Solid-state NMR for the study of membrane systems: the use of anisotropic interactions

Effects of ionic liquids on biomembranes: A review on recent biophysical studies

Ionic liquids (ILs) have been emerged as a versatile class of compounds that can be easily tuned to achieve desirable properties for various applications. The ability of ILs to interact with biomembranes has attracted significant interest, as they have been shown to modulate membrane properties in ways that may have implications for various biological processes. This review provides an overview of recent studies that have investigated the interaction between ILs and biomembranes. We discuss the effects of ILs on the physical and chemical properties of biomembranes, including changes in membrane fluidity, permeability, and stability. We also explore the mechanisms underlying the interaction of ILs with biomembranes, such as electrostatic interactions, hydrogen bonding, and van der Waals forces. Additionally, we discuss the future prospects of this field.

Solid-state NMR study of structural heterogeneity of the apo WT mouse TSPO reconstituted in liposomes

In the last decades, ligand binding to human TSPO has been largely used in clinical neuroimaging, but little is known about the interaction mechanism. Protein conformational mobility plays a key role in the ligand recognition and both, ligand-free and ligand-bound structures, are mandatory for characterizing the molecular binding mechanism. In the absence of crystals for mammalian TSPO, we have exploited solid-state nuclear magnetic resonance (ssNMR) spectroscopy under magic-angle spinning (MAS) to study the apo form of recombinant mouse TSPO (mTSPO) reconstituted in lipids. This environment has been previously described to permit binding of its high-affinity drug ligand PK11195 and appears therefore favourable for the study of molecular dynamics. We have optimized the physical conditions to get the best resolution for MAS ssNMR spectra of the ligand-free mTSPO. We have compared and combined various ssNMR spectra to get dynamical information either for the lipids or for the mTSPO. Partial assignment of residue types suggests few agreements with the published solution NMR assignment of the PK11195-bound mTSPO in DPC detergent. Moreover, we were able to observe some lateral chains of aromatic residues that were not assigned in solution. ¹³C double-quantum NMR spectroscopy shows remarkable dynamics for ligand-free mTSPO in lipids which may have significant implications on the recognition of the ligand and/or other protein partners.

Membrane composition modulates the interaction between a new class of antineoplastic agents deriving from aromatic 2-chloroethylureas and lipid bilayers: a solid-state NMR study

We have investigated the interaction between a new class of antineoplastic agents derived from arylchloroethylureas (CEU) with three different model membranes by (31)P and (2)H solid-state NMR spectroscopy. First, we have prepared model membranes that mimic the mitochondrial inner (Mito IM) and outer (Mito OM) membranes and the endoplasmic reticulum membrane (End Ret). Our results indicate that the effects of the CEU derivatives on lipid bilayers are related to their cytotoxic activity. More specifically, a strong correlation is observed between the drug location in both the mitochondrial inner and outer membranes and its cytotoxicity. In addition, the results indicate that the lipid composition of the model membrane has a very important influence on the effects of CEUs. More specifically, a high proportion of cardiolipin in the mitochondrial inner membrane gives this system the highest fluidity and consequently, this model membrane is more rigidified by the presence of CEUs compared to the mitochondrial outer and endoplasmic reticulum membranes. Finally, the results propound a hypothesis for the location of CEUs in membranes.

Secondary Structures of Peptides and Proteins via NMR Chemical-Shielding Anisotropy (CSA) Parameters

Complete nuclear magnetic resonance (NMR) chemical-shielding tensors, sigma, have been computed at different levels of density-functional theory (DFT), within the gauge-including atomic orbital (GIAO) formalism, for the atoms of the peptide model For-L-Ala-NH2 as a function of the backbone dihedral angles phi and psi by employing a dense grid of 10 degrees. A complete set of rigorously orthogonal symmetric tensor invariants, {sigma iso, rho, tau}, is introduced, where sigma iso is the usual isotropic chemical shielding, while the newly introduced rho and tau parameters describe the magnitude and the orientation/shape of the chemical-shielding anisotropy (CSA), respectively. The set {sigma iso, rho, tau} is unaffected by unitary transformations of the symmetric part of the shielding tensor. The mathematically and physically motivated {rho, tau} anisotropy pair is easily connected to more traditional shielding anisotropy measures, like span (Omega) and skew (kappa). The effectiveness of the different partitions of the CSA information in predicting conformations of peptides and proteins has been tested throughout the Ramachandran space by generating theoretical NMR anisotropy surfaces for our For-L-Ala-NH2 model. The CSA surfaces, including Omega(phi, psi), kappa(phi, psi), rho(phi, psi), and tau(phi, psi) are highly structured. Individually, none of these surfaces is able to distinguish unequivocally between the alpha-helix and beta-strand secondary structural types of proteins. However, two- and three-dimensional correlated plots, including Omega versus kappa, rho versus tau, and sigma iso versus rho versus tau, especially for 13Calpha, have considerable promise in distinguishing among all four of the major secondary structural elements.

The structural properties of the transmembrane segment of the integral membrane protein phospholamban utilizing 13C CPMAS, 2H, and REDOR solid-state NMR spectroscopy

Solid-state NMR spectroscopic techniques were used to investigate the secondary structure of the transmembrane peptide phospholamban (TM-PLB), a sarcoplasmic Ca(2+) regulator. (13)C cross-polarization magic angle spinning spectra of (13)C carbonyl-labeled Leu39 of TM-PLB exhibited two peaks in a pure 1-palmitoyl-2-oleoyl-phosphocholine (POPC) bilayer, each due to a different structural conformation of phospholamban as characterized by the corresponding (13)C chemical shift. The addition of a negatively charged phospholipid (1-palmitoyl-2-oleoylphosphatidylglycerol (POPG)) to the POPC bilayer stabilized TM-PLB to an alpha-helical conformation as monitored by an enhancement of the alpha-helical carbonyl (13)C resonance in the corresponding NMR spectrum. (13)C-(15)N REDOR solid-state NMR spectroscopic experiments revealed the distance between the (13)C carbonyl carbon of Leu39 and the (15)N amide nitrogen of Leu42 to be 4.2+/-0.2A indicating an alpha-helical conformation of TM-PLB with a slight deviation from an ideal 3.6 amino acid per turn helix. Finally, the quadrupolar splittings of three (2)H labeled leucines (Leu28, Leu39, and Leu51) incorporated in mechanically aligned DOPE/DOPC bilayers yielded an 11 degrees +/-5 degrees tilt of TM-PLB with respect to the bilayer normal. In addition to elucidating valuable TM-PLB secondary structure information, the solid-state NMR spectroscopic data indicates that the type of phospholipids and the water content play a crucial role in the secondary structure and folding of TM-PLB in a phospholipid bilayer.

Tethered or adsorbed supported lipid bilayers in nanotubes characterized by deuterium magic angle spinning NMR spectroscopy

2H solid-state NMR experiments were performed under magic angle spinning on lipid bilayers oriented into nanotubes arrays, as a new method to assess the geometrical arrangement of the lipids. Orientational information is obtained from the intensities of the spinning sidebands. The lipid bilayers are formed by fusion of small unilamellar vesicles of DMPC-d54 inside a nanoporous anodic aluminum oxide, either by direct adsorption on the support or by tethering through a streptavidin/biotin linker. The results support that the quality of the lipid bilayers alignment is clearly in favor of the tethering rather than an adsorbed strategy.

The alignment, structure and dynamics of membrane-associated polypeptides by solid-state NMR spectroscopy

Solid-state NMR spectroscopy is being developed at a fast pace for the structural investigation of immobilized and non-crystalline biomolecules. These include proteins and peptides associated with phospholipid bilayers. In contrast to solution NMR spectroscopy, where complete or almost complete averaging leads to isotropic values, the anisotropic character of nuclear interactions is apparent in solid-state NMR spectra. In static samples the orientation dependence of chemical shift, dipolar or quadrupolar interactions, therefore, provides angular constraints when the polypeptides have been reconstituted into oriented membranes. Furthermore, solid-state NMR spectroscopy of aligned samples offers distinct advantages in allowing access to dynamic processes such as topological equilibria or rotational diffusion in membrane environments. Alternatively, magic angle sample spinning (MAS) results in highly resolved NMR spectra, provided that the sample is sufficiently homogenous. MAS spinning solid-state NMR spectra allow to measure distances and dihedral angles with high accuracy. The technique has recently been developed to selectively establish through-space and through-bond correlations between nuclei, similar to the approaches well-established in solution-NMR spectroscopy.

Susceptibility corrections in solid-state NMR experiments with oriented membrane samples. Part I: applications

Chemical shift referencing of solid-state NMR experiments on oriented membranes has to compensate for bulk magnetic susceptibility effects that are associated with the non-spherical sample shape, as described in the accompanying paper [J. Magn. Reson. 164 (2003) 115-127]. The resulting frequency deviations can be on the order of 10 ppm, which is serious for nuclei with a narrow chemical shift anisotropy such as 1H or 13C, and in some cases even 19F. Two referencing schemes are proposed here to compensate for these effects: A flat (0.4 mm) glass container with an isotropic reference molecule dissolved in a thin film of liquid is stacked on top of the oriented membrane sample. Alternatively, the intrinsic proton signal of the hydrated lipid can be used for chemical shift referencing. Further aspects related to magnetic susceptibility are discussed, such as air gaps in susceptibility-matched probeheads, the benefits of shimming, and limitations in the accuracy of orientational constraints. A biological application is illustrated by a series of experiments on the antimicrobial peptide PGLa, aimed at understanding its concentration-dependent membranolytic effect. To address a wide range of molar peptide/lipid ratios between 1:3000 and 1:8, multilayers of hydrated DMPC containing a 19F-labeled peptide were oriented between stacked glass plates. Maintaining an approximately constant amount of peptide gives rise to thick samples (18 plates) at low, and thin samples (3 plates) at high peptide/lipid ratio. Accurate referencing was critical to reveal a small but significant change over 5 ppm in the anisotropic chemical shift of the 19F label on the peptide, indicative of a change in the orientation and/or dynamics of PGLa in the membrane.

Bilayer sample for fast or slow magic angle oriented sample spinning solid-state NMR spectroscopy

An alternative setup for Magic Angle Oriented Spinning Spectroscopy is proposed. Samples were prepared by orienting lipid bilayers onto polymer films, which were wrapped into a spiral so as to fit into 4 or 7 mm MAS rotors. This geometry resulted in narrow line widths and a higher upper spinning limit when compared to the conventional MAOSS setup with stacked glass plates. Whereas orientational information was extracted from low spinning spectra, fast spinning will be applicable to high-resolution multidimensional NMR pulse sequences.

Peptide structural analysis by solid-state NMR spectroscopy

Solid-state nmr spectroscopy provides a robust method for investigating polypeptides that have been prepared by chemical synthesis and that are immobilized by strong interactions with solid surfaces or large macroscopic complexes. Solid-state nmr spectroscopy has been widely used to investigate membrane polypeptides or peptide aggregates such as amyloid fibrils. Whereas magic angle spinning solid-state nmr spectroscopy allows one to measure distances and dihedral angles with high accuracy, static membrane samples that are aligned with respect to the magnetic field direction allow one to determine the secondary structure of bound polypeptides and their orientation with respect to the bilayer normal. Peptide dynamics and the effect of polypeptides on the macroscopic phase preference of phospholipid membranes have been investigated in nonoriented samples. Investigations of the structure and topology of membrane channels, peptide antibiotics, signal sequences as well as model systems that allow one to dissect the interaction contributions in phospholipid membranes will be presented in greater detail.