Pulsed NMR Study Research Articles

Pulse NMR Study on the Mobility of Water Molecules in Various Starch Gel Samples

Pulse NMR Study on the Mobility of Water Molecules in Various Starch Gel Samples

Hydroxyl Motion in Mg(OH)2

We report on pulsed 1H NMR studies of the hydroxyl OH groups in magnesium hydroxide Mg(OH)2 at 77–355 K at 42.5772 MHz. The Fourier-transformed NMR spectra show the superposition of broad and narrow components. The broad NMR spectrum is assigned to dipole-coupled protons on a rigid lattice in the bulk Mg(OH)2, while the narrow NMR spectrum is assigned to extrinsic protons, e.g., conduction protons facilitated by lattice defects. We found a monotonically decreasing linewidth of the broad NMR spectrum on heating. The monotonic decrease in the linewidth is associated with hopping protons around a threefold axis (rotational hydroxyl protons).

Pulsed NMR Study of Water Behavior in Retrogradation Process of Rice and Rice Starch

Pulsed NMR Study of Water Behavior in Retrogradation Process of Rice and Rice Starch

Tailoring the Network Properties of Ca2+ Crosslinked Aloe vera Polysaccharide Hydrogels for in Situ Release of Therapeutic Agents

Properties of Aloe vera galacturonate hydrogels formed via Ca(2+) crosslinking have been studied in regard to key parameters influencing gel formation including molecular weight, ionic strength, and molar ratio of Ca(2+) to COO(-) functionality. Dynamic oscillatory rheology and pulsed field gradient NMR (PFG-NMR) studies have been conducted on hydrogels formed at specified Ca(2+) concentrations in the presence and absence of Na(+) and K(+) ions in order to assess the feasibility of in situ gelation for controlled delivery of therapeutics. Aqueous Ca(2+) concentrations similar to those present in nasal and subcutaneous fluids induce the formation of elastic Aloe vera polysaccharide (AvP) hydrogel networks. By altering the ratio of Ca(2+) to COO (-) functionality, networks may be tailored to provide elastic modulus (G') values between 20 and 20000 Pa. The Aloe vera polysaccharide exhibits time-dependent phase separation in the presence of monovalent electrolytes. Thus the relative rates of calcium induced gelation and phase separation become major considerations when designing a system for in situ delivery applications where both monovalent (Na(+), K(+)) and divalent (Ca(2+)) ions are present. PFG-NMR and fluorescence microscopy confirm that distinctly different morphologies are present in gels formed in the presence and absence of 0.15 M NaCl. Curve fitting of theoretical models to experimental release profiles of fluorescein labeled dextrans indicate diffusion rates are related to hydrogel morphology. These studies suggest that for efficient in situ release of therapeutic agents, polymer concentrations should be maintained above the critical entanglement concentration ( Ce, 0.60 wt %) when [Ca(2+)]/[COO(-)] ratios are less than 1. Additionally, the monovalent electrolyte concentration in AvP solutions should not exceed 0.10 M prior to Ca(2+) crosslinking.

2 石炭層へのガス吸蔵挙動(構造・物性(1))

2 石炭層へのガス吸蔵挙動(構造・物性(1))

Pulsed NMR Study of the Curing Process of Epoxy Resin

To analyze a curing process of epoxy resin in terms of molecular motion, we adapted a pulsed NMR method. Three kinds of (1)H spin-spin relaxation times (T(2L) (long), T(2S) (short) and T(2M) (intermediate)) were estimated from observed solid echo train signals as the curing process proceeded. A short T(2S) value below 20 micros suggests the existence of a motion-restricted chain, that is, cured elements of resin, and its fraction, P(S), sigmoidally increased with the curing time. On the other hand, the fraction of T(2L), P(L), decreased with the reaction time reciprocally against P(S), suggesting the disappearance of highly mobile molecules raised from pre-cured resin. The spin-lattice relaxation time, T(1), was also measured to check another aspect of molecular motion in the process. T(1) of the mixed epoxy resin and curing agent gradually increased just after mixing both of them. This corresponds to an increment of a less-mobile fraction, of which the correction time is more than 10(-6) s, and also means that the occurrence of a network structure whose mobility is strongly restricted by chemically bonded bridges between the epoxy resin and curing agent. The time courses of these parameters coincided with those of IR peaks pertinent to the curing reaction. Therefore, pulsed NMR is a useful tool to monitor the hardening process of epoxy resin in real time non-distractively in terms of the molecular motion of protons.

Pulsed field gradient NMR study of phenol binding and exchange in dispersions of hollow polyelectrolyte capsules

The distribution and exchange dynamics of phenol molecules in colloidal dispersions of submicron hollow polymeric capsules is investigated by pulsed field gradient NMR (PFG-NMR). The capsules are prepared by layer-by-layer assembly of polyelectrolyte multilayers on silica particles, followed by dissolution of the silica core. In capsule dispersion, (1)H PFG echo decays of phenol are single exponentials, implying fast exchange of phenol between a free site and a capsule-bound site. However, apparent diffusion coefficients extracted from the echo decays depend on the diffusion time, which is typically not the case for the fast exchange limit. We attribute this to a particular regime, where apparent diffusion coefficients are observed, which arise from the signal of free phenol only but are influenced by exchange with molecules bound to the capsule, which exhibit a very fast spin relaxation. Indeed, relaxation rates of phenol are strongly enhanced in the presence of capsules, indicating binding to the capsule wall rather than encapsulation in the interior. We present a quantitative analysis in terms of a combined diffusion-relaxation model, where exchange times can be determined from diffusion and spin relaxation experiments even in this particular regime, where the bound site acts as a relaxation sink. The result of the analysis yields exchange times between free phenol and phenol bound to the capsule wall, which are on the order of 30 ms and thus slower than the diffusion controlled limit. From bound and free fractions an adsorption isotherm of phenol to the capsule wall is extracted. The binding mechanism and the exchange mechanism are discussed. The introduction of the global analysis of diffusion as well as relaxation echo decays presented here is of large relevance for adsorption dynamics in colloidal systems or other systems, where the standard diffusion echo decay analysis is complicated by rapidly relaxing boundary conditions.

Pulsed NMR Study of Network Formation in the Course of Bulk Polymerization of Methyl Acrylate

The proton spin-spin relaxation time (T2) during the bulk polymerization of methyl acrylate was measured as a function of the reaction time at various temperatures. Three kinds of T2 (T2L (long), T2S (short) and T2M (intermediate)) were obtained as the polymerization proceeded. The fraction of T2S (FS) increased sigmoidally at a certain reaction time, while that of T2L (FL) decreased reciprocally. The former corresponded to the amount of a polymer whose molecular weight was sufficiently high enough to cause a tight entanglement that produced a transient network structure; the latter reflected a decrease in the mixture of the monomer and the low molecular weight of the polymer. T2M is considered to arise from a relatively mobile region of the entanglement. The relationship between the fractions of T2S + T2M and the polymer yield was found to be linear, which led us to monitor the polymer yield in real time during the polymerization in a non-distractive manner. 13C DD (dipolar decoupling)/MAS (magic angle spinning) NMR spectra were also measured to monitor the polymerization process in terms of the molecular motions between the main chain and the side chain in the formation of a network structure. The 13C DD/MAS NMR spectra show that the side chain motion became restricted as well as the main chain when the "Trommosdorff effect" (gel effect) was observed, and a part of the monomers were trapped in the network structure.

Pulse NMR Study of Refrigerated Cooked Rice and Comparison with Physicochemical Properties

The behavior of water molecules in refrigerated cooked rice was examined by measuring spin-spin relaxation time T2 using a pulse NMR spectrometer. T2 decay curve was analyzed using two-component and non-linear regression analysis and the presence of three kinds of water components with different T2 were revealed. In the case of fresh cooked rice, the observed T2 and the relative population were 0.9 ms (short T2) and 13%, 26 ms (intermediate T2) and 76%, and 200 ms (long T2) and 11%, respectively. The short T2 component was ascribed to the water molecules hydrated directly on the starch molecules. Intermediate T2 component was dominant and ascribed to the water molecules accommodated in the starch matrix inside the rice grain. The mobility of water molecules was suppressed remarkably during the storage at 0°C while it was almost independent of the refrigerated storage below −18°C. The long T2 component was ascribed to those grains accommodated in the surface luster film that was formed in the initial stage of the cooking process and composed of low molecular weight polysaccharides. The relaxation property observed by NMR was well consistent with the physicochemical properties.

Experiments on A-like to B phase transitions of [formula omitted] confined to aerogel

We have done pulsed and CW NMR studies of A-like to B phase transition in superfluid 3 He confined within aerogel with 98% porosity. The experiments were performed at pressure of 25.5 bar and in magnetic field of 284 Oe and 1 kOe . A kinetics of the transition is found to be different for pure 3 He in aerogel and for the cell preplated with 4 He .

De-regulation of D-3-phosphoglycerate dehydrogenase by domain removal.

Escherichia coli 3-phosphoglycerate dehydrogenase (PGDH) catalyzes the first step in serine biosynthesis, and is allosterically inhibited by serine. Structural studies revealed a homotetramer in which the quaternary arrangement of subunits formed an elongated ellipsoid. Each subunit consisted of three domains: nucleotide, substrate and regulatory. In PGDH, extensive interactions are formed between nucleotide binding domains. A second subunit-subunit interaction occurs between regulatory domains creating an extended beta sheet. The serine-binding sites overlap this interface. In these studies, the nucleotide and substrate domains (NSDs) were subcloned to identify changes in both catalytic and physical properties upon removal of a subunit-subunit interface. The NSDs did not vary significantly from PGDH with respect to kinetic parameters with the exception that serine no longer had an effect on catalysis. Temperature dependent dynamic light scattering (DLS) revealed the NSDs aggregated > 5 degrees C before PGDH, indicating decreased stability. DLS and gel filtration studies showed that the truncated enzyme formed a tetramer. This result negated the hypothesis that the removal of the regulatory domain would create an enzyme mimic of the unregulated, closely related dimeric enzymes. Expression of the regulatory domain, to study conformational changes induced by serine binding, yielded a product that by CD spectra contained stable secondary structure. DLS and pulsed field gradient NMR studies of the regulatory domain showed the presence of higher oligomers instead of the predicted dimer. We have concluded that the removal of the regulatory domain is sufficient to eliminate serine inhibition but does not have the expected effect on the quaternary structure.

Models of the low-spin iron(III) hydroperoxide intermediate of heme oxygenase: magnetic resonance evidence for thermodynamic stabilization of the d(xy) electronic state at ambient temperatures.

The (13)C pulsed ENDOR and NMR study of [meso-(13)C-TPPFe(OCH(3))(OO(t)Bu)](-) performed in this work shows that although the unpaired electron in low-spin ferrihemes containing a ROO(-) ligand resides in a d(pi) orbital at 8 K, the d(xy) electron configuration is favored at physiological temperatures. The variable temperature NMR spectra indicate a dynamic situation in which a heme with a d(pi) electron configuration and planar porphyrinate ring is in equilibrium with a d(xy) electron configuration that has a ruffled porphyrin ring. Because of the similarity in the EPR spectra of the hydroperoxide complexes of heme oxygenase, cytochrome P450, and the model heme complex reported herein, it is possible that these two electron configurations and ring conformations may also exist in equilibrium in the enzymatic systems. The ruffled porphyrinate ring would aid the attack of the terminal oxygen of the hydroperoxide intermediate of heme oxygenase (HO) on the meso-carbon, and the large spin density at the meso-carbons of a d(xy) electron configuration heme suggests the possibility of a radical mechanism for HO. The dynamic equilibrium between the ruffled (d(xy)) and planar (d(pi)) conformers observed in the model complexes also suggests that a flexible heme binding cavity may be an important structural motif for heme oxygenase activity.

Pulsed field gradient NMR study of the diffusion of n-hexane confined in hydroxylated and dehydroxylated MCM-41 of various pore diameters

The pore dimension of a reference MCM-41 material was reduced in a controlled manner by successive silylation with tetrachlorosilane. Also, the surface of these materials was modified by temperature treatment, resulting in a reduction of the concentration of surface silanol groups. Pulsed field gradient (PFG) NMR diffusion measurements of n-hexane confined in these modified MCM-41 samples have been performed and the results discussed within the Gaussian phase distribution and short gradient pulse approximations. The observed increase of the mean residence time with increasing diffusion time of the confined n-hexane suggests a distribution of channel lengths to exist.

On the roles of protein and starch in the aging of non-waxy rice flour

Changes in pasting and thermal properties of non-waxy rice flour and its isolated starch during storage at 25, 35 and 45°C were compared. Aging had no apparent effect on the pasting behaviour of the isolated starch, but markedly affected that of the flour. Peak viscosity ( V p) of rice flour pastes generally increased with both temperature and time of storage, but reached a plateau within 4 weeks of storage at 45°C. Fresh flour paste exhibited lower V p, a slower rise in apparent viscosity, and much better stability than an isolated starch paste. The addition of isolated oryzenin to isolated rice starch resulted in pasting behaviour which more closely approximated that of an extensively aged flour. DSC scans of fresh flour at ∼13% moisture revealed a weak heat-irreversible endothermic event over the temperature range from 47 to 66°C which was attributed to the denaturation of oryzenin. This transition was shifted to higher temperatures with increasing storage temperature and time. It also became increasingly skewed and broadened until it was no longer detectable after the flour had been stored at 45°C for 8 weeks. Starch gelatinization characteristics of both flour and isolated starch, as determined by DSC, were apparently unaffected by aging. However, rice flour exhibited significantly higher onset temperature of gelatinization, but lower gelatinization enthalpy, than isolated rice starch. Similarly, pulsed NMR studies showed no apparent effect of aging on retrogradation behaviour of rice flour or starch gels. These results suggest that modification of the protein component, rather than starch, was primarily responsible for rheological changes associated with aging of rice flour.

NMR Studies of Submonolayer 3He Adsorbed on Hexagonal Boron Nitride

Results are reported for high frequency pulsed NMR studies of 3He adsorbed on hexagonal boron nitride. The commensurate coverage, corresponding to a 0.75 fraction of a monolayer was studied. The temperature dependence of the nuclear spin-spin relaxation time was measured in the range of 0.1-2 K. From the relaxation times, the quantum mechanical exchange rates are calculated for a triangular array of 3He atoms.

Model peptide studies of sequence repeats derived from the intracrystalline biomineralization protein, SM50. I. GVGGR and GMGGQ repeats.

We report solution-state pulsed field gradient nmr studies of a native sequence-derived 23-residue peptidomimetic, N alpha-acetyl-QPGVGGRQPGMGGQPGVGGRQPG-C alpha-amid, that incorporates the prevalent GVGGR and GMGGQ repeats found in the sea urchin embryo intracrystalline spicule matrix protein, SM50 (Strongylocentrotus purpuratus). These repeats are sequence homologues of elastin protein repeats (VPGVG, VGGVG, and APGVGV) and spider dragline silk protein repeats (GPGG, GQGG, and QPGYG). Using rotating frame nuclear Overhauser effect (ROE) connectivities, CH alpha proton conformational shifts, 3JNH-CH alpha coupling constants, amide temperature shift coefficients, and pulsed field gradient ROE spectroscopy solvent exchange measurements, we find that the 23-mer peptidomimetic possesses a multiple beta-turn structure in aqueous solution, in equilibria with an extended or coil structure (60% beta-turn: 40% random coil). The GVGGR sequence adopts a double beta-turn conformation that is stabilized by two hydrogen bonds (R7-->V4, R20-->V17; G6-->G3, G19-->G16). The GMGGQ region adopts a single beta-turn conformation that is stabilized by a hydrogen bond involving residues Q14 and M11. Repeating beta-turn structures, or beta-spirals, may play an important role with regard to matrix assembly, protein stability, molecular elasticity, and/or protein-crystal recognition within the spicule mineralized matrix.

Model peptide studies of sequence repeats derived from the intracrystalline biomineralization protein, SM50. I. GVGGR and GMGGQ repeats

We report solution-state pulsed field gradient nmr studies of a native sequence-derived 23-residue peptidomimetic, Nα–acetyl–QPGVGGRQPGMGGQPGVGGRQPG–Cα–amide, that incorporates the prevalent GVGGR and GMGGQ repeats found in the sea urchin embryo intracrystalline spicule matrix protein, SM50 (Strongylocentrotus purpuratus). These repeats are sequence homologues of elastin protein repeats (VPGVG, VGGVG, and APGVGV) and spider dragline silk protein repeats (GPGG, GQGG, and QPGYG). Using rotating frame nuclear Overhauser effect (ROE) connectivities, CHα proton conformational shifts, 3JNH-CHα coupling constants, amide temperature shift coefficients, and pulsed field gradient ROE spectroscopy solvent exchange measurements, we find that the 23-mer peptidomimetic possesses a multiple β-turn structure in aqueous solution, in equilibria with an extended or coil structure (60% β-turn: 40% random coil). The GVGGR sequence adopts a double β-turn conformation that is stabilized by two hydrogen bonds (R7 → V4, R20 → V17; G6 → G3, G19 → G16). The GMGGQ region adopts a single β-turn conformation that is stabilized by a hydrogen bond involving residues Q14 and M11. Repeating β-turn structures, or β-spirals, may play an important role with regard to matrix assembly, protein stability, molecular elasticity, and/or protein–crystal recognition within the spicule mineralized matrix. © 1999 John Wiley & Sons, Inc. Biopoly 49: 303–312, 1999

Water Diffusion in Polymer-like Reverse Micelles. 2. Composition Dependence

A pulsed field gradient FT 1H NMR study on the system soybean lecithin/water/perdeuterated cyclohexane, covering a wide range of compositions, is presented. The water diffusion was found to be Gaussian with a well-defined, time-independent, single diffusion coefficient. After the contribution from the diffusion of water dissolved in the cyclohexane has been properly taken into account, all the data follow the same master curve as a function of the water to lecithin molar ratio (W0). The dominating mechanism for the water diffusion was found to be the motion inside giant wormlike reverse aggregates mediated by an interaggregates exchange with a characteristic time on the order of microseconds. For all the lecithin concentrations investigated, increasing the value of W0 increases the water diffusion up to W0 = 15, where a structural transition toward spherical aggregates causes a decrease in the water diffusion.

Peculiar spin echo in thick3Hefilms on boron nitride

Results are reported for pulsed NMR studies of thick films (25 layers) of ${}^{3}\mathrm{He}$ physisorbed on carefully prepared hexagonal boron nitride substrates. Peculiar doublet structures in the NMR spin-echo shapes have been observed for temperatures $0.9l~Tl~1.2\mathrm{K}.$ These echo shapes are interpreted in terms of a two-component system: (a) a solid phase (up to 4.6 layers) near the substrate, and (b) a liquid phase near the free surface of the film.

A pulsed gradient spin echo NMR study of guest encapsulation by hydrogen-bonded tetraurea calix[4]arene dimers

Diffusion coefficients, as obtained by the PGSE NMR technique, were used to probe encapsulation of guests in self-assembled tetraurea calix[4]arene dimers in organic solvents and to demonstrate their destruction by DMSO.