hormone (TRH), the epimer incorporating D-His, and their analogues where trans- and cis-4-hydroxy-L-proline replace L-proline (Pro). In all six compounds the observed overall conformation of the major conformer around the Pro-His amide bond, and the observed increase of the cis/trans ratio between the conformers when L-His is replaced by D-His, can be accommodated by assuming that a ten-membered ring is formed by hydrogen bonding between the N-H of the Pro carboxamide function and the N pi-atom of the His imidazole nucleus.

Lipid extraction efficiency is variable when different methods are used on the same tissue. After a systematic comparison on erythrocytes, we found that this variability was due to several factors, such as the addition of alcohol and chloroform as a mixture, the use of repeated extractions or partition procedures after the extraction. Most important, however, was the uneven distribution of solvents and lipids in an apparently monophasic extraction system. This formation reduces lipid extraction and causes accumulation of lipids in the extract supernatant, which leads to an overestimation of the extracted lipid content. We recommend a one-step 18.75 volume of methanol-chloroform (1.5:1 v/v) method for lipid extraction from erythrocytes, with methanol added before chloroform. This method combines simplicity in the extraction procedure, complete extraction of cholesterol and total phospholipids, and reliability in the quantitation of the extracted lipids.

The synthesis of two analogues of kainic acid (KA) incorporating photo-activatible moieties attached either on the gamma-carboxy function (gamma-amide 1) or the isopropenyl side-chain (amide 2) is described. The synthesis of the former amide involves coupling of N-(tert-butoxycarbonyl)-protected alpha-diphenylmethyl kainate with 2-(4-azidobenzamido)ethylamine (5) followed by trifluoroacetic acid mediated complete deprotection. Amide 2 was synthesized by palladium-mediated allylic amination, with 4,4'-dimethoxybenzhydrylamine (DMBA), of N-(9-fluorenylmethoxycarbonyl)-protected dimethyl kainate, followed by splitting the DMB-group with formic acid, coupling with N-hydroxysuccinimidoyl 4-azidobenzoate and finally complete deprotection by saponification. Preliminary pharmacological studies in chicken brain membranes showed that amide 2 is a stronger inhibitor of [3H]KA binding on chicken cerebellar membranes than is amide 1 and that amide 2 has specificity only for the cerebellar, as opposed to the telencephalon, type of non-NMDA binding sites.

From the fruits of Sambucus canadensis four anthocyanin glycosides have been isolated by successive application of an ion-exchange resin, droplet-counter chromatography and gel filtration. The structure of the novel, major (69.8%) pigment, cyanidin 3-O-[6-O-(E-p-coumaroyl-2-O-(beta-D-xylopyranosyl)-beta-D- glucopyranoside]-5-O-beta-D-glucopyranoside, was determined by means of chemical degradation, chromatography and spectroscopy, especially homo- and heteronuclear two-dimensional NMR techniques. The other anthocyanins were identified as cyanidin 3-sambubioside-5-glucoside (22.7%), cyanidin 3-sambubioside (2.3%) and cyanidin 3-glucoside (2.1%).

The 1 H and 13 C NMR spectra of trans-4-hydroxy-N-9-fluorenylmethoxycarbonyl-L-proline show a 56: 44 mixture of two conformers due to hindered rotation around the partial C-N double bond. A combination of homo- and heteronuclear 1D and 2D NMR techniques provided the assignments of the 1 H and 13 C resonances

The 1H and 13C NMR spectra of the anthraquinones physcion, emodin, chrysophanol, aloe-emodin and rhein were assigned. The previously preferred assignments for C-2 and C-4 in physcion were reversed. The NOE technique was used for the assignment of the 1H signals and SEFT and/or heteronuclear decoupling for 13C resonances.

Principal component analysis (PCA) is applied to /sup 13/C nuclear magnetic resonance spectra of the naphtha fraction of crude oil from wells located on the Norwegian shelf. Unsupervised PCA correlates oil samples from the same geographical area. The correlation follows from properties related to the composition of the oils, e.g., long-chained vs. short-chained alkanes, branching, cyclization, and aromatization. Several of the major constituents present in the oils are identified. The link between chemical composition of the crude oils and the geochemical processes of biodegradation water washing, and maturation suggests a simplified characterization of the oil samples. Also, by use of component scores the possibility of communication between the wells is uncovered. Separate modeling of replicated spectra of each sample by the use of supervised PCA (SIMCA) confirmed the results of the unsupervised classification. 39 references, 8 figures, 1 table.

The high-resolution proton-coupled 13C NMR spectra of 2-chloro- and 2-phenyl-4-methyl-1,3,2-dithiarsolane and 2-chloro- and 2-phenyl-1,3,2-dithiarsolane have been obtained and completely analysed. The spectral analysis has provided all the 13C chemical shifts and 13C1H coupling constants, including their relative signs. The results of this work are adequately interpreted in terms of two rapidly equilibrating half-chair conformers with the 4-methyl pseudo-axial or pseudo-equatorial. The cis isomers appear to be conformationally biased towards the form with the 4-methyl pseudo-equatorial. The two one-bond 13C1H coupling constants of the methylene group are distinctly different. For both the cis and trans isomers, the largest one-bond coupling always involves the methylene proton situated syn to the substituent at arsenic. The observed variation in the geminal and vicinal 13C1H coupling constants are discussed in terms of a stereospecific inductive effect of the sulphur hetero-atoms and conformational factors.

/sup 13/C NMR spin-lattice relaxation times of dominant resonances observed in the spectra of the oil samples were determined for solutions without and with a paramagnetic relaxation reagent, Cr(acac)/sub 3/, added. The average carbon chain length of the straight chain alkanes which are present in the petroleum samples can be estimated from the relaxation data, and it agrees with that determined from the NMR intensity measurements. In the aliphatic region of the /sup 13/C spectra, electron-nuclear spin-lattice relaxa

The temperature dependence of the one-bond 13 C 1 H coupling constant of the methyl groups in pivalic acid, tert -butyl chloride and hexamethylethane has been studied in the liquid and plastic crystalline phases. A steady decrease in the coupling constant with falling temperature in the plastic crystalline phase has been observed for these organic solids. A maximum change in the 13 C 1 H coupling constant of 25 Hz has been found after deduction of the effect of overlap of the broadened lines in the methyl quartet. The CNDO/2 calculations indicate that the temperature dependence of the coupling constant is not caused by intramolecular transitions. The significant reduction of the 13 C 1 H coupling constant is largely attributed to intramolecular dipole-dipole interactions due to a slight anisotropic tumbling of the molecules in the plastic phase.

The 1H and 13C NMR spectra of a series of 1,3,2-dioxarsolanes have been obtained at 2.1 T and some at 9.4 T. The chemical shifts and spin-spin coupling constants have been obtained from complete spectral analyses of the 1H and proton-coupled 13C spectra. The spectral data are interpreted on the basis of two rapidly interconverting half-chair conformers with a pseudoaxial substituent at arsenic. Unique assignment of syn/anti or cis/trans geometries have been made from 1H or 13C NMR spectroscopy alone. The syn and trans isomers of the 4-methyl- and 4,5-dimethyl-1,3,2-dioxarsolanes, respectively, appear to be conformationally biased towards the forms with pseudoequatorial methyl groups. The general trends in the geminal and vicinal 1H1H and 13C1H coupling constants are interpreted in terms of stereospecific, electronegativity and lone-pair effects of the oxygen heteroatoms and conformational factors. The NMR data on the 1,3,2-dioxarsolanes are discussed with reference to related 1,3-dithoxa- and 1,3-dithia- five-membered rings with As, P, S or C at the 2-position.

NMR line widths (13C) and spin–lattice relaxation times (13C and 2H) were measured over a wide temperature region in liquid and solid tert-butyl chloride (1) and tert-butyl nitrate (2). Molecular self-diffusion, with activation energies in the range 29–37 kJ mol−1, is responsible for the considerable line narrowing observed for the 13C resonances in solid I of these substances. An analysis of the 13C and 2H spin–lattice relaxation times (T1) in the liquid and solid phases of 1 and 2 is reported. Rotational correlation times, activation energies and pre-exponential factors for the appropriate motions were obtained from the Arrhenius equation. The 13C and 2H T1 data suggest that the overall reorientation occurring in solid I of 1 and 2 is slightly retarded by the melting process. The 13C T1 data show that the faster motion in the ordered phases of 1 and 2 is the uniaxial C3′ reorientation with an activation energy of 12–14 kJ mol−1, while the slower motion is the C3 methyl reorientation with an activation energy of 12–13 kJ mol−1.

Use of the paramagnetic relaxation reagents, tris-(acetylacetonato)chromium(III) [Cr(acac) 3 ], in 13 C spin-lattice relaxation time measurements proves capable of differentiating the alkyl groups of short-chain alkylbenzenes and paraffinic hydrocarbons. A specific hydrogen-bond interaction between Cr(acac) 3 and a very weak hydrogen-bond donor, such as an α-benzylic group, is the most important factor controlling the observed electron-nuclear spin-lattice relaxation times ( T 1 e s) of carbon- 13 nuclei.

Proton and 13C line widths and 13C spin-lattice relaxation times (T1) were measured over a wide temperature region in liquid and solid tert-butyl bromide (1) and tert-butyl cyanide (2). A striking feature of the proton and 13C NMR spectra of 1 is the abrupt line narrowing at the phase I phase II transition point. In 2, however, the line width does not vary with temperature in phase I, indicating that the translational motion is too slow to affect the line width. An analysis of spin-lattice relaxation times in both the liquid and solid phases is reported. It is shown that 13C T1 measurements can distinguish between internal methyl group reorientation (C3 motion) and uniaxial molecular reorientation of tert-butyl groups (C3′ motion) if these motions are independent and responsible for the T1 relaxation over a large part of the temperature region. The 13C spin-lattice relaxation mechanism in the liquid and disordered phases of 1, and the liquid phase of 2, is governed by overall molecular tumbling with an activation energy of 8–11 kJ mol−1. The 13C T1 data show that the faster motion in the ordered phases of 1 and 2 is the C3′ reorientation with an activation energy of 15–18 kJ mol−1 while the slower motion is the C3 reorientation with an activation energy of ca 19 kJ mol−1.

Determination en fonction de la temperature des dioxarsolanes-1,3,2 afin d'etudier les mecanismes de relaxation presents dans ces systemes, et l'effet de la substitution de methyl sur les temps T 1

Proton and 13C line widths and spin-lattice relaxation times, T1, have been measured for liquid and solid 2,2-dichloropropane (1) and 2,2-dibromopropane (2). Molecular self-diffusion, with activation energies in the range 18-24 kJ mol−1, is responsible for the considerable line narrowing observed for the NMR spectra in solid I of these substances. The NMR data reveal a previously unreported phase transition point at 208 K for 2. An analysis of the proton and 13C T1 times in both the liquid and solid phases is reported. Effective correlation times, activation energies and pre-exponential factors for the appropriate motions have been obtained from the Arrhenius equation. The proton and 13C spin-lattice relaxation mechanisms in the liquid and solid I phases are governed by overall molecular reorientations, with activation energies in the range 9–11 kJ mol−1. The rotational correlation time increases slightly at the liquid solid I boundary from 3.2 to 3.6 ps for 1 and from 5.1 to 5.4 ps for 2. The correlation time is observed to decrease by five orders of magnitude at the solid I solid II transition point of 2. The dominant motion causing spin-lattice relaxation in phase II is methyl group reorientation with an activation energy of 18 kJ mol−1.

Etude RMN de 17 O, 1 H et 13 C sur l'acide pivalique presentant des changements aux transitions de phase; influence de la temperature sur les vibrations de valence IR de OH et C=0, pres du point de transition de phase plastique-cassant

A 13C electron–nuclear spin–lattice relaxation study showed that the interaction between alkyl-substituted aromatics and a paramagnetic relaxation reagent, tris(acetylacetonato)chromium(III), is purely electrostatic, i.e. of a dipole–dipole or dipole–induced dipole type. Except for aromatics having bulky alkyl groups, the preferred chelate position is close to the positive end of the dipole, i.e. close to the α-position. However, the major electron–nuclear mechanism is non-specific, and the relaxation rate is mainly controlled by the mutual translational/rotational diffusion of the two species. Paramagnetic shifts induced on addition of Cr(acac)3 support the suggested interaction mechanism.

13C NMR spin-lattice relaxation times (T1) have been measured as a function of temperature for three 2-phenyl-1,3,2-dioxarsolanes. The T1 data have been interpreted in terms of isotropic overall molecular reorientation, internal rotation about the AsPh bond and internal methyl rotation. Information on the relative order of magnitude of the overall and internal reorientations has been obtained. The internal rotation of the phenyl ring is thus 2—4 times faster than the overall molecular tumbling. Further, the internal rotation of the arsolane ring appears to be comparable to, or slightly faster than, the phenyl ring rotation. The association process which is believed to take place in the unsubstituted parent compound is reflected in its very high activation energy for overall reorientation (39 kJ mol−1 compared with 24-25 kJ mol−1 for the methyl derivatives). The rotation of the methyl groups appears to be considerably more hindered in the cis isomer of 2-phenyl-4,5-dimethyl-1,3,2-dioxarsolane than in the trans isomer and, in particular, the mono-4-methyl compound.

/sup 13/C solvent shifts relative to internal tetramethylsilane (TMS) are measured for 12 hydrocarbons in 11 hydrocarbon solvents and are found to be generally less than 0.1 ppm. Relative to TMS, the solvent shifts for methyl carbons are small, indicating a similarity between the methyl groups of TMS and those of hydrocarbons. For other carbon types the solvent shifts are relatively large. On average, a more representative hydrocarbon would provide a better chemical shift reference for fossil oil analysis. Relative peak heights in a steady-state /sup 1/H broad-band decoupled /sup 13/C spectrum of light hydrocarbons are discussed. /sup 13/C analyses of light naphta fractions from a crude oil show that unambiguous identification of the main components is feasible.