Dilute bicellar solutions for structural NMR work

Mechanisms of membrane protein crystallization in 'bicelles'

Despite remarkable progress, mainly due to the development of LCP and ‘bicelle’ crystallization, lack of structural information remains a bottleneck in membrane protein (MP) research. A major reason is the absence of complete understanding of the mechanism of crystallization. Here we present small-angle scattering studies of the evolution of the “bicelle” crystallization matrix in the course of MP crystal growth. Initially, the matrix corresponds to liquid-like bicelle state. However, after adding the precipitant, the crystallization matrix transforms to jelly-like state. The data suggest that this final phase is composed of interconnected ribbon-like bilayers, where crystals grow. A small amount of multilamellar phase appears, and its volume increases concomitantly with the volume of growing crystals. We suggest that the lamellar phase surrounds the crystals and is critical for crystal growth, which is also common for LCP crystallization. The study discloses mechanisms of “bicelle” MP crystallization and will support rational design of crystallization.

Advances in NMR Spectroscopy of Weakly Aligned Biomolecular Systems

The measurement and application of residual dipolar couplings (RDCs) in solution NMR studies of biological macromolecules has become well established over the past quarter of a century. Numerous methods for generating the requisite anisotropic orientational molecular distribution have been demonstrated, each with its specific strengths and weaknesses. In parallel, an enormous number of pulse schemes have been introduced to measure the many different types of RDCs, ranging from the most widely measured backbone amide ¹⁵N-¹H RDCs, to ¹H-¹H RDCs and couplings between low-γ nuclei. Applications of RDCs range from structure validation and refinement to the determination of relative domain orientations, the measurement of backbone and domain motions, and de novo structure determination. Nevertheless, it appears that the power of the RDC methodology remains underutilized. This review aims to highlight the practical aspects of sample preparation and RDC measurement while describing some of the most straightforward applications that take advantage of the exceptionally precise information contained in such data. Some emphasis will be placed on more recent developments that enable the accurate measurement of RDCs in larger systems, which is key to the ongoing shift in focus of biological NMR spectroscopy from structure determination toward gaining improved understanding of how molecular flexibility drives protein function.

Effects of fluidity and charge density on the morphology of a bicellar mixture - A SANS study

The spontaneously formed structures of physiologically relevant lipid model membranes made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and 1,2-hexanoyl-sn-glycero-3-phosphocholine have been evaluated in depth using small angle neutron scattering. Although a common molar ratio of long- to short- chain phospholipids (~4) as reported in many bicellar mixtures was used, discoidal bicelles were not found as the major phase throughout the range of lipid concentration and temperature studied, indicating that the required condition for the formation of bicelle is the immiscibility between the long- and short- chain lipids, which were in the gel and L_α phases, respectively, in previous reports. In this study, all lipids are in the L_α phase. The characterization outcome suggests that the spontaneous structures tie strongly with the physical parameters of the system such as melting transition temperature of the long-chain lipid, total lipid concentration and charge density of the system. Multilamellar vesicles, unilamellar vesicles, ribbons and perforated lamellae can be obtained based on the analysis of the small angle neutron scattering results, leading to the construction of structural diagrams. This report provides the important map to choose suitable lipid systems for the structural study of membrane-associated proteins, design of theranostic nanocarriers or other related research fields.

Systematic Characterization of DMPC/DHPC Self-Assemblies and Their Phase Behaviors in Aqueous Solution

Self-assemblies composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) form several kinds of structures, such as vesicle, micelle, and bicelle. Their morphological properties have been studied widely, but their interfacial membrane properties have not been adequately investigated. Herein, we report a systematic characterization of DMPC/DHPC assemblies at 20 °C. To investigate the phase behavior, optical density OD500, size (by dynamic light scattering), membrane fluidity 1/PDPH (using 1,6-diphenyl-1,3,5-hexatriene), and membrane polarity GP340 (using 6-dodecanoyl-N,N-dimethyl-2-naphthylamine) were measured as a function of molar ratio of DHPC (XDHPC). Based on structural properties (OD500 and size), large and small assemblies were categorized into Region (i) (XDHPC < 0.4) and Region (ii) (XDHPC ≥ 0.4), respectively. The DMPC/DHPC assemblies with 0.33 ≤ XDHPC ≤ 0.67 (Region (ii-1)) showed gel-phase-like interfacial membrane properties, whereas DHPC-rich assemblies (XDHPC ≥ 0.77) showed disordered membrane properties (Region (ii-2)). Considering the structural and interfacial membrane properties, the DMPC/DHPC assemblies in Regions (i), (ii-1), and (ii-2) can be determined to be vesicle, bicelle, and micelle, respectively.

Multivariate Analysis of Mixed Lipid Aggregate Phase Transitions Monitored Using Raman Spectroscopy

The phase behavior of aqueous 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC)/1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) mixtures between 8.0 ℃ and 41.0 ℃ were monitored using Raman spectroscopy. Temperature-dependent Raman matrices were assembled from series of spectra and subjected to multivariate analysis. The consensus of pseudo-rank estimation results is that seven to eight components account for the temperature-dependent changes observed in the spectra. The spectra and temperature response profiles of the mixture components were resolved by applying a variant of the non-negative matrix factorization (NMF) algorithm described by Lee and Seung (1999). The rotational ambiguity of the data matrix was reduced by augmenting the original temperature-dependent spectral matrix with its cumulative counterpart, i.e., the matrix formed by successive integration of the spectra across the temperature index (columns). Successive rounds of constrained NMF were used to isolate component spectra from a significant fluorescence background. Five major components exhibiting varying degrees of gel and liquid crystalline lipid character were resolved. Hydrogen-bonded water networks exhibiting varying degrees of organization are associated with the lipid components. Spectral parameters were computed to compare the chain conformation, packing, and hydration indicated by the resolved spectra. Based on spectral features and relative amounts of the components observed, four components reflect long chain lipid response. The fifth component could reflect the response of the short chain lipid, DHPC, but there were no definitive spectral features confirming this assignment. A minor component of uncertain assignment that exhibits a striking response to the DMPC pre-transition and chain melting transition also was recovered. While none of the spectra resolved exhibit features unequivocally attributable to a specific aggregate morphology or step in the gelation process, the results are consistent with the evolution of mixed phase bicelles (nanodisks) and small amounts of worm-like DMPC/DHPC aggregates, and perhaps DHPC micelles, at low temperature to suspensions of branched and entangled worm-like aggregates above the DMPC gel phase transition and perforated multi-lamellar aggregates at high temperature.

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.

Lipid concentration and molar ratio boundaries for the use of isotropic bicelles

Bicelles are model membranes generally made of long-chain dimyristoylphosphatidylcholine (DMPC) and short-chain dihexanoyl-PC (DHPC). They are extensively used in the study of membrane interactions and structure determination of membrane-associated peptides, since their composition and morphology mimic the widespread PC-rich natural eukaryotic membranes. At low DMPC/DHPC (q) molar ratios, fast-tumbling bicelles are formed in which the DMPC bilayer is stabilized by DHPC molecules in the high-curvature rim region. Experimental constraints imposed by techniques such as circular dichroism, dynamic light scattering, or microscopy may require the use of bicelles at high dilutions. Studies have shown that such conditions induce the formation of small aggregates and alter the lipid-to-detergent ratio of the bicelle assemblies. The objectives of this work were to determine the exact composition of those DMPC/DHPC isotropic bicelles and study the lipid miscibility. This was done using (31)P nuclear magnetic resonance (NMR) and exploring a wide range of lipid concentrations (2-400 mM) and q ratios (0.15-2). Our data demonstrate how dilution modifies the actual DMPC/DHPC molar ratio in the bicelles. Care must be taken for samples with a total lipid concentration ≤250 mM and especially at q ∼ 1.5-2, since moderate dilutions could lead to the formation of large and slow-tumbling lipid structures that could hinder the use of solution NMR methods, circular dichroism or dynamic light scattering studies. Our results, supported by infrared spectroscopy and molecular dynamics simulations, also show that phospholipids in bicelles are largely segregated only when q > 1. Boundaries are presented within which control of the bicelles' q ratio is possible. This work, thus, intends to guide the choice of q ratio and total phospholipid concentration when using isotropic bicelles.

High anisotropy of flow-aligned bicellar membrane systems

In recent years, multi-lipid bicellar systems have emerged as promising membrane models. The fast orientational diffusion and magnetic alignability made these systems very attractive for NMR investigations. However, their alignment was so far achieved with a strong magnetic field, which limited their use with other methods that require macroscopic orientation. Recently, it was shown that bicelles could be aligned also by shear flow in a Couette flow cell, making it applicable to structural and biophysical studies by polarized light spectroscopy. Considering the sensitivity of this lipid system to small variations in composition and physicochemical parameters, efficient use of such a flow-cell method with coupled techniques will critically depend on the detailed understanding of how the lipid systems behave under flow conditions. In the present study we have characterized the flow alignment behavior of the commonly used dimyristoyl phosphatidylcholine/dicaproyl phosphatidylcholine (DMPC/DHPC) bicelle system, for various temperatures, lipid compositions, and lipid concentrations. We conclude that at optimal flow conditions the selected bicellar systems can produce the most efficient flow alignment out of any lipid systems used so far. The highest degree of orientation of DMPC/DHPC samples is noticed in a narrow temperature interval, at a practical temperature around 25 °C, most likely in the phase transition region characterized by maximum sample viscosity. The change of macroscopic orientation factor as function of the above conditions is now described in detail. The increase in macroscopic alignment observed for bicelles will most likely allow recording of higher resolution spectra on membrane systems, which provide deeper structural insight and analysis into properties of biomolecules interacting with solution phase lipid membranes.

Solution NMR studies of cell-penetrating peptides in model membrane systems

Cell-penetrating peptides (CPPs) are a class of short, often cationic peptides that have the capability to translocate across cellular membranes, and although the translocation most likely involves several pathways, they interact directly with membranes, as well as with model bilayers. Most CPPs attain a three-dimensional structure when interacting with bilayers, while they are more or less unstructured in aqueous solution. To understand the relationship between structure and the effect that CPPs have on membranes it is of great importance to investigate CPPs at atomic resolution in a suitable membrane model. Moreover, the location in bilayers is likely to be correlated with the translocation mechanism. Solution-state NMR offers a unique possibility to investigate structure, dynamics and location of proteins and peptides in bilayers. This review focuses on solution NMR as a tool for investigating CPP-lipid interactions. Structural propensities and cell-penetrating capabilities can be derived from a combination of CPP solution structures and studies of the effect that the peptides have on bilayers and the localization in a bilayer.

Dependence of bicellar system phase behavior and dynamics on anionic lipid concentration

Bicellar dispersions of chain perdeuterated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC-d54) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) were prepared with the molar fraction of DHPC held fixed at 20% and varying amounts of DMPC replaced by the anionic lipid 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG). (2)H NMR spectra were examined to assess the effect of added DMPG on mixture phase behavior and morphology. Quadrupole echo decay and quadrupole-Carr-Purcell-Mieboom-Gill echo train measurements provided information about slow motions contributing to echo decay in the high temperature phases. The spectra and quadrupole echo decay properties of DMPC-d54/DHPC (4:1) and DMPC-d54/DMPG/DHPC (3:1:1) were qualitatively similar. With increasing DMPG concentration, the transition between the magnetically orientable phase and the higher temperature phase became increasingly distinct, and the spectral shape and echo decay characteristics of the high temperature bicellar phase became increasingly similar to those of DMPC-d54 in the liquid crystalline phase. The observation that DMPG changes spectra in the orientable phase incrementally while increasing the distinction between the orientable and high temperature bicellar phases provides new insights into how DMPG influences bicellar mixture morphology.

Dynamic properties of bicellar lipid mixtures observed by rheometry and quadrupole echo decay

In bicellar dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), the transition from isotropic reorientation to partial orientational order, on warming, is known to coincide with a sharp increase in viscosity. In this work, cone-and-plate rheometry, (2)H NMR spectroscopy, and quadrupole echo decay observations have been used to obtain new insights into the dynamics of phases observed in bicellar DMPC/DHPC mixtures. Samples with 25% of the DMPC component deuterated were used to correlate rheological measurements with phase behavior observed by (2)H NMR spectroscopy. Mixtures containing only normal DMPC (DMPC/DHPC) or only chain perdeuterated DMPC (DMPC-d(54)/DHPC) were used to refine rheology and quadrupole echo decay measurements respectively. The viscosity peaked at 4-9 Pa·s, just above the isotropic-to-nematic transition, and then dropped as samples were warmed through the nematic-to-lamellar transition. Quadrupole echo decay times above the nematic-to-lamellar transition were significantly longer than typically observed in the liquid crystalline phase of saturated lipid multilamellar vesicles. This may indicate a damping of slow bilayer undulations resulting from the coupling of opposite bilayer surfaces by DHPC-lined pores.

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.

Structural Versatility of Bicellar Systems and Their Possibilities as Colloidal Carriers

Bicellar systems are lipid nanostructures formed by long- and short-chained phospholipids dispersed in aqueous solution. The morphological transitions of bicellar aggregates due to temperature, composition and time variations have been revised in this work. To this end, two bicellar systems have been considered; one formed by dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl- phosphatidylcholine (DHPC) and another formed by dipalmitoyl-phosphatidylcholine (DPPC) and DHPC. The relationship between the magnetic alignment, the morphology of the aggregates and the phase transition temperature (T_m) of lipids is discussed. In general terms, the non-alignable samples present rounded objects at temperature below the T_m. Above this temperature, an increase of viscosity is followed by the formation of large elongated aggregates. Alignable samples presented discoidal objects below the T_m. The best alignment was achieved above this temperature with large areas of lamellar stacked bilayers and some multilamellar vesicles. The effect of the inclusion of ceramides with different chain lengths in the structure of bicelles is also revised in the present article. A number of physical techniques show that the bicellar structures are affected by both the concentration and the type of ceramide. Systems are able to incorporate 10% mol of ceramides that probably are organized forming domains. The addition of 20% mol of ceramides promotes destabilization of bicelles, promoting the formation of mixed systems that include large structures. Bicellar systems have demonstrated to be morphologically stable with time, able to encapsulate different actives and to induce specific effects on the skin. These facts make bicellar systems good candidates as colloidal carriers for dermal delivery. However, water dilution induces structural changes and formation of vesicular structures in the systems; stabilization strategies have been been explored in recent works and are also updated here.

Bicosomes: bicelles in dilute systems

Bicelles are discoidal phospholipid nanostructures at high lipid concentrations. Under dilute conditions, bicelles become larger and adopt a variety of morphologies. This work proposes a strategy to preserve the discoidal morphology of bicelles in environments with high water content. Bicelles were formed in concentrated conditions and subsequently encapsulated in liposomes. Later dilution of these new structures, called bicosomes, demonstrated that lipid vesicles were able to isolate and protect bicelles entrapped inside them from the medium. Characterization of systems before and after dilution by dynamic light-scattering spectroscopy and cryo-transmission electron microscopy showed that free bicelles changed in size and morphology, whereas encapsulated bicelles remained unaltered by the effect of dilution. Free and entrapped bicelles (containing the paramagnetic contrast agent gadodiamide) were injected into rat brain lateral ventricles. Coronal and sagittal visualization was performed by magnetic resonance imaging. Whereas rats injected with free bicelles did not survive the surgery, those injected with bicosomes did, and a hyperintensity effect due to gadodiamide was observed in the cerebrospinal fluid. These results indicate that bicosomes are a good means of preserving the morphology of bicelles under dilution conditions.

Bicellar mixture phase behavior examined by variable-pressure deuterium NMR and ambient pressure DSC

Variable-pressure deuterium nuclear magnetic resonance ((2)H NMR) has been used to study the pressure-temperature phase diagram of bicellar mixtures containing 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC). Spectra were obtained for DMPC-d(54)/DHPC (3:1), DMPC-d(54)/DHPC (4.4:1), DMPC/DHPC-d(22) (3:1), and DMPC/DHPC-d(22) (4.4:1) in the range 10-68 degrees C at ambient pressure, 66 MPa, 102 MPa, and 135 MPa. Isotropic-to-nematic and nematic-to-lamellar transition temperatures were found to rise with pressure at approximately 0.15 and approximately 0.14 degrees C/MPa, respectively, for DMPC-d(54)/DHPC (3:1) and at at approximately 0.19 and approximately 0.18 degrees C/MPa, respectively, for DMPC-d(54)/DHPC (4.4:1). Pressure had little effect on the range of DMPC-d(54) chain orientational order through the nematic phase temperature range, but the behavior of chain orientational order at the nematic-to-lamellar transition was found to vary slightly with pressure. Comparison of differential scanning calorimetry (DSC) observations with ambient-pressure (2)H NMR observations of DMPC-d(54) in the bicellar mixtures suggests that absorption of heat persists for a few degrees above the onset of axially symmetric DMPC-d(54) reorientation.

Penetration and growth of DPPC/DHPC bicelles inside the stratum corneum of the skin

The effect of dipalmitoyl phosphatidylcholine (DPPC)/dihexanoyl phosphatidylcholine (DHPC) bicelles on the microstructure of pig stratum corneum (SC) in vitro was evaluated. The physicochemical characterization of these nanoaggregates revealed small disks with diameters around 15 nm and a thickness of 5.4 nm. Upon dilution, the bicelles grow and transform into vesicles. Cryogenic scanning electron microscopy (cryo-SEM) images of the SC pieces treated with this system showed vesicles of about 200 nm and lamellar-like structures in the intercellular lipid areas. These vesicles probably resulted from the growth and molecular rearrangement of the DPPC/DHPC bicelles after penetrating the SC. The presence of lamellar-like structures is ascribed to the interaction of the lipids from bicelles with the SC lipids. The bicellar system used is suitable to penetrate the skin SC and to reinforce the intercellular lipid areas, constituting a promising tool for skin applications.

Dynamic behavior of tea catechins interacting with lipid membranes as determined by NMR spectroscopy

Interaction between tea catechins, such as epicatechin gallate (ECg) and epigallocatechin gallate (EGCg), and isotropic bicelle model lipid membranes was investigated by solution NMR techniques. (1)H NMR measurements provided signals from the B-ring and the galloyl moiety in ECg and EGCg that were obviously shifted, and whose proton T1 relaxation times were shortened upon interaction of the catechins with the bicelles. These results indicate that the B-ring and the galloyl moiety play an important role in this interaction. Nuclear Overhauser effect spectrometry experiments demonstrated that the B-ring and the galloyl moiety are located near the gamma-H in the phospholipid trimethylammonium group. On the basis of these findings, we propose that ECg and EGCg interact with the surface of lipid membranes via the choline moiety.

Three-dimensional structure of the transmembrane domain of Vpu from HIV-1 in aligned phospholipid bicelles

The three-dimensional backbone structure of the transmembrane domain of Vpu from HIV-1 was determined by solid-state NMR spectroscopy in two magnetically-aligned phospholipid bilayer environments (bicelles) that differed in their hydrophobic thickness. Isotopically labeled samples of Vpu(2-30+), a 36-residue polypeptide containing residues 2-30 from the N-terminus of Vpu, were incorporated into large (q = 3.2 or 3.0) phospholipid bicelles composed of long-chain ether-linked lipids (14-O-PC or 16-O-PC) and short-chain lipids (6-O-PC). The protein-containing bicelles are aligned in the static magnetic field of the NMR spectrometer. Wheel-like patterns of resonances characteristic of tilted transmembrane helices were observed in two-dimensional (1)H/(15)N PISEMA spectra of uniformly (15)N-labeled Vpu(2-30+) obtained on bicelle samples with their bilayer normals aligned perpendicular or parallel to the direction of the magnetic field. The NMR experiments were performed at a (1)H resonance frequency of 900 MHz, and this resulted in improved data compared to lower-resonance frequencies. Analysis of the polarity-index slant-angle wheels and dipolar waves demonstrates the presence of a transmembrane alpha-helix spanning residues 8-25 in both 14-O-PC and 16-O-PC bicelles, which is consistent with results obtained previously in micelles by solution NMR and mechanically aligned lipid bilayers by solid-state NMR. The three-dimensional backbone structures were obtained by structural fitting to the orientation-dependent (15)N chemical shift and (1)H-(15)N dipolar coupling frequencies. Tilt angles of 30 degrees and 21 degrees are observed in 14-O-PC and 16-O-PC bicelles, respectively, which are consistent with the values previously determined for the same polypeptide in mechanically-aligned DMPC and DOPC bilayers. The difference in tilt angle in C14 and C16 bilayer environments is also consistent with previous results indicating that the transmembrane helix of Vpu responds to hydrophobic mismatch by changing its tilt angle. The kink found in the middle of the helix in the longer-chain C18 bilayers aligned on glass plates was not found in either of these shorter-chain (C14 or C16) bilayers.

Size and shape of fast-tumbling bicelles as determined by translational diffusion

In this study, the size and shape of an isotropic bicelle have been determined by measuring the translational diffusion as a function of the volume fraction of the lipids. A linear relation between the diffusion coefficients is obtained for both DMPC and DHPC in the bicelles. The slope of this linear function, which is strongly shape-dependent, is found to be different for the two molecules. This difference is direct evidence that the two molecules are not fully mixed in the bicelle. The shape- combined with the size-dependence of the diffusion coefficient allows us to calculate both the size and shape of the bicelle.

The orientation effect of gramicidin A on bicelles and Eu3+-doped bicelles as studied by solid-state NMR and FT-IR spectroscopy

We have explored the effect of gramicidin A (gA) on bicelle (Bic) orientation in the absence and presence of Eu(3+) by (31)P and (2)H NMR at different DMPC/gA ratios. FT-IR spectroscopy was used to assess the lipid chain ordering and verify the transmembrane peptide conformation. Our results show a time-dependent flipping of the bilayer normal alignment at high temperatures and high proportion of gA. The results are explained by both the diamagnetic susceptibility anisotropy of the beta(6.3) helical peptides and viscosity of the lipid mixture. The concentration effect of gramicidin on Bic/Eu(3+) is compared to that on Eu(3+)-doped DMPC liposomes. The Bic/Eu(3+) system is no longer oriented in the presence of gA and adopts a vesicular morphology while the peptide incorporation induces the formation of ellipsoidal DMPC/Eu(3+) assemblies aligned with their normal parallel to the magnetic field. The difference is explained in terms of lipid chain disorder and size of the bilayers.

Magnetic resonance investigations of lipid motion in isotropic bicelles

The dynamics of DMPC in different isotropic bicelles have been investigated by NMR and EPR methods. The local dynamics were obtained by interpretation of 13C NMR relaxation measurements of DMPC in the bicelles, and these results were compared to EPR spectra of spin-labeled lipids. The overall size of the bicelles was investigated by PFG NMR translational diffusion measurements. The dynamics and relative sizes were compared among three different bicelles: [DMPC]/[DHPC] = 0.25, [DMPC]/[DHPC] = 0.5, and [DMPC]/[CHAPS] = 0.5. The local motion is found to depend much more strongly on the choice of the detergent, rather than the overall size of the bicelle. The results provide an explanation for differences in apparent dynamics for different peptides, which are bound to bicelles. This in turn determines under what conditions reasonable NMR spectra can be observed. A model is presented in which extensive local motion, in conjunction with the overall size, affects the spectral properties. An analytical expression for the size dependence of the bicelles, relating the radius of the bilayer region with physical properties of the detergent and the lipid, is also presented.

Electron paramagnetic resonance studies of magnetically aligned phospholipid bilayers utilizing a phospholipid spin label: The effect of cholesterol

X-band EPR spectroscopy has been employed to study the dynamic properties of magnetically aligned phospholipid bilayers (bicelles) utilizing a variety of phosphocholine spin labels (n-PCSL) as a function of cholesterol content. The utilization of both perpendicular and parallel aligned bicelles in EPR spectroscopy provides a more detailed structural and orientational picture of the phospholipid bilayers. The magnetically aligned EPR spectra of the bicelles and the hyperfine splitting values reveal that the addition of cholesterol increases the phase transition temperature and alignment temperature of the DMPC/DHPC bicelles. The corresponding molecular order parameter, Smol, of the DMPC/DHPC bicelles increased upon addition of cholesterol. Cholesterol also decreased the rotational motion and increased the degree of anisotropy in the interior region of the bicelles. This report reveals that the dynamic properties of DMPC/DHPC bicelles agree well with other model membrane systems and that the magnetically aligned bicelles are an excellent model membrane system.

Comprehensive examination of mesophases formed by DMPC and DHPC mixtures

Mixtures of long- and short-chain phospholipids, specifically 14:0 and 6:0 phosphatidylcholines (DMPC and DHPC), have been used successfully in NMR studies as magnetically alignable substrates for membrane-associated proteins. However, recent publications have shown that the phase behavior of these mixtures is much more complex than originally thought. Using polarized light microscopy and small-angle neutron scattering, phase diagrams of DMPC/DHPC mixtures at molar ratios of 2, 3.2, and 5 have been determined. Generally, at temperatures below the main-chain melting transition of DMPC (T(M) = 23 degrees C), an isotropic phase of disk-like micelles is found. At high temperatures (T > 50 degrees C), a lamellar phase consisting of either multilamellar vesicles (MLV) or extended lamellae is formed, which at low lipid concentrations (e.g., MLV) coexists with an excess of water. At intermediate temperatures and lipid concentrations, a chiral nematic phase made up of worm-like micelles was observed.

Reinvestigation by phosphorus NMR of lipid distribution in bicelles

Mixtures of dimyristoyl-phosphatidylcholine (DMPC) and dihexanoyl-phosphatidylcholine (DHPC) in water form disks also called bicelles and different bilayer organizations when the mol ratio of the two lipids and the temperature are varied. The spontaneous alignment in a magnetic field of these bilayers above the transition temperature T(m) of DMPC is an attractive property that was successfully used to investigate protein structure by NMR. In this article, we have attempted to give an overview of all structural transformations of DMPC/DHPC mixtures that can be inferred from broad band (31)P-NMR spectroscopy between 5 and 60 degrees C. We show that above a critical temperature, T(v), perforated vesicles progressively replace alignable structures. The holes in these vesicles disappear above a new temperature threshold, T(h). The driving force for these temperature-dependent transformations that has been overlooked in previous studies is the increase of DHPC miscibility in the bilayer domain above T(m). Accordingly, we propose a new model (the "mixed bicelle" model) that emphasizes the consequence of the mixing. This investigation shows that the various structures of DMPC in the presence of increasing mol ratios of the short-chain DHPC is reminiscent of the observation put forward by several laboratories investigating solubilization and reconstitution of biological membranes.

Spontaneously formed monodisperse biomimetic unilamellar vesicles: the effect of charge, dilution, and time

Using small-angle neutron scattering and dynamic light scattering, we have constructed partial structural phase diagrams of lipid mixtures composed of the phosphatidylcholines dimyristoyl and dihexanoyl doped with calcium ions (Ca2+) and/or the negatively charged lipid, dimyristoyl phosphatidylglycerol (DMPG). For dilute solutions (lipid concentration < or =1 wt %), spontaneously forming unilamellar vesicles (ULVs) were found, and their polydispersity was determined to be approximately 20%. The stability of the Ca2+- or DMPG-doped ULVs was monitored over a period of 4 days and their structural parameters (e.g., average outer radius, <Ro>) were found to be insensitive to the lipid concentration (Clp). However, doping the dimyristoyl/dihexanoyl system with both Ca2+ and DMPG resulted in ULVs whose <Ro> was found to be Clp dependent. The <Ro> of DMPG-doped ULVs remained unchanged over an extended period of time (at least 4 days), a good indication of their stability.

Lateral diffusion of PEG-Lipid in magnetically aligned bicelles measured using stimulated echo pulsed field gradient 1H NMR

Lateral diffusion measurements of PEG-lipid incorporated into magnetically aligned bicelles are demonstrated using stimulated echo (STE) pulsed field gradient (PFG) proton (1H) nuclear magnetic resonance (NMR) spectroscopy. Bicelles were composed of dimyristoyl phosphatidylcholine (DMPC) plus dihexanoyl phosphatidylcholine (DHPC) (q = DMPC/DHPC molar ratio = 4.5) plus 1 mol % (relative to DMPC) dimyristoyl phosphatidylethanolamine-N-[methoxy(polyethylene glycol)-2000] (DMPE-PEG 2000) at 25 wt % lipid. 1H NMR STE spectra of perpendicular aligned bicelles contained only resonances assigned to residual HDO and to overlapping contributions from a DMPE-PEG 2000 ethoxy headgroup plus DHPC choline methyl protons. Decay of the latter's STE intensity in the STE PFG 1H NMR experiment (g(z) = 244 G cm(-1)) yielded a DMPE-PEG 2000 (1 mol %, 35 degrees C) lateral diffusion coefficient D = 1.35 x 10(-11) m2 s(-1). Hence, below the "mushroom-to-brush" transition, DMPE-PEG 2000 lateral diffusion is dictated by its DMPE hydrophobic anchor. D was independent of the diffusion time, indicating unrestricted lateral diffusion over root mean-square diffusion distances of microns, supporting the "perforated lamellae" model of bicelle structure under these conditions. Overall, the results demonstrate the feasibility of lateral diffusion measurements in magnetically aligned bicelles using the STE PFG NMR technique.

Magnetically alignable phase of phospholipid "bicelle" mixtures is a chiral nematic made up of wormlike micelles

We have studied the phase behavior of binary mixtures of long- and short-chain lipids, namely, dimyristoyl phosphatidylcholine (DMPC) and dihexanoyl phosphatidylcholine (DHPC), using optical microscopy and small-angle neutron scattering. Samples with a total lipid content of 25 wt %, corresponding to ratios Q ([DMPC]/[DHPC]) of 5, 3.2, and 2, are found to exhibit an isotropic (I) --> chiral nematic (N) --> lamellar phase sequence on increasing temperature. The I-N transition coincides with the chain melting transition of DMPC at Q = 5 and 3.2, but the N phase forms at a higher temperature for Q = 2. All three samples form multilamellar vesicles in the lamellar phase. Our results show that disklike "bicellar" aggregates occur only in the lower temperature isotropic phase and not in the higher temperature magnetically alignable N phase, where they were previously believed to exist. The N phase is found to consist of long, flexible wormlike micelles, their entanglement resulting in the very high viscosity of this phase.

Direct observation and characterization of DMPC/DHPC aggregates under conditions relevant for biological solution NMR

We have used cryo-transmission electron microscopy (cryo-TEM) for inspection of aggregates formed by dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC) in aqueous solution at total phospholipid concentrations cL < or = 5% and DMPC/DHPC ratios q < or = 4.0. In combination with ocular inspections, we are able to sketch out this part of phase-diagram at T = 14-80 degrees C. The temperature and the ratio q are the dominating variables for changing sample morphology, while cL to a lesser extent affects the aggregate structure. At q = 0.5, small, possibly disc-shaped, aggregates with a diameter of approximately 6 nm are formed. At higher q-values, distorted discoidal micelles that tend to short cylindrical micelles are observed. The more well-shaped discs have a diameter of around 20 nm. Upon increasing q or the temperature, long slightly flattened cylindrical micelles that eventually branch are formed. A holey lamellar phase finally appears upon further elevation of q or temperature. The implications for biological NMR work are two. First, discs prepared as membrane mimics are frequently much smaller than predicted by current "ideal bicelle" models. Second, the q approximately 3 preparations used for aligning water-soluble biomolecules in magnetic fields consist of perforated lamellar sheets. Furthermore, the discovered sequence of morphological transitions may have important implications for the development of bicelle-based membrane protein crystallization methods.

Electron paramagnetic resonance studies of magnetically aligned phospholipid bilayers utilizing a phospholipid spin label

X-band electron paramagnetic resonance (EPR) spectroscopy was used to study the structural and dynamic properties of magnetically aligned phospholipid bilayers utilizing a variety of phosphocholine spin labels (PCSL) as a function oftemperature. 1-Palmitoyl-2-[n-(4,4-dimethyloxazolidine-N-oxyl)stearoyl]-sn-glycero-3-phosphocholine (n-PCSL) in which a nitroxide group was attached to the different acyl chain positions of the phospholipid (n = 5, 7, 12, and 14) were used as an EPR spin probe to investigate magnetically aligned phospholipid bilayers from the plateau (near to the headgroup) region to the end of the acyl chain (center of the bilayers). The addition of certain types of paramagnetic lanthanide ions changes the overall magnetic susceptibility anisotropy tensor of the bicelles, such that the bicelles flip with their bilayer normal either parallel or perpendicular to the magnetic field. The present study reveals for the first time that, in the case of the n-PCSL, the bilayer normal is aligned parallel and perpendicular to the magnetic field in the presence of lanthanide ions having positive delta(chi) (e.g., Tm3+) and negative delta(chi) (e.g., Dy3+), respectively. The magnetic alignment of the bilayers and the corresponding segmental molecular order parameter, S(mol), were investigated as a function of the temperature. The S(mol) values decrease in the following order, 5-PCSL > 7-PCSL > 12-PCSL > 14-PCSL, for the magnetically aligned phospholipid bilayers. Also, the variable temperature study indicates that, by increasing the temperature, the order parameters S(mol) decreased for all the n-PCSLs. The results indicate that magnetically aligned phospholipid bilayers represent an excellent model membrane system for X-band EPR studies.

A multidimensional 1H NMR investigation of the conformation of methionine-enkephalin in fast-tumbling bicelles

Enkephalins are pentapeptides found in the central nervous system. It is believed that these neuropeptides interact with the nerve cell membrane to adopt a conformation suitable for their binding to an opiate receptor. In this work, we have determined the three-dimensional structure of methionine-enkephalin (Menk) in fast-tumbling bicelles using multidimensional (1)H NMR. Bicelles were selected as model membranes because both their bilayer organization and composition resemble those of natural biomembranes. The effect of the membrane composition on the peptide conformation was explored using both zwitterionic (PC bicelles) and negatively charged bicelles (Bic/PG). Pulsed field gradient experiments allowed the determination of the proportion of Menk bound to the model membranes. Approximately 60% of the water-soluble enkephalin was found to associate to the bicellar systems. Structure calculations from torsion angle and NOE-based distance constraints suggest the presence of both micro - and delta-selective conformers of Menk in each system and slightly different conformers in PC bicelles and Bic/PG. As opposed to previous studies of enkephalins in membrane mimetic systems, our results show that these opiate peptides could adopt several conformations in a membrane environment, which is consistent with the flexibility and poor selectivity of enkephalins.

Structure and positioning comparison of two variants of penetratin in two different membrane mimicking systems by NMR

The Antennapedia homeodomain protein of Drosophila has the ability to penetrate biological membranes and the third helix of this protein, residues 43-58, known as penetratin (RQIKIWFQNRRMKWKK-amide) has the same translocating properties as the entire protein. The variant, RQI KIFFQNRRMKFKK-amide, here called penetratin (W48F,W56F) does not have the same ability. We have determined a solution structure of penetratin and investigated the position of both peptides in negatively charged bicelles. A helical structure is seen for residues Lys46 through Met54. The secondary structure of the variant penetratin(W48F,W56F) in bicelles appears to be very similar. Paramagnetic spin-label studies and analysis of NOEs between penetratin and the phospholipids show that penetratin is located within the bicelle surface. Penetratin (W48F,W56F) is also located inside the phospholipid bicelle, however, with its N-terminus more deeply inserted than that of wild-type penetratin. The subtle differences in the way the two peptides interact with a membrane in an equilibrium situation could be important for their translocating ability. As a comparison we have also investigated the secondary structure of penetratin(W48F,W56F) in SDS micelles and the results show that the structure is very similar in SDS and bicelles. In contrast, penetratin(W48F,W56F) and penetratin appear to be located differently in SDS micelles. This clearly shows the importance of using realistic membrane mimetics for investigating peptide-membrane interactions.

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.

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.

Methods for studying transmembrane peptides in bicelles: consequences of hydrophobic mismatch and peptide sequence

We have shown that bicelles prepared from dilauryl phosphatidylcholine (DLPC) and dipalmitoyl phosphatidylcholine (DPPC) align in a magnetic field under conditions similar to the more common dimyristoyl phosphatidylcholine (DMPC) bicelles. In addition, a model transmembrane peptide, P16, with a hydrophobic stretch of 24 A, and specific alanine-d(3) labels, was incorporated into all of the different bicelles. The long-chain phospholipid (DLPC, DMPC, or DPPC) remained unperturbed upon incorporation of the peptide while the quadrupolar splitting of the short-chain phospholipid along the bicelle rim increased by varying degrees in the different bicelle systems. The change in quadrupolar splitting of the short-chain phospholipids was attributed to changes in either fluidity of the planar region of the bicelle or differences in overall lipid packing. When the hydrophobic stretch of the bilayer was 22.8 (DMPC) or 26.3 A (DPPC), the peptide tilt was found to be transmembrane (33-35 degrees with respect to the bicelle normal). When the hydrophobic stretch of the bilayer was 19.5 A (DLPC), the peptide quadrupolar splittings suggested a loss of transmembrane orientation. When tryptophan was incorporated in the middle of the transmembrane region, the transmembrane orientation was also lost.

SANS study on the effect of lanthanide ions and charged lipids on the morphology of phospholipid mixtures. Small-angle neutron scattering

The structural phase behavior of phospholipid mixtures consisting of short-chain (dihexanoyl phosphatidylcholine) and long-chain lipids (dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol), with and without lanthanide ions was investigated by small-angle neutron scattering (SANS). SANS profiles were obtained from 10 degrees C to 55 degrees C using lipid concentrations ranging from 0.0025 g/ml to 0.25 g/ml. The results reveal a wealth of distinct morphologies, including lamellae, multi-lamellar vesicles, unilamellar vesicles, and bicellar disks.

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

The morphologies of three dilute liquid crystalline phases, which are widely used for biological NMR spectroscopy, are investigated by the study of tracer self-diffusion. The aqueous liquid crystalline media investigated include the common phospholipid bicelle medium, a phase consisting of a mixture of pentaethyleneglycol mono dodecyl ether and hexanol, and a medium containing cetylpyridinium bromide and hexanol. Threonine and water were used as tracer molecules for probing the aqueous environment, and tetramethylsilane (TMS) was for probing the lipophilic environment. Pulsed field gradient NMR was used to measure tracer self-diffusion rates in three orthogonal directions. Although results for the water-soluble tracers in bicelle media do not contradict the widely accepted disk-shaped bicelle model, the high TMS diffusion rate observed in the bilayer plane requires extensive transient edge-to-edge contacts of such disks. This morphology is essentially that of a heavily perforated lamellar bilayer phase and explains why this medium remains liquid crystalline well below the Onsager limit for disk-shaped nematogens. Below 25 degrees C, a bicelle mixture consisting of dimyristoyl phosphatidyl choline and dihexanoyl phosphatidyl choline remains isotropic, but tracer diffusion obstruction indicates that the particles are significantly oblate. The diffusion anisotropy in the penta(ethyleneglycol) mono dodecyl ether liquid crystals confirms the previously proposed alpha-lamellar phase. However, weak inhibition of aqueous-phase self-diffusion in the z direction points to the presence of bridge- or caplike obstructions, and the bilayers appear slightly permeable to water. If the previously proposed concentric cylinder superstructure of bilayers applies, the diffusion data indicate that the most outer cylinder must have a diameter greater than 50 microm. The tracer self-diffusion data for the cetylpyridinium bromide/hexanol medium is only compatible with a planar alpha-lamellar phase, with its local director orthogonal to the magnetic field, and a very large domain size over which the director remains parallel.

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.

Europium III binding and the reorientation of magnetically aligned bicelles: insights from deuterium NMR spectroscopy

Solid-state deuterium ((2)H) NMR spectroscopy was used to study the reorientation of magnetically ordered bicelles in the presence of the paramagnetic lanthanide Eu(3+). Bicelles were composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) plus 1,2-dihexanoyl-sn-glycero-3-phosphocholine plus either the anionic lipid 1,2-dimyristoyl-sn-3-phosphoglycerol, or the cationic lipid 1,2-dimyristoyl-3-trimethyl ammonium propane. Alignment of the bicelles in the magnetic field produced (2)H NMR spectra consisting of a pair of quadrupole doublets, one from the alpha-deuterons and one from the beta-deuterons of DMPC-alpha,beta-d(4). Eu(3+) addition induced the appearance of a second set of quadrupole doublets, having approximately twice the quadrupolar splittings of the originals, and growing progressively in intensity with increasing Eu(3+), at the expense of the intensity of the originals. The new resonances were attributed to bicelles having a parallel alignment with respect to the magnetic field, as opposed to the perpendicular alignment preferred in the absence of Eu(3+). Therefore, the equilibrium degree and kinetics of reorientation could be evaluated from the (2)H NMR spectra. For more cationic initial surface charges, higher amounts of added Eu(3+) were required to induce a given degree of reorientation. However, the equilibrium degree of bicellar reorientation was found to depend solely on the amount of bound Eu(3+), regardless of the bicelle composition. The kinetics of reorientation were a function of lipid concentration. At high lipid concentration, a single fast rate of reorientation (minutes) described the approach to the equilibrium degree of orientation. At lower lipid concentrations, two rates processes were discernible: one fast (minutes) and one slow (hours). The data indicate, therefore, that bicelle reorientation is a phase transition made critical by bicelle-bicelle interactions.

Solution structure of DinI provides insight into its mode of RecA inactivation

The Escherichia coli RecA protein triggers both DNA repair and mutagenesis in a process known as the SOS response. The 81-residue E. coli protein DinI inhibits activity of RecA in vivo. The solution structure of DinI has been determined by multidimensional triple resonance NMR spectroscopy, using restraints derived from two sets of residual dipolar couplings, obtained in bicelle and phage media, supplemented with J couplings and a moderate number of NOE restraints. DinI has an alpha/beta fold comprised of a three-stranded beta-sheet and two alpha-helices. The beta-sheet topology is unusual: the central strand is flanked by a parallel and an antiparallel strand and the sheet is remarkably flat. The structure of DinI shows that six negatively charged Glu and Asp residues on DinI's kinked C-terminal alpha-helix form an extended, negatively charged ridge. We propose that this ridge mimics the electrostatic character of the DNA phospodiester backbone, thereby enabling DinI to compete with single-stranded DNA for RecA binding. Biochemical data confirm that DinI is able to displace ssDNA from RecA.