Manipulation of the director in bicellar mesophases by sample spinning: a new tool for NMR spectroscopy

Spatial reorientation experiments for NMR of solids and partially oriented liquids

Motional reorientation experiments are extensions of Magic Angle Spinning (MAS) where the rotor axis is changed in order to average out, reintroduce, or scale anisotropic interactions (e.g. dipolar couplings, quadrupolar interactions or chemical shift anisotropies). This review focuses on Variable Angle Spinning (VAS), Switched Angle Spinning (SAS), and Dynamic Angle Spinning (DAS), all of which involve spinning at two or more different angles sequentially, either in successive experiments or during a multidimensional experiment. In all of these experiments, anisotropic terms in the Hamiltonian are scaled by changing the orientation of the spinning sample relative to the static magnetic field. These experiments vary in experimental complexity and instrumentation requirements. In VAS, many one-dimensional spectra are collected as a function of spinning angle. In SAS, dipolar couplings and/or chemical shift anisotropies are reintroduced by switching the sample between two different angles, often 0° or 90° and the magic angle, yielding a two-dimensional isotropic-anisotropic correlation spectrum. Dynamic Angle Spinning (DAS) is a related experiment that is used to simultaneously average out the first- and second-order quadrupolar interactions, which cannot be accomplished by spinning at any unique rotor angle in physical space. Although motional reorientation experiments generally require specialized instrumentation and data analysis schemes, some are accessible with only minor modification of standard MAS probes. In this review, the mechanics of each type of experiment are described, with representative examples. Current and historical probe and coil designs are discussed from the standpoint of how each one accomplishes the particular objectives of the experiment(s) it was designed to perform. Finally, applications to inorganic materials and liquid crystals, which present very different experimental challenges, are discussed. The review concludes with perspectives on how motional reorientation experiments can be applied to current problems in chemistry, molecular biology, and materials science, given the many advances in high-field NMR magnets, fast spinning, and sample preparation realized in recent years.

Modulation of cross polarization in motionally averaged solids by Variable Angle Spinning NMR

In systems where the dipolar couplings are partially averaged by molecular motion, cross-polarization is modulated by sample spinning. The cross-polariation efficiency in Variable Angle Spinning (VAS) and Switched Angle Spinning (SAS) experiments on mobile samples is therefore strongly dependent on the spinning angle. We describe simulations and experimental measurements of these effects over a range of spinning angles from 0° to 90°.

Pneumatic switched angle spinning NMR probe with capacitively coupled double saddle coil

Switched angle spinning (SAS) experiments can be used for generating isotropic-anisotropic correlations in oriented samples in a single experiment. In order for these methods to become widespread, specialized hardware is required. Here we describe the electronic and mechanical design and performance of a double-resonance SAS probe. Unlike many previous SAS probe implementations, the focus here is on systems where the dipolar couplings are partially averaged by molecular motion. This probe has a moving double saddle coil capacitively coupled to the stationary circuit. Angle switching is accomplished by a steam engine-type pneumatic mechanism. The speed and stability of the switching hardware for SAS experiments are demonstrated using spectra of model compounds.

Controlled alignment of lamellar lyotropic mesophases by rotation in a magnetic field

We demonstrate a versatile approach to align lamellar lyotropic mesophases with the use of magnetic fields. It is based on continuous rotation of the sample on an axis perpendicular to the magnetic field direction during a single cooling ramp across the order-disorder transition of the system. The process yields materials with near-perfect, nondegenerate alignment of lamellar stacks along the axis of rotation. We use a model tetraethylene glycol dodecyl ether-water system to investigate the influence of magnetic field strength, cooling rate and the speed of sample rotation on the degree of alignment as quantitatively determined by small-angle X-ray scattering. This approach offers broad utility for the alignment of other soft mesophases relevant in several emerging applications.

Bicelles: A natural 'molecular goniometer' for structural, dynamical and topological studies of molecules in membranes

Major biological processes occur at the biological membrane. One of the great challenges is to understand the function of chemical or biological molecules inside the membrane; as well of those involved in membrane trafficking. This requires obtaining a complete picture of the in situ structure and dynamics as well as the topology and orientation of these molecules in the membrane lipid bilayer. These led to the creation of several innovative models of biological membranes in order to investigate the structure and dynamics of amphiphilic molecules, as well as integral membrane proteins having single or multiple transmembrane segments. Because the determination of the structure, dynamics and topology of molecules in membranes requires a macroscopic alignment of the system, a new membrane model called 'bicelles' that represents a crossover between lipid vesicles and classical micelles has become very popular due to its property of spontaneous self-orientation in magnetic fields. In addition, crucial factors involved in mimicking natural membranes, such as sample hydration, pH and salinity limits, are easy to control in bicelle systems. Bicelles are composed of mixtures of long chain (14-18 carbons) and short chain phospholipids (6-8 carbons) hydrated up to 98% with buffers and may adopt various morphologies depending on lipid composition, temperature and hydration. We have been developing bicelle systems under the form of nano-discs made of lipids with saturated or biphenyl-containing fatty acyl chains. Depending on the lipid nature, these membranous nano-discs may be macroscopically oriented with their normal perpendicular or parallel to the magnetic field, providing a natural 'molecular goniometer' for structural and topological studies, especially in the field of NMR. Bicelles can also be spun at the magic angle and lead to the 3D structural determination of molecules in membranes.

REDOR recoupling in polymer-stabilized liquid crystals undergoing MAS — Two-dimensional NMR applications with strongly aligned proteins

We recently introduced a technique, termed SAD-REDOR, for recoupling residual dipolar couplings in a single-domain polymer-stabilized liquid crystal (PSLC) undergoing magic-angle spinning (MAS). The experiment was demonstrated with 1H–15N dipolar couplings in a small molecule. Here, we show the applicability of the technique to a strongly aligned protein and we describe a novel two-dimensional experiment to generate in-phase and anti-phase (IPAP) doublets in the 1H dimension of an HSQC NMR spectrum. This pulse sequence, SAD-IPAP, was tested on a sample of 15N-labeled ubiquitin (5 mmol/L) in a polyacrylamide-stabilized Pf1 phage liquid crystal (20 mg/mL). 15N–1H residual dipolar couplings (RDCs) were measured with the SAD-IPAP pulse sequence at spinning speeds of 1000 and 1250 Hz. RDSs were also measured using the conventional HSQC-IPAP method in a sample of 15N-ubiquitin dissolved in a solution of Pf1 phage (1 mg/mL). The resulting RDCs were fitted to the solution structure of ubiquitin to estimate the alignment tensor and to determine the accuracy of the measured couplings. The results highlight the benefits of SAD-REDOR for the measurement of RDCs in strongly aligned macromolecules.Key words: residual dipolar couplings (RDCs), polymer-stabilized liquid crystals (PSLCs), rotational echo double resonance (REDOR) recoupling, magic-angle spinning (MAS), ubiquitin, biomolecular nuclear magnetic resonance (NMR), proteins.

Biphenyl bicelle disks align perpendicular to magnetic fields on large temperature scales: a study combining synthesis, solid-state NMR, TEM, and SAXS

A phosphatidylcholine lipid (PC) containing a biphenyl group in one of its acyl chains (1-tetradecanoyl-2-(4-(4-biphenyl)butanoyl)-sn-glycero-3-PC, TBBPC) was successfully synthesized with high yield. Water mixtures of TBBPC with a short-chain C(6) lipid, dicaproyl-PC (DCPC), lead to bicelle systems formation. Freeze-fracture electron microscopy evidenced the presence of flat bilayered disks of 800 A diameter for adequate composition, hydration, and temperature conditions. Because of the presence of the biphenyl group, which confers to the molecule a positive magnetic anisotropy Delta chi, the disks align with their normal, n, parallel to the magnetic field B(0), as directly detected by (31)P, (14)N, (2)H solid-state NMR and also using small-angle x-ray scattering after annealing in the field. Temperature-composition and temperature-hydration diagrams were established. Domains where disks of TBBPC/DCPC align with their normal parallel to the field were compared to chain-saturated lipid bicelles made of DMPC(dimyristoylPC)/DCPC, which orient with their normal perpendicular to B(0). TBBPC/DCPC bicelles exist on a narrow range of long- versus short-chain lipid ratios (3%) but over a large temperature span around room temperature (10-75 degrees C), whereas DMPC/DCPC bicelles exhibit the reverse situation, i.e., large compositional range (22%) and narrow temperature span (25-45 degrees C). The two types of bicelles present orienting properties up to 95% dilution but with the peculiarity that water trapped in biphenyl bicelles exhibits ordering properties twice as large as those observed in the saturated-chains analog, which offers very interesting properties for structural studies on hydrophilic or hydrophobic embedded biomolecules.

Current Applications of Bicelles in NMR Studies of Membrane-Associated Amphiphiles and Proteins,

This review covers current trends in studies of membrane amphiphiles and membrane proteins using both fast tumbling bicelles and magnetically aligned bicelle media for both solution state and solid state NMR. The fast tumbling bicelles provide a versatile biologically mimetic membrane model, which in many cases is preferable to micelles, both because of the range of lipids and amphiphiles that may be combined and because radius of curvature effects and strain effects common with micelles may be avoided. Drug and small molecule binding and partitioning studies should benefit from their application in fast tumbling bicelles, tailored to mimic specific membranes. A wide range of topology and immersion depth studies have been shown to be effective in fast tumbling bicelles, while residual dipolar couplings add another dimension to structure refinement possibilities, particularly for situations in which the peptide is uniformly labeled with 15N and 13C. Solid state NMR studies of polytopic transmembrane proteins demonstrate that it is possible to express, purify, and reconstitute membrane proteins, ranging in size from single transmembrane domains to seven-transmembrane GPCRs, into bicelles. The line widths and quality of the resulting 15NH dipole-15N chemical shift spectra demonstrate that there are no insurmountable obstacles to the study of large membrane proteins in magnetically aligned media.

Applications of variable-angle sample spinning experiments to the measurement of scaled residual dipolar couplings and 15N CSA in soluble proteins

NMR spectra of ubiquitin in the presence of bicelles at a concentration of 32% w/v have been recorded at 700 MHz under sample spinning conditions at the magic angle (54.7 degrees ) and at an angle of 45.5 degrees . At the magic angle, the 1H-15N HSQC spectrum of ubiquitin in bicelles is virtually indistinguishable from the one recorded on the protein in solution. Spinning the sample at the magic angle creates an isotropic environment with no preferred bicelle orientations, thus allowing the determination of scalar coupling constants. For an angle of rotation of 45.5 degrees , the bicelles orient with their normal perpendicular to the spinning axis leading to the observation of strong residual dipolar couplings and chemical shift variations of the 15N resonances.

Lipid membrane-induced optimization for ligand-receptor docking: recent tools and insights for the "membrane catalysis" model

Cells in living organisms are regulated by chemical and physical stimuli from their environment. Often, ligands interact with membrane receptors to trigger responses and Sargent and Schwyzer conceived a model to describe this process, "membrane catalysis". There is a notion that the physical organization of membranes can control the response of cells by speeding up reactions. We revisit the "membrane catalysis" model in the light of recent technical, methodological and theoretical advances and how they can be exploited to highlight the details of membrane mediated ligand-receptor interactions. We examine the possible effects that ligand concentration causes in the membrane catalysis and focus our attention in techniques used to determine the partition constant. The hypothetical diffusional advantage associated with membrane catalysis is discussed and the applicability of existing models is assessed. The role of in-depth location and orientation of ligands is explored emphasizing the contribution of new analysis methods and spectroscopic techniques. Results suggest that membranes can optimize the interaction between ligands and receptors through several different effects but the relative contribution of each must be carefully investigated. We certainly hope that the conjugation of the methodological and technical advances here reported will revive the interest in the membrane catalysis model.

Adiabatic-passage cross polarization in N-15 NMR spectroscopy of peptides weakly associated to phospholipids: determination of large RDC

Structural information can be extracted from one-bond residual dipolar couplings (RDC) measured in NMR spectra of systems in field-ordered media. RDC can be on the order of J-couplings if the anisotropy of alignment is approximately 10(-2), 10-fold stronger than that typically used for structural studies of water-soluble proteins. In such systems the performance of (1)H--> (15)N polarization transfer methods of the INEPT type is not satisfactory. In this study we show the effectiveness of adiabatic-passage cross-polarization (APCP) in transferring the (1)H--> (15)N polarization in the bicelle-associated peptide Leucine Enkephalin (Lenk). APCP is efficient both in static samples and in samples spun at the magic angle (MAS) or any other angle of the spinning axis to the magnetic field (variable-angle spinning, VAS). The anisotropic spectrum of an aligned static sample and the isotropic spectrum of the sample under MAS provide a set of possible values for the (1)H-(15)N RDC of phospholipid-associated Lenk. The unambiguous determination of the (1)H-(15)N RDC was accomplished by means of VAS experiments.

Assignment of 13C resonances of a nematic liquid crystal using off-magic angle spinning

A novel method for assigning the resonances in the 13C NMR spectrum of a static liquid crystalline sample in its nematic phase is proposed. The method is based on the fact that the carbon chemical shifts in the isotropic phase and in the oriented phase under static and off-magic angle spinning (OMAS) conditions are uniquely related by the tensorial property of the CSA tensor, requiring just one OMAS spectrum and the assignment in the isotropic phase. A computational procedure is proposed to take into account deviations arising out of non-ideal experimental conditions and the assignments are made by identifying the minimum in the differences in the frequencies between calculated and experimental line positions. Practical implementation of the method has also been demonstrated in the case of the liquid crystal N-(4-ethoxybenzylidene)-4-n-butylaniline.

Recoupling of residual dipolar couplings in single-domain polymer-stabilized liquid crystals undergoing magic-angle spinning

Measurement of dipolar couplings, chemical shift anisotropies, and quadrupole couplings in oriented media such as liquid crystals are of great importance for extraction of structural parameters in biological macromolecules. Here, we introduce a new technique, SAD-REDOR, that consists of recoupling heteronuclear dipolar couplings in molecules dissolved in a single-domain liquid crystal or other oriented medium through the combined use of magic-angle spinning and rotor-synchronized radiofrequency pulses. This application of the REDOR pulse sequence to oriented media offers several advantages such as selectivity over the type of coupling recovered and tunable scaling of the interaction. The effectiveness of the technique is demonstrated both theoretically and experimentally, using the recently developed polyacrylamide-stabilized Pf1 phage medium and 15N-labeled benzamide as the aligned molecule.

Using switched angle spinning to simplify NMR spectra of strongly oriented samples

This contribution describes a method that manipulates the alignment director of a liquid crystalline sample to obtain anisotropic magnetic interaction parameters, such as dipolar coupling, in an oriented liquid crystalline sample. By changing the axis of rotation with respect to the applied magnetic field in a spinning liquid crystalline sample, the dipolar couplings present in a normally complex strong coupling spectrum are scaled to a simple weak coupling spectrum. This simplified weak coupling spectrum is then correlated with the isotropic chemical shift in a switched angle spinning (SAS) two-dimensional (2D) experiment. This dipolar-isotropic 2D correlation was also observed for the case where the couplings are scaled to a degree where the spectrum approaches strong coupling. The SAS 2D correlation of C(6)F(5)Cl in the nematic liquid crystal I52 was obtained by first evolving at an angle close to the magic angle (54.7 degrees ) and then directly detecting at the magic angle. The SAS method provides a 2D correlation where the weak coupling pairs are revealed as cross-peaks in the indirect dimension separated by the isotropic chemical shifts in the direct dimension. Additionally, by using a more complex SAS method which involves three changes of the spinning axis, the solidlike spinning sideband patterns were correlated with the isotropic chemical shifts in a 2D experiment. These techniques are expected to enhance the interpretation and assignment of anisotropic magnetic interactions including dipolar couplings for molecules dissolved in oriented liquid crystalline phases.

Measurement of 15N chemical shift anisotropy in a protein dissolved in a dilute liquid crystalline medium with the application of magic angle sample spinning

The chemical shifts of nuclei that have chemical shielding anisotropy, such as the 15N amide in a protein, show significant changes in their chemical shifts when the sample is altered from an isotropic state to an aligned state. Such orientation-dependent chemical shift changes provide information on the magnitudes and orientation of the chemical shielding tensors relative to the molecule's alignment frame. Because of the extremely high sensitivity of the chemical shifts to the sample conditions, the changes in chemical shifts induced by adding aligned bicelles do not arise only from the protein alignment but should also include the accumulated effects of environmental changes including protein-bicelle interactions. With the aim of determining accurate 15N chemical shielding tensor values for solution proteins, here we have used magic angle sample spinning (MAS) to observe discriminately the orientation-dependent changes in the 15N chemical shift. The application of MAS to an aligned bicelle solution removes the torque that aligns the bicelles against the magnetic field. Thus, the application of MAS to a protein in a bicelle solution eliminates only the molecular alignment effect, while keeping all other sample conditions the same. The observed chemical shift differences between experiments with and without MAS therefore provide accurate values of the orientation-dependent 15N chemical shifts. From the values for ubiquitin in a 7.5% (w/v) bicelle medium, we determined the 15N chemical shielding anisotropy (CSA) tensor. For this evaluation, we considered uncertainties in measuring the 1H-15N dipolar couplings and the 15N chemical shifts and also structural noise present in the reference X-ray structure, assuming a random distribution of each NH bond vector in a cone with 5 degrees deviation from the original orientation. Taking into account these types of noise, we determined the average 15N CSA tensor for the residues in ubiquitin as Delta sigma=-162.0+/-4.3 ppm, eta=0.18+/-0.02, and beta=18.6+/-0.5 degrees, assuming a 1H-15N bond length of 1.02 A. These tensor values are consistent with those obtained from solid-state NMR experiments.

Capture and manipulation of magnetically aligned Pf1 with an aqueous polymer gel

The magnetic alignment of the Pseudomonas bacteriophage Pf1 is captured indefinitely in a gel of the aqueous triblock copolymer Pluronic F-127. In addition to preserving high-resolution liquids NMR spectra for dissolved solutes, the gel prevents the reorientation of the phage allowing mechanical manipulation of the angle between the axis of the phage alignment and the static magnetic field. The residual 2H quadrupolar couplings for several solutes dissolved in this material as a function of the angle Theta between the non-spinning sample tube and the static magnetic field are consistent with the value of P(2)(cosTheta)=(3cos(2)Theta-1)/2. The variable-angle correlation spectrum for these solutes is shown to separate residual quadrupolar effects from isotropic chemical shifts. Finally, the compatibility of Pluronic F-127 with NMR studies of aqueous biological macromolecules is demonstrated in a measurement of residual dipolar couplings in an 15N-labeled nucleic acid.

Phospholipid Bicelles That Align with Their Normals Parallel to the Magnetic Field

We have recently reported phospholipid bicelles (bilayered micelles) that have positive anisotropy of the magnetic susceptibility and align with their normals parallel to an external magnetic field [J. Am. Chem. Soc. 2001, 123, 1537]. Improvements have been made via the synthesis of a new phospholipid, 1-dodecanoyl-2-(4-(4-biphenyl)butanoyl)-sn-glycero-3-phosphocholine (DBBPC). Bicelles can be formed by mixing DBBPC with a short-chain phospholipid, 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) in a ratio between 5.1:1 and 6.5:1 in an aqueous medium. The (31)P NMR spectra clearly show that these bicelles align with their principal axes parallel to the magnetic field within a wide temperature range. The (31)P chemical shifts indicate that the conformation of the polar headgroup in these bicelles may be different from that in common bicelles. The phase behavior of a mixture of DBBPC/DHPC with 6:1 mole ratio was investigated in the temperature range of 10-75 degrees C using (31)P, (2)H, and (23)Na NMR. At lower temperatures (10-54 degrees C), the system is dominated by the bicellar phase. At higher temperatures (54-75 degrees C), isotropic micelles are formed and coexist with the bicelles. The partial alignment of maltotriose in the DBBPC/DHPC system was studied at three temperatures, and the (1)H-(13)C dipolar coupling constants are compared with those obtained for two other bicelle solutions.

13C NMR spectral assignments for nematic liquid crystals by 2D chemical shift gamma-encoding NMR

A novel method to accomplish 13C NMR spectral assignments for nematic liquid crystals is proposed. The two-dimensional (2D) isotropic/anisotropic chemical shift correlation spectrum is observed in which the anisotropic shift parameters are represented as sharp lines by gamma-encoding. The 13C spectral assignments can be made from the 2D spectrum with the aid of the 13C isotropic shift assignments for the same compound in the isotropic liquid state. The experiments were performed on p-methoxybenzilidene-p-n-butylaniline.

Towards high-resolution 1H-NMR in biological membranes: magic angle spinning of bicelles

Proton line narrowing in biomembranes spun at the magic angle, for spinning speeds greater than 7 kHz, was investigated in two ways: increasing the field strength from 200 to 800 MHz and changing the membrane fluidity. The resolution that one can obtain on natural lipid membranes under the form of liposomes is 0.019 ppm at 800 MHz. On the other hand, spinning bicelles (disk-like model membranes made of synthetic long and short chain lipids) at the magic angle decreases the line width by an additional factor of 3 provided the bicelle is subjected to large orientational disorder. This leads to proton line widths of the order of 6 Hz at 500 MHz. The conjunction of high field, magic angle spinning and use of bicelle membranes should prove to be useful to solve membrane protein structure in a membrane environment.

Two-dimensional NMR correlations of liquid crystals using switched angle spinning

We present the first application of switched angle spinning (SAS) to correlate the first-order dipolar spectrum of a liquid crystalline sample with the isotropic magic angle spinning (MAS) spectrum in a two-dimensional experiment. In this experiment we are able to select the degree of dipolar couplings introduced via mechanical manipulations of the liquid crystal director in a single oriented sample. The (19)F SAS-COSY correlation of iodotrifluoroethylene, an AMX spin system, dissolved in the nematic liquid crystal 4-octylphenyl-2-chloro-4-(4-heptylbenzoyloxy)-benzoate provides assignment of both the J and dipolar couplings in a single experiment. This work demonstrates the use of oriented samples and sample spinning to resolve homonuclear dipolar couplings using isotropic chemical shifts.