Dynamic processes and chemical composition of Lepidium sativum seeds determined by means of field-cycling NMR relaxometry and NMR spectroscopy

Proton nuclear magnetic resonance (NMR) techniques, such as field-cycling relaxometry, wide-line NMR spectroscopy, and magic angle spinning NMR spectroscopy, were applied to study the seeds of cress, Lepidium sativum. Field-cycling NMR relaxometry was used for the first time to investigate the properties of the whole molecular system of dry cress seeds. This method not only allowed the dynamics to be studied, but was also successful in the differentiation among the solid (i.e., carbohydrates, proteins, or fats forming a solid form of lipids) and liquid-like (oil compounds) components of the seeds. The ¹H NMR relaxation dispersion of oils was interpreted as a superposition of intramolecular and intermolecular contributions. The intramolecular part was described in terms of a Lorentzian spectral density function, whereas a log–Gaussian distribution of correlation times was applied for the intermolecular dipole–dipole contribution. The models applied led to very good agreement with the experimental data and demonstrate that the contribution of the intermolecular relaxation to the overall relaxation should not be disregarded, especially at low frequencies. A power-law frequency dependence of the proton relaxation dispersion was used for the interpretation of the solid components. From the analysis of the ¹H wide-line NMR spectra of the liquid-like component of hydrated cress seeds, we can conclude that the contribution of oil protons should always be taken into account when evaluating the spin–lattice relaxation times values or measuring the moisture and oil content. The application of ¹H magic angle spinning NMR significantly improves resolution in the liquid-like spectrum of seeds and allows the determination of the chemical composition of cress seeds.

Spatial structure of biomolecules by nuclear relaxation and molecular modelling methods

Nuclear magnetic resonance spectroscopy (NMR) has become the most frequently used technique for the investigation of the structure of biomacromolecules in solution. In particular, selective relaxation techniques have been used in the analysis of homonuclear dipolar connectivities (selective and biselective proton spin-lattice relaxation experiments) and heteronuclear couplings (selective {H} C-NOE, biselective carbon R₁ experiments). An accurate determination of selected i-j internuclear distances has made it possible to identify the preferred solution structure. A different experimental technique based on two-dimensional NOE spectra has made it possible to define the complex network of dipolar interactions that exists in biomacromolecules and hence the conformational properties in solution.     

Precise molecular weight determination and structure characterization of end-functionalized polymers: An NMR approach <Emphasis Type="Italic">via</Emphasis> combination of one-dimensional and two-dimensional techniques

We present here the application of one-dimensional and two-dimensional NMR techniques to characterize the structure of methoxyl end-functionalized polystyrenes (PS). The peaks in ¹H-NMR spectra corresponding to main-chain, side-chain and chain-end groups are assigned by ¹H-¹H gCOSY, ¹H-¹³C gHSQC and gHMBC spectra. For the first time, the spin-lattice relaxation time (T ₁) of protons of the chain-ends is revealed to be affected more by polymer molecular weight (MW) than by the protons of the main-chains and the side-chains (almost independent from MW). As a result, a much higher delay time (d1) for chain-ends (d1 > 20T ₁) is needed for quantitative NMR measurement when using end-group estimation method to obtain the MW of PS, which is in accordance with the value estimated by GPC. An improved method for the polymer MW determination is established, by combination of different NMR techniques to distinguish the peaks, and a large d1 setting to achieve quantitative NMR analysis.     

Intermolecular nuclear relaxation in paramagnetic solutions: from free radicals to rare earths

The principles of the intermolecular relaxation of a nuclear spin by its fluctuating magnetic dipolar interactions with the electronic spins of the paramagnetic surrounding species in solution are briefly recalled. It is shown that a very high dynamic nuclear polarization (DNP) of solvent protons is obtained by saturating allowed transitions of free radicals with a hyperfine structure, and that this effect can be used in efficient Earth field magnetometers. Recent work on trivalent lanthanide Ln³⁺ aqua complexes in heavy water solutions is discussed, including paramagnetic shift and relaxation rate measurements of the ¹H NMR lines of probe solutes. This allows a determination of the effective electronic magnetic moments of the various Ln³⁺ ions in these complexes, and an estimation of their longitudinal and transverse electronic relaxation times T_1e and T_2e. Particular attention is given to Gd(III) hydrated chelates which can serve as contrast agents in magnetic resonance imaging (MRI). The full experimental electronic paramagnetic resonance (EPR) spectra of these complexes can be interpreted within the Redfield relaxation theory. Monte-Carlo simulations are used to explore situations beyond the validity of the Redfield approximation. For each Gd(III) complex, the EPR study leads to an accurate prediction of T_1e, which can be also derived from an independent relaxation dispersion study of the protons of the probe solutes.     

Square Planar Platinum(II) Complexes with N,S-Donor Ligands: Synthesis,Characterisation, DNA Interaction and Cytotoxic Activity

The platinum(II) complexes with N,S-donor ligand have been synthesised and characterised by physiological techniques like elemental, electronic, Fourier transform infrared, hydrogen-1 nuclear magnetic resonance (¹H NMR) and liquid chromatography–mass spectrometry spectra. The synthesised complexes have been checked for their DNA binding ability by absorption titration and viscosity measurement, and the results show that the complexes binds to herring sperm DNA (HS DNA) via covalent mode of binding. The DNA cleavage activity of synthesised complexes has been carried out by gel electrophoresis experiment using supercoiled form of pUC19 DNA, showing the unwinding of the negatively charged supercoiled DNA. Brine shrimp (Artemia cysts) lethality bioassay technique has been applied for the determination of toxic property of synthesised complexes in terms of micromolars.     

Monitoring of metabolic profiling and water status of Hayward kiwifruits by nuclear magnetic resonance

The metabolic profiling of kiwifruit (Actinidia deliciosa, Hayward cultivar) aqueous extracts and the water status of entire kiwifruits were monitored over the season (June-December) using nuclear magnetic resonance (NMR) methodologies. The metabolic profiling of aqueous kiwifruit extracts was investigated by means of high field NMR spectroscopy. A large number of water-soluble metabolites were assigned by means of 1D and 2D NMR experiments. The change in the metabolic profiles monitored over the season allowed the kiwifruit development to be investigated. Specific temporal trends of aminoacids, sugars, organic acids and other metabolites were observed.The water status of kiwifruits was monitored directly on the intact fruit measuring the T₂ spin-spin relaxation time by means of a portable unilateral NMR instrument, fully non-invasive. Again, clear trends of the relaxation time were observed during the monitoring period.The results show that the monitoring of the metabolic profiling and the monitoring of the water status are two complementary means suitable to have a complete view of the investigated fruit.     

Nuclear Magnetic Resonance Relaxivities: Investigations of Ultrahigh‐Spin Lanthanide Clusters from 10 MHz to 1.4 GHz

Paramagnetic relaxation enhancement is often explored in magnetic resonance imaging in terms of contrast agents and in biomolecular nuclear magnetic resonance (NMR) spectroscopy for structure determination. New ultrahigh‐spin clusters are investigated with respect to their NMR relaxation properties. As their molecular size and therefore motional correlation times as well as their electronic properties differ significantly from those of conventional contrast agents, questions about a comprehensive characterization arise. The relaxivity was studied by field‐dependent longitudinal and transverse NMR relaxometry of aqueous solutions containing Fe^III₁₀Dy^III₁₀ ultrahigh‐spin clusters (spin ground state 100/2). The high‐field limit was extended to 32.9 T by using a 24 MW resistive magnet and an ultrahigh‐frequency NMR setup. Interesting relaxation dispersions were observed; the relaxivities increase up to the highest available fields, which indicates a complex interplay of electronic and molecular correlation times.     

The use of pulsed NMR to determine morphology of macromolecules

The study of the T₂ relaxation of nuclear spins, using a pulsed NMR sequence, can provide information on many aspects of molecular weight and mobility, morphology, crosslinking, entanglements, as well as on the influence of temperature and of solvents. This relatively new technique of analysis, based largely on irradiated polymers of known arrangements can also be extended to biopolymers, and determine for example the influence of additives, of radiation or chemical rearrangement, on the configuration and mobility of these macromolecules. The method of T₂ relaxation by spin-spin interaction is quite distinct from the very widely utilised T₁ spin-lattice interaction which depends primarily on local environment, suitable for the determination of chemical structure. The T₂ pulsed technique is particularly appropriate to analyse complex systems, as for example partially crystalline specimen, network-non network composites, molecular rearrangements occurring during melting or crystallization oriented and reinforced polymers dissolved or entangled high molecular weight chain molecules. Many of the data provided by this techniques can now be understood, but in some systems, basic work still remains to be done to elucidate the meaning of the information which it provides.     

Solvent Effect and Reactivity Trend in the Aerobic Oxidation of 1,3‐Propanediols over Gold Supported on Titania: NMR Diffusion and Relaxation Studies

In recent work, it was reported that changes in solvent composition, precisely the addition of water, significantly inhibits the catalytic activity of Au/TiO₂ catalyst in the aerobic oxidation of 1,4‐butanediol in methanol due to changes in diffusion and adsorption properties of the reactant. In order to understand whether the inhibition mechanism of water on diol oxidation in methanol is generally valid, the solvent effect on the aerobic catalytic oxidation of 1,3‐propanediol and its two methyl‐substituted homologues, 2‐methyl‐1,3‐propanediol and 2,2‐dimethyl‐1,3‐propanediol, over a Au/TiO₂ catalyst has been studied here using conventional catalytic reaction monitoring in combination with pulsed‐field gradient nuclear magnetic resonance (PFG‐NMR) diffusion and NMR relaxation time measurements. Diol conversion is significantly lower when water is present in the initial diol/methanol mixture. A reactivity trend within the group of diols was also observed. Combined NMR diffusion and relaxation time measurements suggest that molecular diffusion and, in particular, the relative strength of diol adsorption, are important factors in determining the conversion. These results highlight NMR diffusion and relaxation techniques as novel, non‐invasive characterisation tools for catalytic materials, which complement conventional reaction data.     

Scalar relaxation of homonuclear multiple-quantum coherences and relative signs of 14N-1H spin-spin coupling constants in E-N-phenylformamide

Homonuclear multiple-quantum NMR relaxation rates for E-N-phenylformamide, whose interesting part consists of two protons A and M differently coupled to a rapidly relaxing ¹⁴N nucleus are reported. The zero- and double-quantum linewidths depend on the cross correlation spectral densities and allow the determination of ¹J and ²J¹⁴N-¹H spin-spin coupling constants and their relative signs. We show that the two coupling constants have the same signs and are likely to be positive. Note that only one of the resonances of the AM pair needs to be significantly broadened due to scalar interactions in order for the cross correlation spectral density to become important for the multiple-quantum relaxation rates.     

Conformation and dynamics of fibrous and membrane proteins as revealed by high-resolution solid-state nuclear magnetic resonance

We examined conformation and dynamics of a variety of fibrous and membrane proteins by means of the conformation-dependent displacements of ¹³C nuclear magnetic resonance (NMR) chemical shifts and their relaxation parameters, respectively, as recorded by high-resolution solid-state ¹³C NMR. Determination of three-dimensional structure of atomic resolution is also briefly described for a simple peptide on the basis of precise measurements of interatomic distances.     

13C Fourier transform NMR and partially relaxed Fourier transform NMR relaxation study on the ethanol–manganese(II) system

The paramagnetic contribution to the ¹³C and ¹H nuclear relaxation rates in the ethanol-Mn(II) system has been calculated. Both T₁ and T₂ experiments have been performed by means of Fourier transform and partially relaxed Fourier transform NMR spectroscopy. The correlation times for the dipolar and scalar parts have been discussed and evaluated. Calculations of distances and of hyperfine coupling constants have been carried out.     

Assignment of proton NMR resonances and conformational analysis of [Lys8]-vasopressin homologues

The assignment of proton NMR signals and the conformational analysis of N-(Gly)–[Lys⁸]–vasopressin and N-(Gly Gly Gly) [Lys⁸]-vasopressin in dimethyl sulfoxide solution by means of one- and two-dimensional NMR techniques are presented. Dihedral angles obtained from ³J (HN C α-H) vicinal couplings, intramolecular distances estimated from nuclear Overhauser enhancements (NOEs) and two-dimensional experiments in the rotating frame (ROESY), as well as temperature dependences of the amide protons, suggest a more or less rigid ring conformation with an inverse γ-turn for both cyclic peptides.     

The effect of degassing on the measurement of transverse relaxation times in molten polymers

Three commercially produced polymer samples (polyethylene, polypropylene, and polystyrene) have been analyzed in the melt state using proton nuclear magnetic resonance (NMR) T₂ relaxation methods using the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo pulse sequence. Samples were run with exposure to air and again after extensive degassing at a vacuum of 10⁻⁴ mmHg for periods of not less than 96 h. The comparison is made by initially considering the presence of microscopic voids in the samples as a source of local field inhomogeneity and how they affect the T₂ relaxation behavior. For PP and PS samples, degassing caused a decrease in all T₂ time constants associated with the multicomponent decays. The component intensities each of the time constants was also significantly altered. For the PE sample, degassing caused a decrease in the time constants associated with the amorphous material in the molten polymer. Examination of the fastest relaxing component of the three component decay showed approximate invariance in the T₂ decay constant. This result supports our previously reported model in which that fast relaxing component is attributable to regions of local order or high segmental density within the molten polyethylene, a remnant of the crystalline material which exists in the premelting bulk polymer. The results of this research are of particular significance to those who wish to use this NMR technique as a quantitative method of determination of NMR distinct morphological regions within heterogeneous polymers. © 1996 John Wiley & Sons, Inc.     

On the single relaxation time approximation (SRTA) in the kinetic theory of gas phase NMR

The use of single-relaxation-time approximations (SRTA) in describing nuclear magnetic relaxation (NMR) is discussed and the proper identification of the relaxation parameters with collision cross sections is addressed. Two different SRTA curves giving the variation of the longitudinal relaxation time T₁ with number density n are evaluated using the extract numerical close-coupled results of Shafer and Gordon for molecular hydrogen infinitely dilute in helium and both SRTA curves are compared with that obtained from the full matrix inversion procedure.     

Identification of halogen substituents in natural products by measurement of 13c spin-lattice relaxation times and integrated intensities

A method is proposed for differentiating brominated carbons from chlorinated carbons by means of natural-abundance ¹³C NMR spectroscopy. The basis of the method is that the spin-lattice relaxation behaviour of brominated carbons is influenced by carbon-bromine scalar interactions, which can lead to shortened ¹³C spin-lattice relaxation times and reduced values of the nuclear Overhauser enhancement. C-Cl scalar interactions make a negligible contribution to the spin-lattice relaxation of chlorinated carbons. These effects are illustrated by measurement of the ¹³C spin-lattice relaxation times and integrated intensities of chloro-, bromo and iodobenzene and chloro-, bromo- and iodocyclohexane. The method is then tested on four polyhalogenated marine natural products. The results indicate that ¹³C relaxation measurements can be used to distinguish brominated carbons from chlorinated carbons in the case of halogenated quaternary carbons, sp² hydridized methine carbons and some sp³ hydridized methine carbons, but not in the case of halogenated methylene carbons or gem-dihalo substituted methine carbons.