2H NMR Spin-lattice Relaxation Times Research Articles

2H and 13C NMR studies of molecular orientation and dynamics in liquid crystal 6BA

The dynamics and orientation of dimers accompanying the formation and destruction of hydrogen bonds in the nematic phases of 4- n-hexylbenzoic acid (6BA) were studied by 13C and 2H NMR. The orientational order parameter S in the nematic phase was estimated from the quadrupole splitting of the 2H NMR spectrum. The intermolecular interaction energy for the molecular order in the nematic phase decreased with increasing temperature. The flexibility of dimers due to the destruction of the hydrogen bond is closely related to a decrease in the intermolecular interaction energy. The proportion of 2H NMR spin–lattice relaxation time ( T 1) to S, which reveals the coupling of the orientational fluctuations with the hydrogen bonding processes, was observed.

An NMR Data Base for Simulations of Membrane Dynamics

Validation of molecular simulations requires comparison with experimental data to confirm and test computational predictions. One rich area of computational effort is in the field of membrane biophysics. Here we report a comprehensive NMR data base containing the results of 13C and 2H NMR spin-lattice relaxation times (T1Z) and segmental order parameters (SCD) for various saturated, unsaturated, and biological membrane phospholipids. Relaxation rates recorded as a function of field strength (Larmor frequency) provide information about molecular dynamics.

Quantitative analysis of 2H NMR T1Q, T1Z and T2 relaxation times in the SmA phase of a liquid crystal dendrimer

In this work the 2H NMR spin-lattice relaxation times, T1Z and T1Q, and the spin-spin relaxation times measured by means of the quadrupolar echo sequence, T2, have been analyzed in order to study the dynamic processes of a liquid crystalline dendrimer labelled on the aromatic core of the lateral mesogenic units in the SmA phase. Several theoretical models describing both internal and overall molecular diffusion motions have been used to reproduce the temperature dependence of the spectral densities J(0)(0), J1(omega0) and J2(2omega0) experimentally determined at the Larmor frequency omega0 of 61.3 MHz. This analysis identifies in the internal rotation of the deuterated phenyl ring around its para axis the main contribution to the longitudinal relaxation. Moreover, the best fitting of the minimum in the trend of both T1Z and T1Q was obtained by introducing a distribution of activation energies for such internal motion, thus confirming previous hypothesis of dynamic heterogeneity. A much slower motion, affecting the sole transverse relaxation and ascribable to the reorientation of the whole macromolecular dendrimers, was modelled through the so called "slowly relaxing local structure" approach.

Significantly Contrasting Solid State Dynamics of the Racemic and Enantiomerically Pure Crystalline Forms of an Amino Acid

Solid state dynamic properties have been investigated for the racemic (DL) and enantiomerically pure (L) crystalline forms of the amino acid serine [HO x CH2 x CH(NH3(+)) x CO2(-)] using 2H NMR line shape analysis and 2H NMR spin-lattice relaxation time measurements for samples of DL-serine and L-serine deuterated in the NH3(+) and OH groups. 2H NMR line shape analysis indicates that, for both L-serine and DL-serine, the ND3(+) group undergoes a 3-site 120 degrees jump motion, with jump frequencies in the intermediate motion regime (10(3) s(-1) to 10(8) s(-1)) in the temperature range 153-313 K. However, at a given temperature, the jump frequency is substantially higher for L-serine (e.g., at 233 K, the jump frequency is 5.0 x 10(6) s(-1) for L-serine and 6.0 x 10(4) s(-1) for DL-serine). The OD group is not dynamic on the 2H NMR time scale within the temperature range studied. The results from both 2H NMR line shape analysis (LA) and 2H NMR spin-lattice relaxation time measurements (SLR) indicate that the activation energy for the 3-site 120 degree jump motion of the ND3(+) group is significantly higher for DL-serine [38.0 +/- 1.0 kJ mol(-1) (LA); 39.7 +/- 0.8 kJ mol(-1) (SLR)] than for L-serine [23.4 +/- 0.8 kJ mol(-1) (LA); 23.8 +/- 0.3 kJ mol(-1) (SLR)]. The difference in activation energies between DL-serine and L-serine is substantially greater than any reported previously for an amino acid in different crystal forms.

Phase behaviour and dynamics in ethylammonium halide salts

1H and 2H NMR spin-lattice relaxation times (T1) have been studied as a function of temperature to obtain activation energies for the threefold reorientations of CH3 groups (Ea = 22.83, 18.95, and 18.39 kJ mol–1) and NH3 groups (Ea = 47.64, 45.31, and 28.74 kJ mol–1) in the β-phases of ethylammonium chloride, -bromide, and -iodide, respectively, for comparison with barriers determined from Raman torsional frequencies. T1, 2H NMR lineshapes, and DSC studies have been used to clarify the existence of a β → α phase transition above room temperature in all three salts, and the absence of a transition at low temperature. 2H NMR lineshape analysis indicates a rapid n-fold (probably fourfold) reorientation of the whole molecule in the α-phase. This motion occurs about an axis in the molecular CCN mirror plane in the bromide and iodide. In the chloride, the axis is in the plane perpendicular to the bisector of the DCD bond angle.Key words: ethylammonium halides, solid state 1H and 2H NMR, molecular dynamics, rotational barriers, phase behaviour.

Diffusion of hydrogen isotopes in the monohydride phase of Ti 1− zV zH xD y studied by 1H and 2H NMR spin–lattice relaxation times

The 1H and 2H spin–lattice relaxation times in the monohydride phase of the V–H–D and Ti 0.1V 0.9–H–D systems have been re-analyzed by Bloembergen–Purcell–Pound (BPP) equations with a log-normal distribution of correlation times, and the obtained parameters have been compared with those of the Ti 0.33V 0.67–H–D system. The mean activation energy and the magnitude of the distribution of H and D diffusions are discussed in relation to the metal compositions, the hydrogen site, the crystal structure and the [H]/[D] ratios.

Pressure and Temperature Effects on 2H-NMR Spin-Lattice Relaxation Times in TBA- and Urea-D2O Solutions.

The 2H NMR spin-lattice relaxation times (T1) of D2O molecules and the 1H chemical shifts (δ) of water in tert-butyl alcohol (TBA) -and urea-D2O solutions up to 8 mol% were measured at 7. 7, 29. 9, and 48. 4°C up to 294 MPa in order to elucidate the pressure and temperature effects on the hydrophobic hydration. The pressure coefficient of T1 in the TBA solution was smaller than that in pure water, and decreased monotonously with an increase in the TBA content. The δ (H2O) in the TBA solution exhibits a downfield shift relative to that in neat D2O and the difference in the δ (H2O) between TBA solution and neat D2O becomes larger with compression. These results suggest that the structure of the hydration shell is not so bulky as the tetrahedral open structure of water. On the other hand, the results in urea solutions were in marked contrast to those in TBA solutions.

Pressure Effect on the Solvation of Tetraalkylammonium Ions in Water.

In order to elucidate the pressure effect on the hydrophobic hydration shell, the limiting molar conductivities of tetraalkylammonium ions (Me4N+ to Bu4N+) in H2O and D2O were determined at 5°C as a function of pressure up to 196. 1 MPa. The rotational correlation times of D2O molecules in pure D2O (τc0) and coordinated to Me4N+ and Bu4N+ ions (τc) were also estimated at 6. 9°C up to 294. 2 MPa from 2H NMR spin-lattice relaxation times (T1). The residual friction coefficient (Δζobs) obtained from the limiting molar conductivity of the ion (λ0) became larger with increasing ionic radius and pressure except for Pr4N+ and Bu4N+ ions in D2O at lower pressure. These results suggest that the structure of the hydrophobic hydration shell is not weakened by pressure as much as that of bulk water, which is in qualitative agreement with the pressure dependence of τc (Bu4N+) /τc0. However, a larger value of τc (Me4N+) than that of τc0 does not always correspond to a negative value of Δζobs (Me4N+) indicating a structure breaking property of this ion.

Dynamics of benzene and pyridine guest molecules in their tri-ortho-thymotide inclusion compounds. Solid-state 2H NMR Studies

Dynamic properties of perdeuteriated benzene (C6D6) and perdeuteriated pyridine (C5D5N) guest molecules in their tri-ortho-thymotide (TOT) inclusion compounds have been studied via variable-temperature wide-line solid-state 2H NMR spectroscopy. The temperature dependences of the quadrupole echo 2H NMR lineshape and the 2H NMR spin–lattice relaxation time for C6D6/TOT in the temperature range 96–293 K are interpreted in terms of in-plane rotation (nearest-neighbour 2π/6 jump model) of the benzene molecule, together with restricted wobbling of the molecular plane. The 2H NMR spin–lattice relaxation time measurements suggest that there are two dynamically inequivalent types of benzene molecule (included in structurally different types of tunnel within the TOT host structure), with their activation energies for in-plane rotation estimated to be 4.0 and 5.9 kJ mol–1. For C5D5N/TOT in the temperature range 90–363 K, three types of motion have been identified: (i) in-plane libration; (ii)π jumps of each pyridine molecule about its two-fold symmetry axis; (iii) restricted wobbling of the molecular plane. On the basis of 2H NMR lineshape analysis of 2H NMR spin–lattice relaxation time measurements, the activation energy for motion (ii) is estimated to be ca. 17-20 kJ mol–1.

Effect of the Binding of Water to Excipients as Measured by 2H-NMR Relaxation Time on Cephalothin Decomposition Rate.

^2H-NMR spectra and the relaxation time of deuterium oxide adsorbed on some pharmaceutical excipients were measured using a high resolution NMR spectrometer in order to clarify the effect of the binding of water to some pharmaceutical excipients on drug decomposition. The decomposition rate of cephalothin was determined in the presence of corn starch and MCC as model excipients under various humidity conditions. The decomposition rate increased with the increasing water content of the excipients. The spin-lattice relaxation time increased with increasing water content similarly to the decomposition rate. This suggests that the 2H-NMR spin-lattice relaxation time can be used as a measure of the water mobility which affects the decomposition rate of drugs in the solid state.

Nuclear magnetic resonance studies of the methylammonium hexahalotellurates

A 2H and 14N NMR study of the solid methylammonium hexahalotellurates, (MA)2TeX6 (MA = CH3ND3+ or CD3NH3+, X = Cl, Br, and I), has been undertaken to characterize the dynamics of the methylammonium (MA) ion as a function of temperature. At room temperature, the MA ion in the hexachlorotellurate (solid II) is confined to C3 jumps about the C—N axis while a small angle libration of the C—N axis is occurring. In the room temperature phase, solid I, of (MA)2TeBr6 and (MA)2TeI6 the MA ions are performing overall reorientations on the ps time scale, averaging the 2H nuclear quadrupolar interactions to zero. Variable temperature 2H NMR spin-lattice relaxation times, T1, indicate an activation energy, EA, for "isotropic" reorientations of the CH3ND3+ ion of 5.2 ± 0.5 and 2.6 ± 0.3 kJ mol−1 for X = Br and I, respectively. Deuterium T1 values for C-deuterated MA ion in the hexaiodotellurate indicate an EA for whole-ion reorientation of 3.1 ± 0.3 kJ mol−1. At any given temperature, the correlation time, τc, derived from the T1 results was found to be the same for the two deuterium-labelled hexaiodotellurates. The similarity of both the EA and the τc values implies correlated motion of the methyl and ammonium groups. The 14N T1 results for solid I of (MA)2TeI6 indicate that C—N axis motions, with an EA = 5.6 ± 0.6 kJ mol−1, are more hindered than N—D or C—D bond dynamics. The 2H NMR spectra for (MA)2TeI6 (solid II) and (MA)2TeBr6 (solids II, III, and IV) are characterized by a Pake doublet line shape. The measured peak-to-peak splittings are less than what is predicted by C3 motion about the molecular symmetry axis. It is possible to model these line shapes by postulating that C3 rotations of the methyl and ammonium groups occur as the C—N axis librates in an effective cone about the position of the static molecular axis. For (CH3ND3)2TeBr6 and (CD3NH3)3TeBr6 the peak-to-peak splittings in the 2H NMR spectra were measured as a function of temperature in solid phases II, III, and IV and were found to be similar. Finally, the 2H NMR line shape relaxation for (MA)2TeBr6 (solid III) displays an orientation dependence indicating that rotations about the C—N axis are discrete rather than diffusive in nature. For solid phase II of (MA)2TeCl6, the line shape is observed to relax isotropically, implying that continuous C3 rotations are taking place. Keywords: 2H and 14N NMR, methylammonium hexahalotellurates, molecular motion.

The dependence of glyceroglycolipid orientation and dynamics on head-group structure

The head-group orientations and molecular dynamics of three glyceroglycolipids in aqueous dispersions, as determined by 2H-NMR, are compared. 1,2- Di-O- tetradecyl-3-O-(α- d - glucopyranosyl)-sn- glycerol (α-DTGL) and 1,2- di-O- tetradecyl-3-O-(α- d - mannopyranosyl)-sn- glycerol (α-DTML), selectively 2H-labelled on the pyranose ring, at the exocyclic hydroxymethyl group, and at C3 of glycerol, have been studied by 2H-NMR and the results compared with those reported earlier for 1,2- di-O- tetradecyl-3-O-(β- d - glucopyranosyl)-sn- glycerol (β-DTGL). The α-glucolipid exhibits a gel-to-liquid crystal phase transition and a lamellar to hexagonal mesophase transition at temperatures which are similar to those of the β-anomer, β-DTGL. However, α-DTGL exhibits head group orientations and molecular ordering in the lamellar and hexagonal phases which differ strikingly with those reported for the corresponding β-glucolipid. Whereas the head group of β-DTGL is extended away from the bilayer surface into the aqueous phase, that of α-DTGL is almost parallel to the bilayer surface. α-DTGL exhibits a molecular order parameter of 0.56 which is substantially greater than that of its anomer, β-DTGL, 0.45. The latter indicates that the head group region of the α-glyceroglucolipid is characterized by smaller angular fluctuations than that of β-DTGL. On entering the hexagonal mesophase the pyranose ring of the β-glucolipid undergoes a large reorientation relative to the motional axis of the head group, whereas the α-anomer exhibits only a small orientational change. 1,2- Di-O- tetradecyl-3-O-(α- d - mannopyranosyl)-sn- glycerol (α-DTML) undergoes a phase transition at 47°C, attributed to the unusual lamellar gel to hexagonal phase transition. The pyranose ring of α-DTML, in a mixture with dimyristoylphosphatidylcholine (1:9 mol ratio) to give a lamellar liquid crystalline phase, is oriented away from the bilayer surface into the aqueous environment and has an S mol of 0.75. The results for α-DTML, 2H-labelled at the C3 position of glycerol, suggest that this segment also has high molecular ordering. α-DTML in a lamellar environment has the least flexible membrane surface of the glyceroglycolipids investigated to date. 2H-NMR spin lattice relaxation times have been used to probe the head group motions of the glycolipids. The results indicate that the rate of head group motion increases in the order α-DTML < α-DTGL < β-DTGL. The glycolipid head group has lower motional rates than the corresponding motion of phospholipids, with the possible exception of cardiolipin.