Regitze R. Vold

Multiple quantum spin-echo spectroscopy

The generation of nuclear magnetic double quantum echoes and their detection by 2-dimensional Fourier transform spectroscopy is reported for partially aligned 1-deuteron systems in a nematic liq. crystal. Phase-cycling techniques which suppress detection of unwanted coherences were developed, and the transverse decay rates of double quantum coherences agree quant. with relaxation results obtained by more conventional methods. [on SciFinder (R)]

Isotropic solutions of phospholipid bicelles: A new membrane mimetic for high-resolution NMR studies of polypeptides

In order to illustrate the utility of phospholipid bicelles [Sanders, C.R. and Schwonek, J.P.(1992) Biochemistry, 31, 8898–8905] as a membrane mimetic for high-resolution NMRstudies, we have recorded two-dimensional 1H NMR spectra of the tetradecameric peptidemastoparan Vespula lewisii in an isotropic aqueous solution of dimyristoyl and dihexanoylphosphatidylcholine. Mastoparan is largely unstructured in water, but assumes a well-definedhelical conformation in association with the bilayers. A pronounced periodicity of thesequential NH chemical shifts provides strong evidence that the helix axis of this shortpeptide is parallel, rather than perpendicular, to the bilayer plane. The bicellar solutions stillrequire in-depth morphological characterization, but they appear to be ideal media for NMRdetermination of the mode of binding and the structure of membrane-associated peptides andproteins.

Structural Evaluation of Phospholipid Bicelles for Solution-State Studies of Membrane-Associated Biomolecules

Several complementary physical techniques have been used to characterize the aggregate structures formed in solutions containing dimyristoylphosphatidylcholine (DMPC)/dihexanoylphosphatidylcholine (DHPC) at ratios of < or =0.5 and to establish their morphology and lipid organization as that of bicelles. (31)P NMR studies showed that the DMPC and DHPC components were highly segregated over a wide range of DMPC/DHPC ratios (q = 0.05-0.5) and temperatures (15 degrees C and 37 degrees C). Only at phospholipid concentrations below 130 mM did the bicelles appear to undergo a change in morphology. These results were corroborated by fluorescence data, which demonstrated the inverse dependence of bicelle size on phospholipid concentration as well as a distinctive change in phospholipid arrangement at low concentrations. In addition, dynamic light scattering and electron microscopy studies supported the hypothesis that the bicellar phospholipid aggregates are disk-shaped. The radius of the planar domain of the disk was found to be directly proportional to the ratio of DMPC/DHPC and inversely proportional to the total phospholipid concentration when the DMPC/DHPC ratio was held constant at 0.5. Taken together, these results suggest that bicelles with low q retain the morphology and bilayer organization typical of their liquid-crystalline counterparts, making them useful membrane mimetics.

Deuterium quadrupole-echo NMR spectroscopy. III. Practical aspects of lineshape calculations for multiaxis rotational processes

A multisite exchange formalism is developed for describing effects of molecular motion on quadrupole-echo spectra. Convenient multiple-frame transformation procedures are developed for constructing the matrix of site frequencies and rates, which properly describe noncommuting rotations about several noncoincident axes. The formalism is illustrated by lineshapes calculated for ortho or meta deuterons on a benzene ring which undergoes 180° flips about the p, p′ axis in the presence of small-angle librational motion. Several different types of motion, fitting this description, are considered and found to yield very different lineshapes. In particular, the difference between a “wobbling” p, p′ axis and a “wobbling” CD bond is illustrated. If reliable comparisons are to be made between experimental spectra and simulated lineshapes, the simulation must include lineshape distortions due to pulses of finite amplitude as well as exchange during pulses of finite width. A computer program is described which includes simulations of both these artifacts as well as very general procedures for including multiple-exchange processes. Timing and accuracy tests are described for typical calculations performed on machines ranging from a personal workstation to a supercomputer.

Magnetically Aligned Phospholipid Bilayers with Positive Ordering: A New Model Membrane System

A stable smectic phospholipid bilayer phase aligned with the director parallel to the magnetic field can be generated by the addition of certain trivalent paramagnetic lanthanide ions to a bicellar solution of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC) in water. Suitable lanthanide ions are those with positive anisotropy of their magnetic susceptibility, namely Eu3+, Er3+, Tm3+, and Yb3+. For samples doped with Tm3+, this phase extends over a wide range of Tm3+ concentrations (6-40 mM) and temperatures (35-90 degrees C) and appears to undergo a transition from a fluid nematic discotic to a fluid, but highly ordered, smectic phase at a temperature that depends on the thulium concentration. As a membrane mimetic, these new, positively ordered phospholipid phases have high potential for structural studies using a variety of techniques such as magnetic resonance (EMR and NMR), small-angle x-ray and neutron diffraction, as well as optical and infrared spectroscopy.

Errors in measurements of transverse relaxation rates

Abstract Errors which may occur in the measurement of transverse relaxation rates in liquids by wideband multiple pulse techniques are described. Numerical analysis of Bloch equations is used to evaluate the effects of imperfect pulses, and results of pulse imperfections are demonstrated experimentally. Effects of field-frequency instability and of diffusion combined with external field gradients are discussed. Methods for avoiding several of these errors are described and applied to the measurement of T2 for degassed benzene. It is experimentally demonstrated that if certain criteria are met, measurements of T2 with accuracy of ±1–2% are feasible.

Bicelles in structure-function studies of membrane-associated proteins.

Bicelles are a novel form of long-chain/short-chain phospholipid aggregates, which are useful for biophysical and biochemical studies of membrane-associated biomolecules. In this work, we review the development of bicelles and their uses in structural characterization (primarily via NMR, circular dichroism, and fluorescence) of membrane-associated peptides. We also show that bicellar phospholipids are substrates for lipolytic enzymes. For this latter work, we employed a 31P NMR enzymatic assay system to examine the kinetic behavior of cobra venom phospholipase A(2) toward a variety of bicellar substrates. This enzyme hydrolyzed all bicelle lipids at rates comparable to those found for the enzyme action on traditional micellar substrates, which are the best substrates for this enzyme. In addition, we found that this PLA(2) showed no significant preference for long-chain or short-chain phospholipids when they were presented as mixtures in bicelles.

Nuclear spin relaxation and molecular dynamics in ordered systems: Models for molecular reorientation in thermotropic liquid crystals

New models are developed to account for rotational motion in liquid crystals. A distinction is made between rotation about a molecule fixed z axis, described by Eulerian angle γ, and about a space fixed z axis, described by Eulerian angle α. Our model allows γ motion to proceed by jumps of arbitrary angular size, while motion about the space fixed axes (α,β motion) is described in terms of small step rotational diffusion in presence of a restoring pseudopotential. Calculations are presented for different forms of the restoring potential including (1−cosn β) for n=2, 4, and 10 where β is the angle between the molecule fixed and space fixed z axes, as well as angular square well potentials with finite and infinite walls. Multiexponential correction functions for α,β motion in the infinite square well (‘‘diffusion‐in‐a‐cone’’) potential as well as the Maier–Saupe potential (cos2 β) are listed in tabular form as a function of the second rank order parameter Szz =〈 P2(cos β)〉. It is shown that spectral density...

Acidic phospholipid bicelles: a versatile model membrane system.

With the aim of establishing acidic bicellar solutions as a useful membrane model system, we have used deuterium NMR spectroscopy to investigate the properties of dimyristoyl/dihexanoylphosphatidylcholine (DMPC/DHPC) bicelles containing 25% (w/w in H(2)O) of either dimyristoylphosphatidylserine (DMPS) or dimyristoylphosphatidylglycerol (DMPG). The addition of the acidic lipid component to this lyotropic liquid crystalline system reduces its range of stability because of poor miscibility of the two dimyristoylated phospholipids. Compared to the neutral bicelles, which are stable at pH 4 to pH 7, acidic bicelles are stable only from pH 5.5 to pH 7. Solid-state deuterium NMR analysis of d(54)-DMPC showed similar ordering in neutral and acidic bicelles. Fully deuterated DMPS or DMPG is ordered in a way similar to that of DMPC. Study of the binding of the myristoylated N-terminal 14-residue peptide mu-GSSKSKPKDPSQRR from pp60(nu-src) to both neutral and acidic bicelles shows the utility of these novel membrane mimetics.

Application of the Jeener-Broekaert pulse sequence to molecular dynamics studies in liquid crystals

Abstract The Jeener-Broekaert pulse sequence is considered as an alternative to selective and nonselective pulse experiments for relaxation measurements in partially ordered spin systems. It is demonstrated that measurements of the decay of the sum and difference magnetizations provide a convenient method for the simultaneous determination of J1(ω0) and J2(2ω0).