Reorientational Correlation Times Research Articles

Measurement of bound water in an aqueous DNA solution using nuclear magnetic resonance and time domain reflectometry

Nuclear magnetic resonance (NMR) and time domain reflectometry (TDR) were employed to investigate the bound water of deoxyribonucleic acid (DNA) in aqueous solution. The NMR transverse magnetization of the water protons decays in the manner described by a monoexponential function which gives a single magnetic relaxation time (T 2 ). However, this fact does not mean that the correlation times for rotational reorientation of both bound are free water molecules are identical

Application of 1H NMR selective and biselective relaxation times. III—conformational analysis of quinidine in solution

AbstractA relaxation method was applied to quinidine (1) where cross‐relaxations (σij) were obtained by 1H selective and biselective T1 measurements and correlation times for molecular reorientation (τc) were evaluated from the frequency dependence of the non‐selective T1. The interproton distances (rij) were estimated using these σij and τc values, and the conformation of 1 was determined by comparing the estimated rij values with those calculated from the molecular model. The resulting proposed conformation had the following dihedral angles: ∠ C‐11C‐10C‐3–C‐4 = 150°, ∠ C‐4′C‐9C‐8C‐7 = 50° and ∠ C‐9′C‐4′C‐9C‐8 = 70°. The conformation of the side‐chain (∠ C‐11C‐10C‐3C‐4) is different from the results for the crystal by x‐ray analyses.

Deuterium NMR relaxation studies of peptide-lipid interactions.

A unique model membrane system composed of a synthetic amphiphilic peptide (Lys2-Gly-Leu16-Lys2-Ala-amide) and a specifically labeled phospholipid (1,2-[7,7-2H2]dipalmitoyl-sn-glycero-3-phosphocholine) has been studied by 2H NMR, using inversion recovery, quadrupolar echo, and modified Jeener-Broekaert sequences, from 213 to 333 K, at molar peptide concentrations of 0, 2, 4, and 6%. Analysis of the experiments, employing a density matrix treatment based on the stochastic Liouville equation, revealed information about the dynamic organization of the lipid in the model membrane system, whose phase behavior has been determined previously [Huschilt et al. (1985) Biochemistry 24, 1377-1386]. The dynamic organization is described in terms of segmental and molecular order parameters and in terms of correlation times corresponding to both internal and overall lipid motions. In the liquid crystalline phase, the molecular order parameter, SZZ, was observed to decrease slightly upon addition of peptide while the conformational order parameter corresponding to the seventh segment, SZ'Z', did not change for any concentration of peptide. In general, the gauche-trans isomerization rate in the middle of the chain was not observed to change upon peptide addition, whereas the whole body reorientational correlation times (tau R parallel and tau R perpendicular) increased by nearly an order of magnitude. The anisotropy ratio (tau R perpendicular/tau R parallel) decreased with peptide added. An additional motion which involves a jump about the axis of the sn-2 chain is also observed to be slowed down significantly in the presence of peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

C60 solid state rotational dynamics and production and EPR spectroscopy of fullerenes containing metal atoms

The narrow single-line NMR spectrum of solid C60 at ambient temperature demonstrates that the molecules rapidly reorient within the crystal lattice. The rate of this motion can be quantitatively probed using solid-state NMR by measuring the relaxation rate, 1/T1. The dominant contribution to this rate at ambient temperature arises from the combination of rotation and chemical shift anisotropy (CSA). The contribution, 1/T1CSA, is isolated by measuring the magnetic field dependence of T1, allowing the molecular reorientational correlation time, tau , to be determined from T1CSA and the CSA tensor components. At 283 K tau =9.1 ps, only three times longer than the calculated tau for free rotation, and shorter than the value measured for C60 in solution (15.5 ps). This tau corresponds to a rotational diffusion constant D=1.8*1010 s-1. Below 260 K a second phase with a much longer reorientation time is observed, consistent with reports of an orientational phase transition in solid C60. In both phases tau shows Arrhenius behaviour with apparent activation energies of 1.4 and 4.2 kcal mol-1 for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The authors succeeded in producing quantities of fullerene cages containing metal atoms sufficient to allow the first characterization of such a species by electron spin resonance spectroscopy. The EPR spectrum of La@C82 shows an octet of lines centered in the region characteristic of fullerene anion radicals, with a small hyperfine splitting which indicates that the spin is predominantly associated with the carbon shell, leaving the lanthanum atom in its preferred +3 oxidation state.

Surface melting of the (0001) face of TIP4P ice

The onset of surface melting of the basal plane (0001) face of TIP4P ice is studied in the temperature range T = 190−250 K using the molecular dynamics (MD) method. The disordering of the surface seen in the calculations can be explained in terms of the disruption of the hydrogen bonding network at the surface. At T = 210 K and lower the surface remains solid-like, as evidenced by non-zero order parameters and no calculable self-diffusion, although the reorientational correlation time (ca. 100 ps) is closer to that of the liquid than that of bulk ice. On the other hand, at T = 230 K (−43°C) and higher the surface is liquid-like, as shown by low order parameters and a high translational and rotational mobility of the molecules. At all temperatures, there is an orientational preference for the water molecule pointing its protons down, which is in agreement with experiment. The polarisation of the surface increases with increasing temperature. For the transition layer, the calculated reorientational correlation times are in rough agreement with NMR experiments, but for T ⩾ 230 K the calculated self-diffusion coefficients are much higher than the values obtained in NMR experiments.

Interactions of La(III) with anions in aqueous solutions. A139La NMR study

The interactions of the La(III) cations with three anions (X), nitrate, chloride and perchlorate, in aqueous solutions in the pH range 4.0–6.5, were studied by139La NMR spectroscopy. A single model, involving the formation of the contact ion-pair (inner-sphere complex) (LaX)2+ was successfully and quantitatively applied to the chemical shift and the transverse relaxation rate data. Both measurements gave values for the thermodynamic equilibrium constants of formation of (LaX)2+ (Kth) in good agreement (average Kth=0.45±0.05; 0.15±0.09; 0.03±0.01, respectively for nitrate, chloride and perchlorate). The complexes are characterized by chemical shifts of −25, 22 and −3.1 ppm and by transverse relaxation rates of 11.2, 5 and 1.65 kHz respectively for nitrate, chloride and perchlorate. The139La quadrupolar relaxation rate is not controlled by the reorientational correlation time. This finding is discussed, and it is suggested that the very fast exchange of water molecules in the first coordination sphere of La(III) is responsible for the time fluctuation of the electric field gradient at the139La nucleus site.

A molecular dynamics simulation of sulphur hexafluoride

Molecular dynamics simulations are performed for the six-centre Lennard-Jones model of SF6 over a temperature range from 225 to 398 K and density from 1·3 to 1·9 g cm-3. The results of simulations are in good agreement with experimental data for the translational diffusion coefficient and correlation times for molecular reorientation and angular momentum, except for the highest temperature where small discrepancies between the simulated and experimental values for the diffusion coefficients are observed. Theoretical models for time correlation functions of molecular reorientations are compared with computer simulation results. It is found that the J-diffusion model reproduces the rotational correlation function of SF6 more satisfactorily than other models.

Molecular Dynamics of the Muonium-C60Radical in SolidC60

The molecular dynamics and electronic structure of the μ+-C60 radical in crystalline C60 have been studied using muon spin rotation and relaxation. At room temperature μ+-C60 appears to be in a state of quasifree rotation. At the critical temperature TS=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on C60, suggesting that the molecular dynamics of μ+-C60 are strongly coupled to those of its C60 neighbors.

C 60 Rotation in the Solid State: Dynamics of a Faceted Spherical Top

The rotational dynamics of C(60) in the solid state have been investigated with carbon-13 nuclear magnetic resonance ((13)C NMR). The relaxation rate due to chemical shift anisotropy (1/9T1(CSA)(1)) was precisely measured from the magnetic field dependence of T(1), allowing the molecular reorientational correlation time, tau, to be determined. At 283 kelvin, tau = 9.1 picoseconds; with the assumption of diffusional reorientation this implies a rotational diffusion constant D = 1.8 x 10(10) per second. This reorientation time is only three times as long as the calculated tau for free rotation and is shorter than the value measured for C(60) in solution (15.5 picoseconds). Below 260 kelvin a second phase with a much longer reorientation time was observed, consistent with recent reports of an orientational phase transition in solid C(60). In both phases tau showed Arrhenius behavior, with apparent activation energies of 1.4 and 4.2 kilocalories per mole for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The results parallel those found for adamantane.

Molecular dynamics ofμ+-C60radical in solidC60

The molecular dynamics and electronic structure of the ${\mathrm{\ensuremath{\mu}}}^{+}$-${\mathrm{C}}_{60}$ radical in crystalline ${\mathrm{C}}_{60}$ have been studied using muon spin rotation and relaxation. At room temperature ${\mathrm{\ensuremath{\mu}}}^{+}$-${\mathrm{C}}_{60}$ appears to be in a state of quasifree rotation. At the critical temperature ${\mathit{T}}_{\mathit{S}}$=260 K the local electronic structure and molecular dynamics change discontinuously as expected for a first-order phase transition. The correlation times for reorientation are remarkably close to those determined by recent NMR experiments on ${\mathrm{C}}_{60}$, suggesting that the molecular dynamics of ${\mathrm{\ensuremath{\mu}}}^{+}$-${\mathrm{C}}_{60}$ are strongly coupled to those of its ${\mathrm{C}}_{60}$ neighbors.

Comparison of the lipid acyl chain dynamics between small and large unilamellar vesicles

13C NMR spin-lattice relaxation (T1) rates and 13C-1H nuclear Overhauser effects (NOEs) were measured in an identical fashion in two lipid preparations having dramatically different curvatures. The T1 times that were obtained at four magnetic field strengths were fit along with the NOEs to simple models for lipid molecular dynamics. The results indicate that phospholipid chain ordering and dynamics are virtually identical in small and large unilamellar vesicles at the time scales sampled by these 13C-NMR studies. The order parameters and reorientational correlation times that characterize the amplitudes and rates of internal acyl chain motions were equal within experimental error for the methylene segments in the middle of the chains. The only significant differences in order parameters and correlation times between the two vesicle types were small and appeared at the ends of the acyl chains. At the carbonyl end the order was slightly higher in small vesicles than large vesicles, and at the methyl end the order was slightly lower for small vesicles. This indicates that in the more planar systems the acyl chains exhibit a slightly flatter order profile than in more highly curved membranes. The use of the same experimental approach in both small and large vesicle systems provided a more reliable and accurate assessment of the effect of curvature on molecular order than has been previously obtained.

Microviscosity of human erythrocytes studied using hypophosphite two-spin order relaxation

A new 31P NMR method is used to probe the cytoplasmic viscosity of human erythrocytes. The method is based on observing two-spin order relaxation of the 31P atom of the hypophosphite ion. This method is superior to our previous method, using the longitudinal relaxation time of the ion, because random field effects such as intermolecular dipole-dipole relaxation can be separated from intramolecular relaxation. This allows a more accurate determination of the effective reorientational correlation time from the measured intramolecular relaxation because it is now unaffected by random field effects. The new method also provides a means by which to estimate the random field effects. Both two-spin order and proton-decoupled T1 measurements were conducted on hypophosphite in water solutions at various temperatures, glycerol solutions of various viscosities, and in erythrocyte samples of various cell volumes. The results show that the effective reorientational correlation time of the hypophosphite ion varies from 7.2 to 15.2 ps in the cytoplasm of cells ranging in volume from 102 to 56 fl cells.

Theory of nuclear spin relaxation in heterogeneous media and application to the cross correlation between quadrupolar and dipolar fluctuations of deuterons in clay gels

The fictitious-spin- 1 2 operator formalism allows a comprehensive study of the longitudinal and transverse nuclear relaxations of a nuclear spin I = 1 in heterogeneous media, in the presence of a residual time-averaged quadrupolar interaction. An extension of this formalism makes possible the calculation of the cross correlation between the quadrupolar and dipolar (paramagnetic) fluctuations. Although this cross correlation is second order relative to the quadrupolar relaxation, its long range of effectiveness ( ∞ 1 r 3 ) is sufficient to provide the sole asymmetric component to the fine broadening of the deuterium doublet. This differential line broadening depends linearly on the applied magnetic induction field and on the paramagnetic spin concentration, as checked experimentally from a lineshape analysis of the 2H doublet observed for D 20 in clay gels. The reorientational correlation time of the adsorbed water at the clay interface is obtained from the measured 2H spin-relaxation rates.

Nuclear spin relaxation and dynamical behaviour of short-chain surfactants in micellar and liquid-crystalline aggregates

The dynamical behaviour and conformations of the octylphosphate anion in micellar solution and liquid-crystalline hexagonal (H1) and lamellar (Lα) phases formed from pyridinium octylphosphate (OCTP) have been investigated by 13C relaxation at several frequencies. In the HI and Lα phases the conformer populations, the rate of conformational changes and the reorientation correlation times were derived from the fits of the 13C–1H dipolar splittings and of the longitudinal relaxation rates. In isotropic micellar solution the same information was only available from the frequency dependence of these rates. The experimental data were interpreted by means of a rotational jump model corresponding to trans⇌gauche isomerizations about one bond at a time. In these phases, the aggregation results in a strong enhancement of the probabilities of the trans rotamer, generally found in the 0.8–0.95 range. The surfactant therefore behaves as a quite rigid entity undergoing a very anisotropic and quasi-axial reorientation, a behaviour expected from the computation of the inertial tensor averaged over all accessible conformers. Similar conclusions have been drawn from the deuteron splittings and longitudinal relaxation rates of an octanoate probe diluted in the OCTP–H2O mesophases. The deuteron longitudinal relaxation of this probe being the same at 13.4 and 46 MHz, the contribution of collective motions (ODF) appears to be negligible under our experimental conditions.

Molecular dynamics and NMR study of methane-water systems

Molecular dynamics of methane-water systems have been studied through computer simulations (MD), nuclear spin relaxation and self diffusion methods (NMR). MD simulation results give support to the concept of hydrophobic hydration by enhancing the water structure around the methane to create a clathrate. Spontaneous contacts between methanes is observed only at high densities, while so-called solvent-separated configurations are present in all simulations. The dynamics of the solvent water molecules is slowed down. In particular, the reorientational motion is sensitive to the presence of methane in solution. The simulated translational diffusion of methanes as solutes is also slowed down, but is higher than observed experimentally. The rotational motion of methanes is very sensitive to variation of the density while, the translational motion of methane is more constant and follows that of water. Methane acts nearly as a free rotor. The simulated reorientational correlation times for methane in water soluti...

Molecular dynamics simulation of methyl group relaxation in water

Recent theoretical studies of stochastic classical and quantum dynamics suggest that quantum effects may be significant in the nuclear magnetic resonance (NMR) relaxation of a methyl group attached to intermediate and large sized molecules at normal temperatures. The magnitude of the effects depends on the reorientational correlation time of the molecule, τ0, the barrier to internal rotation, V, and the correlation time for the collision induced randomization of the internal angular momentum, τω. This note reports the results of molecular dynamics simulations used to estimate τω for a methyl group in water. The simulations indicate that extended diffusion, rather than Fokker–Planck–Langevin, dynamics are appropriate for this system. Using τω determined in the simulation and V=13 KJ/mol (3 Kcal/mol), predictions of the classical and quantum models differ by 40% for the 13C relaxation of a methyl on an intermediate sized molecule, such as a disaccharide. In larger molecules, the differences are smaller due to smaller contributions from the internal dynamics to the NMR relaxation process, unless τω and/or V are larger. Experimental evidence suggests that τω is an order of magnitude or more larger in a hydrocarbon environment than in water. With the larger τω, a plausible example is given where the models’ predictions differ by 80%. More work is needed, however, to better define τω in hydrophobic regions of macromolecules.

Determination of dynamic parameters in amino acids from 17O NMR line width measurements

AbstractThe viscosity and temperature dependence of the 17O NMR line width of glycine, alanine, proline, leucine, histidine and phenylalanine were investigated at pH 2, 7 and 12.5. The observed viscosity/temperature (η/T) dependence of the reorientation correlation time, τc, is compared with that given by hydrodynamic models. It was found that for amino acids the Gierer–Wirtz model is superior to the Stokes–Einstein–Debye and Hu–Zwanzig models. The 17O quadrupole coupling constant for glycine, alanine and histidine was determined in water solution; the activation energies for molecular tumbling were evaluated, and the hydration of the amino acids in their cationic zwitterionic and anionic ionization states is discussed.

Time-resolved coherent Stokes Raman spectroscopy (CSRS) of benzene

Femtosecond time-resolved coherent Stokes Raman spectroscopy (CSRS) signals of benzene- h 6, benzene- d 6, and their 1 : 1 binary mixture are investigated at room temperature with two different polarization geometries. The reorientational correlation time of benzene as well as the vibrational dephasing time of the v 1 ring-breathing mode have been measured precisely. Bohr frequency difference beats have been resolved between the v 1 modes of 12C 6H 6 and 12C 6H 6 and 12C 5 13CH 6 protonated benzenes and the 12C 6D 6 and 12C 5 13CD 6 deuterated benzenes that are present at natural abundances in their neat liquids. There is no evidence for stochastic line broadening phenomena other than in their fast modulation limit even on the 100 fs time scale of these experiments.

NMR longitudinal relaxation study of the fluidity at octadecylsilica surfaces: acetonitrile as a probe of surface viscosity

Deuterium and nitrogen-14 longitudinal (spin-lattice) relaxation measurements are used to assess the surface fluidity under conditions typical of reversed-phase high-performance liquid chromatography. A model of two phases in rapid exchange and values of the observed 2 H and 14 N spin-lattice relaxation time (T 1 ) in suspensions of microporous silica gel (bare and C-18 modified) and acetonitrile-d 3 (CD 3 CN) are used to assess the rotational motion of surface-associated CD 3 CN. With use of bulk theories of relaxation and predicted values of the longitudinal relaxation rate of surface-bound nuclei (R 1,s =1/T 1,s ), the reorientational correlation time, τ c , of surface-associated CD 3 CN is predicted and used to determine a surface microviscosity

Isotope Effects on the Reorientational Motion of Hydrated H2O Molecules in Alkali Metal Bromide Dilute Aqueous Solutions Studied by NMR Spectroscopy

Abstract The spin-lattice relaxation times of 1H and 17O nuclei of hydrated H2O molecules in alkali metal bromide (LiBr, NaBr, KBr, and CsBr) dilute aqueous solutions in the concentration range from 0.2 to 1.0 mol kg−1 at 25 °C were measured by NMR spectroscopy. The reorientational correlation times for the intramolecular 1H–1H (τHH+) and 1H–17O (τOH+) axes, as well as the perpendicular axis (17O nuclei, τ17O+) to the molecular plane of a hydrated H2O molecule were determied. The two reorientational correlation times (τHH+ and τOH+) of pure H2O and hydrated H2O molecules in Li+, Na+, K+, and Cs+ ions were equal, respectively. Comparing previous data for hydrated D2O molecules with that calculated by using correction values (rHH, rOH, and quadrupolar constant) for H2O molecules, the ratios of τD+(D2O)/τOH(H2O) and τ17O+(D2O)/τ17O+(H2O) were found to be of the same order as η(D2O)/η(H2O), except for the τ17O+(D2O)/τ17O+(H2O) value for the Li+ ion.