Kenneth R. Jeffrey

A 7Li nuclear magnetic resonance study of LiCF3SO3 complexed in poly(propylene‐glycol)

7Li nuclear magnetic resonance (NMR) linewidths and spin–lattice relaxation times for poly(propylene‐glycol) complexed with a range of concentrations of LiCF3SO3 are reported over the temperature region from 205 to 405 K. Calculations suggest that the spin–lattice relaxation mechanism is caused by the interaction between the 7Li (I=3/2) quadrupole moment and fluctuations in the surrounding electric field gradients, whereas the line shapes are influenced by both the dipolar and quadrupolar interactions. The motional parameters reported indicate that ion–polymer or ion–ion interactions are important in determining the Li+ cation mobilities. This is reflected in the lengthening of the correlation time with increase in Li+ ion concentration which suggests a decreased mobility for the cations resulting from a transient coordination of the cation to the polymer matrix or ion aggregation. Also, the activation energies in this study (∼0.24 eV) are in agreement with values obtained from recent pulsed field gradien...

A dielectric analysis of liquid and glassy solid glucose/water solutions.

Dielectric relaxation data covering a temperature range from above room temperature to below the glass transition for 40% (w/w) and 75% (w/w) glucose/water solutions in the frequency range between 5 and 13 MHz are presented. These data are used to obtain correlation times for the dielectric relaxation in the viscous liquid and the glass and are compared with correlation times determined from deuterium nuclear spin relaxation times [J. Chem. Phys., 110 (1999) 3472-3483]. The two sets of results have the same temperature dependence, but differ in magnitude by a factor of 3, implying that the relaxation is a small-step rotational diffusion. Both the structural relaxation (alpha process) and the slow beta process are present. In the 40% glucose/water sample, there is a dielectric relaxation attributable to the ice that forms at low temperature. It is shown that the reciprocal of the viscosity, the correlation time derived from the dielectric relaxation, and the dc conductivity have a similar dependence on temperature.

A study of the molecular motion in glucose/water mixtures using deuterium nuclear magnetic resonance

Using separate samples containing deuterated glucose and D2O, the motions of the glucose and the water molecules in the viscous liquid and in the amorphous glass phase of glucose/water mixtures are examined. Spin-lattice relaxation measurements and spectra obtained in the solid confirm the existence of both a crystalline ice phase and an amorphous glass phase. Diffusion measurements performed using the large gradient in the fringe field of a superconducting magnet determine the rates of translational motion of both the sugar and the water molecules over a limited range of temperature in the viscous liquid region. From the deuterium nuclear magnetic resonance spin-lattice relaxation time, T1, the spin-spin relaxation time, T2, and spin-alignment measurements, correlation times for the motion of these molecules are obtained. The correlation time for the α process increases dramatically at Tg, while the β process continues into the low temperature glass phase, following an approximately Arrhenius relationship.

The quadrupolar spectrum of a spin I = 1 in a lipid bilayer in the presence of paramagnetic ions

Abstract The NMR signal from selectively deuterated molecules in a lipid bilayer where there are paramagnetic ions present in the aqueous region is influenced by both the nuclear quadrupole interaction and the dipolar interaction between the deuterium nuclei and the surrounding ions. The quadrupolar split powder pattern is no longer symmetric about the center of the spectrum. The spectra which result from the use of the quadrupole echo pulse sequence are quite complex since this sequence does not refocus the dephasing due to the dipolar interaction. A new pulse sequence which refocuses both the dipolar and quadrupolar interactions is suggested. Both 2H and 14N spectra from lipid molecules with the phosphatidylcholine headgroup obtained with the conventional quadrupole echo sequence and the new sequence are compared with predictions of density matrix theory calculations. There is excellent agreement between the experimental and simulated spectra.

Orientational order in the choline headgroup of sphingomyelin: A 14N-NMR study

An aqueous dispersion of fully hydrated bovine sphingomyelin was studied using 14N-NMR spectroscopy. Spectra were obtained as a function of temperature over the range 15-80 degrees C, in both the liquid crystal and gel phases. In the liquid crystal phase, powder pattern lineshapes were obtained, whose quadrupolar splitting slowly decreases with increasing temperature. The spectra are increasingly broadened as the temperature is lowered through the phase transition into the gel phase. The linewidths and the second moments of these spectra indicate that the onset of a broad phase transition occurs at approx. 35 degrees C, in agreement with previous calorimetric and 31 P-NMR measurements. There is no evidence from the lineshapes for an hexagonal phase in this system, and this conclusion is supported by X-ray diffraction measurements carried out on aqueous dispersions of sphingomyelin in both phases. Assuming that the static nitrogen quadrupole coupling constant is the same for both sphingomyelin and dipalmitoyl-L-alpha-phosphatidylcholine (DPPC), the decrease observed in the quadrupolar splitting of sphingomyelin compared to that of DPPC indicates that the orientational order of the choline headgroup in liquid crystalline sphingomyelin is not the same as that of its counterpart in DPPC. Preliminary relaxation time measurements of T1 and T2 are presented which suggest that there are also dynamic differences between sphingomyelin and DPPC in the choline headgroup.

The use of wide-line [14N]nitrogen NMR as a probe in model membranes

1. Introduction A major aim of bioiogi~al membrane research is to understand the relationship between membrane struc- ture and function at the molecular level. The presently accepted fluid mosaic model [ 1] indicates that the membrane is basically an anisotropic fluid composed of a lipid bilayer in which proteins are embedded. At the molecular level it is necessary to describe both the average conformation of the molecules and their molecular dynamics. There is both translation and rotation of the lipid and protein molecules within the bilayer structure. For the lipids in particular, there are many internal degrees of freedom. Of the many techniques [Z] being used to probe the molecular environment within membranes, nuclear magnetic resonance (NMR) [3,4] has proven to be one of the most useful methods for investigating the dynamical state of the membrane. Work has con- centrated on describing the dynamical behaviour of the lipid molecules and the interaction of these molecules with other membrane components such as various proteins [5,6] and cholesteroi [7,8]. To do this, it is essential to probe various positions in the lipid molecules. Because of the overabundance of hydrogen atoms, proton NMR spectra have been difficult to interpret [9,10] and give little information about the motion of a single segment of the lipid molecule. 3’P NMR [l I] has been successfully used to probe the head group region of phospholipids. Selective incorporation of ‘II [12], 13C [ 131 and tgF [14] into lipid molecules has been the most fruitful method of probing the molecular dynamics. However, considerable time and expertise are necessary for the synthesis of the labelled molecules required for a complete investigation of the lipid dynamics. The use of naturally abundant NMR probes is there- fore advantageous whenever possible. One naturally

Molecular motion in the lyotropic liquid crystal system containing potassium palmitate: A study of proton spin-lattice relaxation times

Abstract Proton magnetic resonance of mixtures of 1 mol potassium palmitate/6.3 mol D2O have yielded information about the phase transitions as well as the frequencies and activation energies associated with the various molecular motions in the temperature range from 100 to 500 K. At low temperature in the coagel phase methyl group rotation occurs. At higher temperature in this same phase a small amplitude motion which could be the diffusion of water or the counterion is detected. At about 300 K there is a phase transition to the gel phase, where both rotation of the soap molecule about the long axis and translational diffusion are observed; diffusion being the slower process. The transition from the gel to liquid crystal phase occurs at about 323 K. The observed 1 (ω 0 ) 1 2 dependence of the spin-lattice relaxation demonstrates the importance of order fluctuations in the liquid crystal phase. A second relaxation mechanism is also observed which is probably caused by conformational changes within the fatty acid chain. Perdeuterated soap molecules and molecules with deuterated methyl groups were used to show that methyl group reorientation is an important relaxation mechanism only in the coagel phase and to probe the effect of the intermolecular nuclear dipole-dipole interaction.

Proton magnetic relaxation study of molecular reorientation in NH4ReO4

Abstract Measurements of the proton T 1 , T 1 ρ , and T 1D in NH 4 ReO 4 are reported as a function of temperature over the range from 60 to 300 K. The temperature dependence of T 1 can be described in terms of the dipolar interaction between protons within an NH 4 + ion made time dependent by rapid classical reorientation at a rate which may be described by an Arrhenius relation. The activation energy for the molecular reorientation is 2.2 kcal mole −1 . Over limited regions of temperature this same model can also be used to explain the T 1 ρ and T 1D results. However, at low temperatures the T 1 ρ data suggest the importance of quantum mechanical tunneling, and at high temperatures the dipolar coupling between the proton and rhenium spins rendered time dependent by the rapid rhenium spin-lattice relaxation dominates both T 1 ρ and T 1D . Due to a strong angular dependence of the relaxation rates associated with the proton-rhenium interaction, nonexponential decays occur for a powdered sample as used in these experiments. From the T 1 ρ and T 1D results it is concluded that the rhenium spin-lattice relaxation is dominated by the quadrupolar Raman spin-phonon interaction. There is no evidence in any of the measurements for a previously postulated order-disorder phase transition in the vicinity of 200 K and involving the NH 4 + ions.

Dynamics of the phosphate group in phospholipid bilayers. A 31P nuclear relaxation time study

The spin-lattice relaxation time of the 31P nucleus in the phosphate group of egg yolk phosphatidylcholine multilamellar dispersions has been investigated at four resonant frequencies (38.9, 81.0, 108.9, and 145.7 MHz) in the temperature range from -30 degrees to 60 degrees C. The observed frequency dependence of the relaxation indicates that both dipolar relaxation and relaxation due to anisotropic chemical shielding are significant mechanisms. The experimental data have thus been modeled assuming both mechanisms and the analysis has allowed the contribution of each to the relaxation to be determined along with the correlation time for the molecular reorientation as a function of temperature. Dipolar relaxation was found to dominate at low nuclear magnetic resonance frequencies while at high frequencies the anisotropic chemical shift dominates. The correlation time of the phosphate group is on the order of 10(-9) s at 60 degrees C and increases to approximately 10(-7) s at -30 degrees C. It is observed that the freezing of the buffer which occurs at approximately -8 degrees C has a significant effect on the phosphate group reorientation. This effect of the freezing is to change the activation energy for the phosphate group reorientation from 16.9 KJ/mol above -8 degrees C to 32.5 KJ/mol below -8 degrees C.

Effects of cholesterol on the orientational order of unsaturated lipids in the membranes of Acholeplasma laidlawii: A 2H-NMR study

We have investigated by 2H-NMR the effects of the incorporation of cholesterol on the orientational order of unsaturated lipid acyl chains in the membranes of acholeplasma laidlawii B. This is the only 2H-NMR study to date of the influence of cholesterol in a biological membrane using specifically labelled fatty acids. We observed the characteristic condensing effect of cholesterol on the lipid acyl chain order in the liquid crystalline phase. In terms of the percentage increase in the quadrupolar splittings, the presence of cholesterol has its greatest effect on the methyl end of the labelled oleoyl chains, with a maximum at the C-14 segment. In absolute terms, the perturbation is greatest in the carboxyl end of the chains. The temperature dependence of the 2H spectra for the cholesterol-containing membranes is very similar to that for the cholesterol-free membranes. The broad phase transition of the membrane lipids, which is characteristic for the samples lacking cholesterol, is apparently little affected by the presence of up to 27 mol% cholesterol. In addition, the temperature of onset of the phase transition is not significantly depressed by the presence of cholesterol.