Switched-angle spinning applied to bicelles containing phospholipid-associated peptides

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.

Investigation of the curvature induction and membrane localization of the influenza virus M2 protein using static and off-magic-angle spinning solid-state nuclear magnetic resonance of oriented bicelles

A wide variety of membrane proteins induce membrane curvature for function; thus, it is important to develop new methods to simultaneously determine membrane curvature and protein binding sites in membranes with multiple curvatures. We introduce solid-state nuclear magnetic resonance (NMR) methods based on magnetically oriented bicelles and off-magic-angle spinning (OMAS) to measure membrane curvature and the binding site of proteins in mixed-curvature membranes. We demonstrate these methods on the influenza virus M2 protein, which not only acts as a proton channel but also mediates virus assembly and membrane scission. An M2 peptide encompassing the transmembrane (TM) domain and an amphipathic helix, M2(21-61), was studied and compared with the TM peptide (M2TM). Static (31)P NMR spectra of magnetically oriented 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) bicelles exhibit a temperature-independent isotropic chemical shift in the presence of M2(21-61) but not M2TM, indicating that the amphipathic helix confers the ability to generate a high-curvature phase. Two-dimensional (2D) (31)P spectra indicate that this high-curvature phase is associated with the DHPC bicelle edges, suggestive of the structure of budding viruses from the host cell. (31)P- and (13)C-detected (1)H relaxation times of the lipids indicate that the majority of M2(21-61) is bound to the high-curvature phase. Using OMAS experiments, we resolved the (31)P signals of lipids with identical headgroups based on their distinct chemical shift anisotropies. On the basis of this resolution, 2D (1)H-(31)P correlation spectra show that the amide protons in M2(21-61) correlate with the DMPC but not DHPC (31)P signal of the bicelle, indicating that a small percentage of M2(21-61) partitions into the planar region of the bicelles. These results show that the amphipathic helix induces high membrane curvature and localizes the protein to this phase, in good agreement with the membrane scission function of the protein. These bicelle-based relaxation and OMAS solid-state NMR techniques are generally applicable to curvature-inducing membrane proteins such as those involved in membrane trafficking, membrane fusion, and cell division.

Cholesterol increases the magnetic aligning of bicellar disks from an aqueous mixture of DMPC and DMPE-DTPA with complexed thulium ions

Aqueous mixtures of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-diethylenetriamine pentaacetate (DMPE-DTPA) with complexed thulium ions (Tm(3+)), and cholesterol with varying molar ratio were studied at different temperatures in the presence and absence of a magnetic field. For mixtures without cholesterol weakly magnetically alignable small disks, so-called bicelles, are formed at temperatures below the phase transition temperature (5-22 °C), as shown by cryo-transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS). In presence of 16 mol % cholesterol the disk size and the magnetic alignability were larger within the entire temperature range studied (5-40 °C). Cholesterol acts as a spacer between DMPE-DTPA with complexed Tm(3+), allowing these molecules to integrate more frequently into the planar part of the bicelles. Replacing DMPC partially by cholesterol thus lead to an increase in magnetic aligning by a higher amount of the magnetic handles (Tm(3+) complexed to DMPE-DTPA) in the plane and by an increased number of phospholipids in the enlarged bicelles. The magnetic aligning was most pronounced at 5 °C. The temperature-dependent structural changes of the DMPC/cholesterol/DMPE-DTPA/Tm(3+) aqueous mixtures are complex, including the transient appearance of holes in the disks at intermediate temperatures.

Formation of kinetically trapped nanoscopic unilamellar vesicles from metastable nanodiscs

Zwitterionic long-chain lipids (e.g., dimyristoyl phosphatidylcholine, DMPC) spontaneously form onion-like, thermodynamically stable structures in aqueous solutions (commonly known as multilamellar vesicles, or MLVs). It has also been reported that the addition of zwitterionic short-chain (i.e., dihexanoyl phosphatidylcholine, DHPC) and charged long-chain (i.e., dimyristoyl phosphatidylglycerol, DMPG) lipids to zwitterionic long-chain lipid solutions results in the formation of unilamellar vesicles (ULVs). Here, we report a kinetic study on lipid mixtures composed of DMPC, DHPC, and DMPG. Two membrane charge densities (i.e., [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities (i.e., [NaCl] = 0 and 0.2 M) are investigated. Upon dilution of the high-concentration samples at 50 °C, thermodynamically stable MLVs are formed, in the case of both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to cause MLVs to unbind. Importantly, in the case of these samples small angle neutron scattering (SANS) data show that, initially, nanodiscs (also known as bicelles) or bilayered ribbons form at low temperatures (i.e., 10 °C), but transform into uniform size, nanoscopic ULVs after incubation at 10 °C for 20 h, indicating that the nanodisc is a metastable structure. The instability of nanodiscs may be attributed to low membrane rigidity due to a reduced charge density and high salinity. Moreover, the uniform-sized ULVs persist even after being heated to 50 °C, where thermodynamically stable MLVs are observed. This result clearly demonstrates that these ULVs are kinetically trapped, and that the mechanical properties (e.g., bending rigidity) of 10 °C nanodiscs favor the formation of nanoscopic ULVs over that of MLVs. From a practical point of view, this method of forming uniform-sized ULVs may lend itself to their mass production, thus making them economically feasible for medical applications that depend on monodisperse lipid-based systems for therapeutic and diagnostic purposes.

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°.

Mechanism of inhibition of enveloped virus membrane fusion by the antiviral drug arbidol

The broad-spectrum antiviral arbidol (Arb) inhibits cell entry of enveloped viruses by blocking viral fusion with host cell membrane. To better understand Arb mechanism of action, we investigated its interactions with phospholipids and membrane peptides. We demonstrate that Arb associates with phospholipids in the micromolar range. NMR reveals that Arb interacts with the polar head-group of phospholipid at the membrane interface. Fluorescence studies of interactions between Arb and either tryptophan derivatives or membrane peptides reconstituted into liposomes show that Arb interacts with tryptophan in the micromolar range. Interestingly, apparent binding affinities between lipids and tryptophan residues are comparable with those of Arb IC50 of the hepatitis C virus (HCV) membrane fusion. Since tryptophan residues of membrane proteins are known to bind preferentially at the membrane interface, these data suggest that Arb could increase the strength of virus glycoprotein's interactions with the membrane, due to a dual binding mode involving aromatic residues and phospholipids. The resulting complexation would inhibit the expected viral glycoprotein conformational changes required during the fusion process. Our findings pave the way towards the design of new drugs exhibiting Arb-like interfacial membrane binding properties to inhibit early steps of virus entry, i.e., attractive targets to combat viral infection.

The structural and topological analysis of membrane-associated polypeptides by oriented solid-state NMR spectroscopy: established concepts and novel developments

Solid-state NMR spectroscopy is a powerful technique for the investigation of membrane-associated peptides and proteins as well as their interactions with lipids, and a variety of conceptually different approaches have been developed for their study. The technique is unique in allowing for the high-resolution investigation of liquid disordered lipid bilayers representing well the characteristics of natural membranes. Whereas magic angle solid-state NMR spectroscopy follows approaches that are related to those developed for solution NMR spectroscopy the use of static uniaxially oriented samples results in angular constraints which also provide information for the detailed analysis of polypeptide structures. This review introduces this latter concept theoretically and provides a number of examples. Furthermore, ongoing developments combining solid-state NMR spectroscopy with information from solution NMR spectroscopy and molecular modelling as well as exploratory studies using dynamic nuclear polarization solid-state NMR will be presented.

Site-specific backbone amide (15)N chemical shift anisotropy tensors in a small protein from liquid crystal and cross-correlated relaxation measurements

Site-specific (15)N chemical shift anisotropy (CSA) tensors have been derived for the well-ordered backbone amide (15)N nuclei in the B3 domain of protein G (GB3) from residual chemical shift anisotropy (RCSA) measured in six different mutants that retain the native structure but align differently relative to the static magnetic field when dissolved in a liquid crystalline Pf1 suspension. This information is complemented by measurement of cross-correlated relaxation rates between the (15)N CSA tensor and either the (15)N-(1)H or (15)N-(13)C' dipolar interaction. In agreement with recent solid state NMR measurements, the (15)N CSA tensors exhibit only a moderate degree of variation from averaged values, but have larger magnitudes in alpha-helical (-173 +/- 7 ppm) than in beta-sheet (-162 +/- 6 ppm) residues, a finding also confirmed by quantum computations. The orientations of the least shielded tensor component cluster tightly around an in-peptide-plane vector that makes an angle of 19.6 +/- 2.5 degrees with the N-H bond, with the asymmetry of the (15)N CSA tensor being slightly smaller in alpha-helix (eta = 0.23 +/- 0.17) than in beta-sheet (eta = 0.31 +/- 0.11). The residue-specific (15)N CSA values are validated by improved agreement between computed and experimental (15)N R(1rho) relaxation rates measured for (15)N-{(2)H} sites in GB3, which are dominated by the CSA mechanism. Use of residue-specific (15)N CSA values also results in more uniform generalized order parameters, S(2), and predicts considerable residue-by-residue variations in the magnetic field strengths where TROSY line narrowing is most effective.

Interactions of tyrosine in Leu-enkephalin at a membrane-water interface: an ultrafast two-dimensional infrared study combined with density functional calculations and molecular dynamics simulations

The interactions of neuropeptides and membranes play an important role in peptide hormone function. Our current understanding of peptide-membrane interactions remains limited due to the paucity of experimental techniques capable of probing such interactions. In this work, we study the nature of opioid peptide-membrane interactions using ultrafast two-dimensional infrared (2D IR) spectroscopy. The high temporal resolution of 2D IR is particularly suited for studying highly flexible opioid peptides. We investigate the location of the tyrosine (Tyr) side chain of leucine-enkephalin (Lenk) in lipid bilayer membranes by measuring spectral diffusion of the phenolic ring vibrational mode in three different systems: Lenk in lipid bilayer membranes (bicelles), Lenk in deuterated water, and p-cresol in deuterated water. Frequency-frequency correlation functions obtained from waiting-time-dependent 2D IR spectra reveal an ultrafast decaying component with an approximately 1 ps time constant that is common for all three systems. On the basis of density functional theory calculations and molecular dynamics simulations, this spectral diffusion component is attributed to hydrogen-bond dynamics of the phenolic hydroxyl group interacting with bulk water. Unlike p-cresol in water, both Lenk systems exhibit static spectral inhomogeneity, which can be attributed to conformational distributions of Lenk that do not interconvert within 4 ps. Our results suggest that the Tyr side chain of Lenk in bicelles is located at the water-abundant region at the membrane-water interface and not embedded into the hydrophobic core.

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.

Variable angle spinning (VAS) experiments for strongly oriented systems: methods development and preliminary results

Variable angle spinning (VAS) experiments provide a useful method for measuring long-range dipolar couplings and obtaining isotropic-anisotropic correlation spectra. These experiments make it possible to obtain correlations between isotropic and anisotropic spectra without altering the chemical composition of the sample. They also allow working with very strongly oriented systems that are not accessible to solution-state techniques. In this communication, we discuss recent hardware developments in our laboratory and show representative data from small molecules in strongly oriented liquid-crystalline samples.

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.

Structural and dynamic studies of proteins by solid-state NMR spectroscopy: rapid movement forward

Starting only a few years ago, many solid-state NMR spectroscopy laboratories have become engaged in solving the complete structures of biological macromolecules using high-resolution methods based on magic angle spinning. These efforts typically involve structurally homogeneous samples, and utilize recently developed pulse sequences for the sequential correlation of resonances, the detection of tertiary contacts and the characterization of torsion angles. Thereby, systems have been studied that evaded other, more established, structure determination methods.

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.

Molecular orientation and conformation of phosphatidylinositides in membrane mimetics using variable angle sample spinning (VASS) NMR

For many biological molecules, determining their geometry as they exist in a membrane environment is a crucial step in understanding their function. Variable angle sample spinning (VASS) NMR provides a new route to obtaining geometry information on membrane-associating molecules; it has been used here to scale and separate anisotropic contributions to phosphorus chemical shifts in NMR spectra of phosphatidylinositol phosphates. The procedure allows spectral assignment via correlation with isotropic chemical shifts and determination of a family of probable headgroup orientations via interpretation of anisotropic shift contributions. The molecules studied include phosphtidylinositol-4-phosphate (PI(4)P) and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2). A membrane-like environment is provided by a dispersion of alkyl-poly(ethylene) glycols and n-alcohols that forms a field-orienting liquid crystal with a director that can be manipulated by varying the sample spinning axis. The experiments presented indicate that the variable angle sample spinning method will provide a direct approach for assignment and extraction of structural information from membrane-associating biomolecules labeled with a wider variety of NMR active isotopes.

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.