Morphology of three lyotropic liquid crystalline biological NMR media studied by translational diffusion anisotropy

Interaction of the neuropeptide met-enkephalin with zwitterionic and negatively charged bicelles as viewed by 31P and 2H solid-state NMR

The interaction of the neuropeptide methionine-enkephalin (Menk) with bicelles was investigated by solid-state NMR. Bicelles composed of dimyristoylphosphatidylcholine (DMPC) and dicaproylphosphatidylcholine (DCPC) were modified to investigate the effect of the lipid headgroup and electrostatic charges on the association with Menk. A total of 10 mol % of DMPC was replaced by zwitterionic phosphatidylethanolamine (DMPE), anionic phosphatidylglycerol (DMPG), or phosphatidylserine (DMPS). The preparation of DMPE-doped bicelles (Bic/PE) is reported for the first time. The (31)P and (2)H NMR results revealed changes in the lipid dynamics when Menk interacts with the bicellar systems. (2)H NMR experiments showed a disordering effect of Menk on the lipid chains in all the bicelles except Bic/PG, whereas the study of the choline headgroups indicated a decreased order of the lipids only in Bic/PE and Bic/PG. Our results suggest that the insertion depth of Menk into bicelles is modulated by their composition, more specifically by the balance between hydrophobic and electrostatic interactions. Menk would be buried at the lipid polar/apolar interface, the depth of penetration into the hydrophobic membrane core following the scaling Bic > Bic/PE > Bic/PS at the slightly acidic pH used in this study. The peptide would not insert into the bilayer core of Bic/PG and would rather remain at the surface.

Motilin-bicelle interactions: membrane position and translational diffusion

The interaction between the peptide hormone motilin and bicelles has been investigated by pulsed field gradient-nuclear magnetic resonance methods and by the use of paramagnetic probes. Diffusion coefficients were measured for motilin, the phospholipids with and without motilin, and for tetramethylsilane. The results show that around 90% of motilin is bound to acidic bicelles and 84% of motilin is bound to neutral bicelles. It is found that the apparent bicelle size is reduced by the presence of motilin. This cannot be explained by changes in 1,2-dihexanoyl-sn-glycero-3-phosphatidylcholine solubility. The use of paramagnetic agents to investigate the position of motilin shows that the turn in the N-terminus of motilin is inserted into the bicelle, while the helix most likely resides within the head-group layer.

Magnetically aligned phospholipid bilayers in weak magnetic fields: optimization, mechanism, and advantages for X-band EPR studies

Our lab is developing a spin-labeled EPR spectroscopic technique complementary to solid-state NMR studies to study the structure, orientation, and dynamics of uniaxially aligned integral membrane proteins inserted into magnetically aligned discotic phospholipid bilayers, or bicelles. The focus of this study is to optimize and understand the mechanisms involved in the magnetic alignment process of bicelle disks in weak magnetic fields. Developing experimental conditions for optimized magnetic alignment of bicelles in low magnetic fields may prove useful to study the dynamics of membrane proteins and its interactions with lipids, drugs, steroids, signaling events, other proteins, etc. In weak magnetic fields, the magnetic alignment of Tm(3+)-doped bicelle disks was thermodynamically and kinetically very sensitive to experimental conditions. Tm(3+)-doped bicelles were magnetically aligned using the following optimized procedure: the temperature was slowly raised at a rate of 1.9K/min from an initial temperature being between 298 and 307K to a final temperature of 318K in the presence of a static magnetic field of 6300G. The spin probe 3beta-doxyl-5alpha-cholestane (cholestane) was inserted into the bicelle disks and utilized to monitor bicelle alignment by analyzing the anisotropic hyperfine splitting for the corresponding EPR spectra. The phases of the bicelles were determined using solid-state 2H NMR spectroscopy and compared with the corresponding EPR spectra. Macroscopic alignment commenced in the liquid crystalline nematic phase (307K), continued to increase upon slowly raising the temperature, and was well-aligned in the liquid crystalline lamellar smectic phase (318K).

Weak alignment offers new NMR opportunities to study protein structure and dynamics

Protein solution nuclear magnetic resonance (NMR) can be conducted in a slightly anisotropic environment, where the orientational distribution of the proteins is no longer random. In such an environment, the large one-bond internuclear dipolar interactions no longer average to zero and report on the average orientation of the corresponding vectors relative to the magnetic field. The desired very weak ordering, on the order of 10(-3), can be induced conveniently by the use of aqueous nematic liquid crystalline suspensions or by anisotropically compressed hydrogels. The resulting residual dipolar interactions are scaled down by three orders of magnitude relative to their static values, but nevertheless can be measured at high accuracy. They are very precise reporters on the average orientation of bonds relative to the molecular alignment frame, and they can be used in a variety of ways to enrich our understanding of protein structure and function. Applications to date have focused primarily on validation of structures, determined by NMR, X-ray crystallography, or homology modeling, and on refinement of structures determined by conventional NMR approaches. Although de novo structure determination on the basis of dipolar couplings suffers from a severe multiple minimum problem, related to the degeneracy of dipolar coupling relative to inversion of the internuclear vector, a number of approaches can address this problem and potentially can accelerate the NMR structure determination process considerably. In favorable cases, where large numbers of dipolar couplings can be measured, inconsistency between measured values can report on internal motions.

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.

Cation modulation of bicelle size and magnetic alignment as revealed by solid-state NMR and electron microscopy

The influence of salts (KCl, NaCl, CaCl(2), and MgCl(2)) on bicelles (bilayered micelles) made of dimyristoylphosphatidylcholine (DMPC, molar fraction X = 78%) and dicaproylphosphatidylcholine (DCPC) was investigated by solid-state (31)P- and (2)H NMR as well as by freeze-fracture electron microscopy. Sizes were determined from (2)H- and (31)P NMR on the basis of a model that incorporated a planar bilayer and a (half-torus) curved rim representing the DMPC and DCPC regions of the bicelle, respectively. Good agreement was shown with sizes determined independently from freeze-fracture electron microscopy images. In the presence of K(+) and Na(+), bicelles have diameters of approximately 300 A while in the presence of Ca(2+) and Mg(2+); their diameter increases to approximately 500 A. Bicelle magnetic alignment is considerably improved by the presence of salts. The optimum salt concentration for such an effect ranges from 50 to 200 mM. Bicelles are magnetically aligned for temperatures roughly ranging from 30 degrees C to 40 degrees C with monovalent cations; this range is slightly extended in the presence of divalent salts. In this temperature range, the dynamics of the long-chain hydrocarbon region of the bicelle (leading to a bicelle thickness of 38 A) and of water is about the same independently of cation nature and concentration. However, at higher temperatures, considerable differences in water dynamics are observed between systems with monovalent and divalent cations. In these conditions, the system consists of a mixture of micelles and extended bilayers, which show residual macroscopic alignment in the magnetic field.

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.

Anisotropic water diffusion in macroscopically oriented lipid bilayers studied by pulsed magnetic field gradient NMR

The anisotropy, D(parallel)/D( perpendicular ), of water diffusion in fully hydrated bilayers of dimyristoylphosphatidylcholine at 29 degrees C has been measured by pulsed magnetic field gradient (pfg) NMR. By using NMR imaging hardware to produce magnetic field gradients in an arbitrary direction with respect to a stack of macroscopically aligned lipid bilayers, translational diffusion of water was measured as a function of the angle between the direction of the magnetic field gradient and the normal of the lipid membrane. The observed diffusion coefficient is found to depend strongly on this angle. The anisotropy cannot be accurately determined due to the very small value of D( perpendicular ), but a lower limit of about 70 can be estimated from the observed diffusion coefficients. The results are discussed in terms of the relatively low permeability of water across the lipid bilayer, instrumental limitations, and/or possible defects in the lamellae.