E. Elliott Burnell

EFFECTS OF TUMBLING AND LATERAL DIFFUSION ON PHOSPHATIDYLCHOLINE MODEL MEMBRANE 31P-NMR LINESHAPES

Two factors determining the isotropic motional averaging of NMR spectra obtained from lipids in model and biological membranes systems are particle tumbling and lateral diffusion. The influence of these motions (of which the magnitudes are detemined by the medium viscosity and temperature) on the 31P-NMR spectra arising from unilamellar dioleoyl phosphatidylcholine vesicles of a defined size are examined. It is shown that the lineshapes obtained are in good agreement with those predicted by the theory of motional narrowing. These results are discussed with regard to order parameter determinations and polymorphic phase identifications as obtained by NMR techniques.

NMR of ordered liquids

I Basics.- 1 Basics of NMR of molecules in uniaxial anisotropic environments.- 2 Density matrix methods in NMR.- 3 Coherent averaging and correlation of anisotropic spin interactions in oriented molecules.- 4 Multiple Quantum NMR Spectroscopy in Orientationally Ordered Fluids.- 5 Spectral Analysis of Orientationally Ordered Molecules.- II NMR of solute atoms and molecules.- 6 NMR of Noble Gases Dissolved in Liquid Crystals.- 7 NMR of partially ordered solutes with emphasis on structure determination.- 8 Observation and interpretation of residual dipolar couplings in biomolecules.- 9 The search for high-resolution NMR methods for membrane peptide structure.- III Theory, models, and simulations.- 10 Solutes as probes of simplified models of orientational order.- 11 Molecular Models of Orientational Order.- 12 Molecular theory of orientational order.- 13 Very Flexible Solutes: Alkyl Chains and Derivatives.- 14 NMR Studies of Solutes in Liquid Crystals: Small Flexible Molecules.- 15 Simulations of Orientational Order of Solutes in Liquid Crystals.- IV Dynamic aspects and relaxation.- 16 Spin relaxation in orientationally ordered molecules.- 17 Low-frequency NMR relaxometry of spatially constrained liquid crystals.- 18 NMR on macroscopically oriented lyotropic systems.- 19 Dynamic NMR in liquid crystals and liquid crystalline solutions.

Molecular solutes in nematic liquid crystals: Orientational order and electric field gradients

Abstract The difference between liquid-crystal and gas-phase values for the nuclear quadrupole coupling constant in D 2 and HD is used to obtain the mean electric field gradient in various liquid crystals. Order parameters for small molecules dissolved in liquid crystals are calculated assuming that the orientational order arises from the interaction of the molecular quadrupole moment with the average field gradient. The results obtained are in good agreement with experimental values for hydrogen and several other solutes.

Size and shape effects on the orientation of rigid molecules in nematic liquid crystals

Order parameters, as measured from N.M.R. experiments, are reported for a series of small rigid molecules dissolved in the liquid crystals N-(p-ethoxybenzylidene)-p′-n-butylaniline, Merck ZLI 1132,...

Nuclear magnetic resonance line shapes in lyotropic liquid crystals and related systems

The shapes of NMR absorption lines, free induction decays, and Jeener echoes in systems such as lyotropic liquid crystals have been investigated theoretically and experimentally. For a system having an axis of symmetry for the molecular motion which makes an angle ϑ with the external magnetic field, the secular part of the dipolar Hamiltonian 〈Hd〉 (ϑ) is proportional to (1/2)(3 cos2ϑ−1). This leads to the characteristic logarithmic absorption line shape for nonoriented systems. The experimental consequences are illustrated with measurements on nonoriented samples of potassium palmitate–water in the lamellar liquid crystal phase.

Effects of interaction between molecular internal motion and reorientation on NMR of anisotropic liquids

Abstract NMR spectra and order parameters of nonrigid partially oriented molecules are expected to show effects due to molecular reorientation following a configurational change. The order parameters are functions of configuration lifetime τv and reorientation rate k. The process is best described in an axis system in which internal motion and molecular reorientation are decoupled. The theory is developed assuming that the configurational change is a stationary Markov process. For slow change, a separate order matrix is required for each configuration; for rapid interconversion, the order parameters for the above axis system are the average of the order parameters for all configurations weighted by the product kτv. Effects of small-amplitude vibrations and large-amplitude conformational changes are discussed. NMR splittings observed in tetrahedral molecules are explained without having to invoke distortion by the liquid-crystal solvent.

NMR spectra of oriented 13C-Acetylene: Redetermination of the shrinkage effects and analysis of the apparent temperature dependence of the molecular geometry

Abstract The NMR spectra of the 13C isomers of acetylene oriented in two nematic liquid crystals (EBBA and Phase IV) have been analyzed in order to determine the distance ratios and the shrinkage effects. In the case of the EBBA samples, the results agree with the IR data except for a small discrepancy in the distance ratios. In the Phase IV samples, we have found an unusually strong temperature dependence of the direct coupling constants and of the corresponding apparent distance ratios. In this case the discrepancies between NMR and IR are as large as 30%. We interpret this phenomenon on the basis of a two-site theory. The following assumptions have to be made in order to explain the apparent anomalies: (i) the enthalpy difference between the two sites is 200–400 cal/mole, (ii) the orientation parameters at the two sites have opposite signs, and (iii) at the site with the lower energy (complex?) the ratio r CH r CC is smaller by 0.5-1%. It is interesting that such modest conditions can lead, if neglected, to a 30% error in the distance ratios. With regard to this fact, it is probable that similar effects are not uncommon.

NMR of deuterium in liquid crystal mixtures

Deuterium NMR spectra of D 2 dissolved in mixtures of the nematic liquid crystals N-( p -ethoxybenzylidene)- p ′- n -butyl-aniline and Merck ZLI 1132 are discussed in terms of a theory based on the interaction of the molecular quadrupole moment with the average electric field gradient. The average field gradient depends upon the liquid crystal composition and hence can be adjusted to various values (including zero) between those of the pure liquid crystals. It is shown that for deuterium dissolved in these phases the molecular quadrupole—average field gradient mechanism can account for most but not all of the orientational ordering.

On the average orientation of molecules undergoing large-amplitude conformational changes in anisotropic liquids

Abstract Description of the average orientation of non-rigid molecules in anisotropic fluids is non-trivial. The assumption that molecular orientation is independent of conformation is discussed. For such a case the Euler angles relating each conformation to the common order-tensor axes must still be known. These common axesare normally not fixed to any rigid segment of the molecule.

Anisotropic short range potentials for solutes in nematic liquid crystals

Recently, it has been suggested that the orientational order of solutes dissolved in nematic liquid crystals results from short range forces which depend on the size and shape of the solute molecule and from the interaction between the molecular quadrupole moment and the average electric field gradient experienced by the molecule. In this paper, we model the size and shape-dependent short range potential by assuming that the contribution to the potential from each surface element of a cavity representing the molecule depends on the orientation of the surface element with respect to the director. We have tried a variety of potentials based on this assumption, and we report here the ones which yielded the best agreement between theory and experiment. One of the potentials is obtained by integrating, along the director, the circumference of a slice cut through the molecule perpendicular to the director; better fits are obtained by adding a term which depends on a Hookes law elastic distortion of the liquid ...