Bicelles: a model membrane system for all seasons?

NMR spectroscopic characterization of the membrane affinity of polyols

Residual dipolar couplings (RDCs) are applied here for the analysis of weak, transient binding events between phosphatidylcholine bilayers and polyols. Large signal responses are observed even for low percentages of 'ligand-receptor complexes', making RDCs a sensitive tool for the analysis of molecular recognition events. The different degree of alignment in solution can be compared as a result of the calculation of the alignment tensor elements. By varying polarity and/or charge of the molecules under investigation, nonspecific hydrophobic effects can be excluded.

Toward bicelle stability with ether-linked phospholipids: temperature, composition, and hydration diagrams by 2H and 31P solid-state NMR

Phosphorus and deuterium wide line NMR was used to determine diagrams of binary mixtures of 1,2-di-O-tetradecyl-sn-glycero-3-phosphocholine (DIOMPC) and 1,2-di-O-hexyl-sn-glycero-3-phosphocholine (DIOHPC) ether-phospholipids. By varying the hydration, h, the temperature, T, and the mole fraction, X, of long-chain ether-phospholipids, we delineated the conditions for which such systems are oriented by the magnetic field, in the presence of 100 mM KCl. The 3D domain is found for X = 62-90%, T = 27-50 degrees C, and h = 70-98%. At 80% hydration, the domain shape (X = 70-90% and T = 27-42 degrees C) is close to that already observed for ester-phospholipids mixtures (Raffard, G.; Steinbruckner, S.; Arnold, A.; Davis, J. H.; Dufourc, E. J. Langmuir 2000, 16, 7655-7662) where disc-shaped bicelles of 300-600 A have been found by electron microscopy (Arnold, A.; Labrot, T.; Oda, R.; Dufourc, E. J. Biophys. J. 2002, 83, 2667-2680). Systems made of ether-linked lipids are much more stable on time and acidic conditions than those made of ester lipids. Assuming that the disc-shaped species are also found with ether lipids, their diameter as determined from integration of phosphorus NMR lines ranges from 240 to 440 A +/- 10%; it is generally independent of hydration and temperature but decreases with decreasing long-chain lipid content, X. The structure and the dynamics of water in the DIOMPC-DIOHPC were characterized by (2)H NMR. Water exchanges between the membrane surface where it is bound and a bulk isotropic pool lead to an average ordered state for temperatures in the bicelle region and above, thus offering a larger thermal span for structural studies of dissolved molecules.

Bicelle membranes and their use for hydrophobic peptide studies by circular dichroism and solid state NMR

Mixtures of dicaproyl- (DC), dimyristoyl- (DM) and 1-tetradecanoyl-2-biphenylbutanoyl-(TBB) phosphatidylcholine (PC) in water produce bicelle membranes that are oriented by magnetic fields. DMPC/DCPC systems orient such that their membrane plane is parallel to the magnetic field, whereas for TBBPC/DCPC, the plane is perpendicular to the field. Partial temperature-composition-hydration diagrams are established using solid-state 31P-NMR. DMPC/DCPC bicelles exist on a large range of composition but on a narrow temperature domain (25-45 degrees C). At converse, TBBPC/DCPC form bicelles on a narrow compositional range but over a large temperature span (10-70 degrees C). The TBBPC/DCPC bicelles are shown to be a very powerful potential tool to study the orientation of hydrophobic helices in membranes using wide line 15N-NMR. The DMPC/DCPC system that undergoes a micelle-to-bicelle transition on going from 10 degrees C to 40 degrees C may be used with circular dichroism to study the state of association of hydrophobic helices within the membrane. Results suggest that the transmembrane fragment of the neu/erbB-2 receptor is monomeric in micellar medium and dimeric/multimeric in bicelle membranes.

Influence of the long-chain/short-chain amphiphile ratio on lateral diffusion of PEG-lipid in magnetically aligned lipid bilayers as measured via pulsed-field-gradient NMR

Lateral diffusion measurements of polyethylene glycol(PEG)-lipid incorporated into magnetically aligned lipid bilayers, composed of dimyristoyl phosphatidylcholine (DMPC) plus dihexanoyl phosphatidylcholine (DHPC) plus 1 mol % (relative to DMPC) dimyristoyl phosphatidylethanolamine-n-[methoxy(polyethylene glycol)-2000] (DMPE-PEG 2000), were performed using stimulated-echo pulsed-field-gradient proton ((1)H) nuclear magnetic resonance spectroscopy. The DMPE-PEG 2000 (1 mol %, 35 degrees C) lateral diffusion coefficient D varied directly with the mole fraction of DMPC, X(DMPC) = q/(1+q) where q = DMPC/DHPC molar ratio, decreasing progressively from D = 1.65 x 10(-11) m(2) s(-1) at q approximately 4.7 to D = 0.65 x 10(-11) m(2) s(-1) at q approximately 2.5. Possible sources of this dependence, including orientational disorder, obstruction, and PEG-lipid sequestration, were simulated using, respectively, a diffusion-in-a-cone model, percolation theory, and a two-phase PEG distribution model. Orientational disorder alone was not capable of reproducing the observations, but in combination with either obstruction or PEG-lipid two-phase distribution models did so satisfactorily. A combination of all three models yielded the most reasonable fit to the observed dependence of lateral diffusion on q. These same effects would be expected to influence lateral diffusion of any bilayer-associating species in such systems.

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.

pH effects on drug interactions with lipid bilayers by liposome electrokinetic chromatography

Liposome electrokinetic chromatography (LEKC) provides convenient and rapid methods for studying drug interactions with lipid bilayers using liposomes as a pseudostationary phase. LEKC was used to determine the effects of pH on the partitioning of basic drugs into liposomes composed of zwitterionic phosphatidylcholine (PC), anionic phosphatidylglycerol (PG), and cholesterol, which mimic the composition of natural cell membranes. An increase in pH results in a smaller degree of ionization of the basic drugs and consequently leads to a lower degree of interaction with the negatively charged membranes. From the LEKC retention data, the fractions of drugs distributed in the bulk aqueous and the liposome phase were determined at various pH values. Finally, lipid mediated shifts in the ionization constants of drugs were examined.

High-resolution NMR spectroscopy of membrane proteins in aligned bicelles

High-resolution solid-state NMR spectra can be obtained from uniformly (15)N-labeled membrane proteins in magnetically aligned bicelles. Fast uniaxial diffusion about the axis of the bilayer normal results in single-line spectra that contain the orientational information necessary for protein structure determination.

Detergent-like properties of magainin antibiotic peptides: a 31P solid-state NMR spectroscopy study

(31)P solid-state NMR spectroscopy has been used to investigate the macroscopic phase behavior of phospholipid bilayers in the presence of increasing amounts of magainin antibiotic peptides. Addition of >1 mol% magainin 2 to gel-phase DMPC or liquid crystalline POPC membranes respectively, results in (31)P NMR spectra that are characterized by the coexistence of isotropic signals and line shapes typical for phospholipid bilayers. The isotropic signal intensity is a function of temperature and peptide concentration. At peptide concentrations >4 mol% of the resulting phospholipid (31)P NMR spectra are characteristic of magnetically oriented POPC bilayers suggesting the formation of small disk-like micelles or perforated sheets. In contrast, addition of magainin to acidic phospholipids results in homogenous bilayer-type (31)P NMR spectra with reduced chemical shift anisotropies. The results presented are in good agreement with the interfacial insertion of magainin helices with an alignment parallel to the surface of the phospholipid bilayers. The resulting curvature strain results in detergent-like properties of the amphipathic helical peptides.

Micellar aggregates formed following the addition of hexafluoroisopropanol to phospholipid membranes

The addition of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) to aqueous phospholipid membranes leads to perturbation of the bilayer. In the case of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), calorimetric and small-angle X-ray scattering analyses indicate that effects are already apparent at bound molar HFIP/lipid ratios of less than 1:150, with a pronounced decrease in the temperature of the main (gel to liquid crystalline) phase transition and a decrease in the intensity of the first- and second-order scattering reflections. As the HFIP concentration is raised further, at bound molar HFIP/lipid ratios >2:1, uniform isotropic particulate structures are formed with higher intrinsic curvature than the parent liposomes. These observations are supported by the results of thin-film experiments and are consistent with the formation of DMPC/HFIP adducts that are detergent-like in nature. In the case of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) the effects are much less marked, with no blebbing observed over a comparable range of HFIP concentrations. Although HFIP interacts strongly with DOPC membranes, it appears that membrane rupture is not promoted as readily with this lipid. Data from electron microscopy, laser correlation spectroscopy, and marker release experiments suggest that some of the immediate (nonequilibrium) effects of HFIP on membranes are the consequence of microinhomogeneity in water/HFIP mixtures. On the basis of our observations, we propose a model for the interaction of HFIP with phospholipid membranes.

Lipid bilayer tethered inside a nanoporous support: a solid-state nuclear magnetic resonance investigation

(31)P and (1)H solid-state nuclear magnetic resonance (NMR) experiments have been designed with the aim of studying directly the formation of supported bilayers tethered inside nanoporous aluminum oxide supports as a model of biomimetic membranes. The static and magic angle spinning (31)P NMR spectra of the supported bilayers have been compared with the experimental and simulated spectra of a simpler model with cylindrical geometry, namely a phospholipid bilayer adsorbed on an oriented polymer sheet. The broadening observed for the nanoporous model is most likely due to the presence of paramagnetic ions in the aluminum oxide. A phospholipid lateral diffusion coefficient of (2.8 +/- 0.4) x 10(-8) cm(2)/s has been measured for the tethered bilayer on a spherical support, indicating a good fluidity as compared with adsorbed membrane models.

Amantadine partition and localization in phospholipid membrane: a solution NMR study

Quantification of membrane partition potential of drug compounds is of great pharmaceutical interest. Here, a novel approach combining liquid-state NMR diffusion measurements and fast-tumbling lipid/detergent bicelles is used to measure accurately the partition coefficient K(p) of amantadine in phospholipid bilayers. Amantadine is found to have a strong membrane partition potential, with K(p) of 27.6 in DMPC and 37.8 in POPC lipids. Electrostatic interaction also plays a major role in the drug's affinity towards biological membrane as introduction of negatively charged POPG dramatically increases its K(p). Saturation transfer difference experiments in small bicelles indicate that amantadine localizes near the negatively charged phosphate group and the hydrocarbon chain of bilayer lipid. The approach undertaken in this study is generally applicable for characterizing interactions between small molecules and phospholipid membranes.

Investigating magnetically aligned phospholipid bilayers with EPR spectroscopy at Q-band (35 GHz): optimization and comparison with X-band (9 GHz)

This paper presents the improvement and advantages of investigating magnetically aligned phospholipid bilayers (bicelles) utilizing electron paramagnetic resonance (EPR) spectroscopy at a microwave frequency of 35 GHz (Q-band) and at a high magnetic field strength of 1.25 T when compared to weaker magnetic fields for X-band EPR studies. The nitroxide spin label 3beta-doxyl-5alpha-cholestane (cholestane or CLS) was inserted into the bicelles and utilized to demonstrate the effects of macroscopic bilayer alignment through the measurement of orientational dependent hyperfine splittings. The effects of different lanthanide ions with varying degree of magnetic susceptibility anisotropy were examined. The requirement of minimal amounts of the Tm3+ and Dy3+ lanthanide ions for well-aligned bicelles were examined for Q-band and compared with amounts required for X-band bicelle alignment studies. At a magnetic field of 1.25 T (when compared to 0.63 T at X-band), the perpendicular and parallel orientation were aligned with lower concentrations of Dy3+ and Tm3+, respectively, and thereby eliminating/minimizing the unwanted effects associated with lanthanide-protein interactions. Thus, it is much easier to magnetically align phospholipid bilayers at Q-band when compared to X-band.

Membrane proteins, lipids and detergents: not just a soap opera

Studying membrane proteins represents a major challenge in protein biochemistry, with one of the major difficulties being the problems encountered when working outside the natural lipid environment. In vitro studies such as crystallization are reliant on the successful solubilization or reconstitution of membrane proteins, which generally involves the careful selection of solubilizing detergents and mixed lipid/detergent systems. This review will concentrate on the methods currently available for efficient reconstitution and solubilization of membrane proteins through the use of detergent micelles, mixed lipid/detergent micelles and bicelles or liposomes. We focus on the relevant molecular properties of the detergents and lipids that aid understanding of these processes. A significant barrier to membrane protein research is retaining the stability and function of the protein during solubilization, reconstitution and crystallization. We highlight some of the lessons learnt from studies of membrane protein folding in vitro and give an overview of the role that lipids can play in stabilizing the proteins.

Homotropic cooperativity of monomeric cytochrome P450 3A4 in a nanoscale native bilayer environment

Mechanistic studies of mammalian cytochrome P450s are often obscured by the phase heterogeneity of solubilized preparations of membrane enzymes. The various protein-protein aggregation states of microsomes, detergent solubilized cytochrome or a family of aqueous multimeric complexes can effect measured substrate binding events as well as subsequent steps in the reaction cycle. In addition, these P450 monooxygenases are normally found in a membrane environment and the bilayer composition and dynamics can also effect these catalytic steps. Here, we describe the structural and functional characterization of a homogeneous monomeric population of cytochrome P450 3A4 (CYP 3A4) in a soluble nanoscale membrane bilayer, or Nanodisc [Nano Lett. 2 (2002) 853]. Cytochrome P450 3A4:Nanodisc assemblies were formed and purified to yield a 1:1 ratio of CYP 3A4 to Nanodisc. Solution small angle X-ray scattering was used to structurally characterize this monomeric CYP 3A4 in the membrane bilayer. The purified CYP 3A4:Nanodiscs showed a heretofore undescribed high level of homotropic cooperativity in the binding of testosterone. Soluble CYP 3A4:Nanodisc retains its known function and shows prototypic hydroxylation of testosterone when driven by hydrogen peroxide. This represents the first functional characterization of a true monomeric preparation of cytochrome P450 monooxygenase in a phospholipid bilayer and elucidates new properties of the monomeric form.

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.

Magnetic field dependence of residual dipolar couplings measured in dilute liquid crystalline media

The effect of the magnetic field strength on the magnitude of residual dipolar couplings introduced by dilute liquid crystal media was investigated. One-bond heteronuclear residual dipolar couplings and residual deuterium quadrupolar splitting of the water were monitored at various static magnetic fields. It is suggested that the value of residual anisotropic NMR parameters resulting from exchange between ordered and non-ordered molecules decreases with increasing magnetic fields.

Effects of small neutral molecules on phospholipid bicelle ordering

The effects of small neutral molecules on the liquid-crystalline ordering of dimyristoyl-phosphatidylcholine (DMPC)/dihexanoyl-phosphatidylcholine (DHPC) bicelles (q = 3.0 and 3.5) were studied via 2H, 31P, and 13C variable-temperature NMR. The addition of chloroform (up to approximately 90 mM, with a lipid concentration of approximately 120 mM) was observed to reduce the temperature onset of bicelle ordering by up to approximately 10 degrees C, likely resulting from the depression of the DMPC phase transition temperature. The temperature for the collapse of the bicelle phase was also significantly reduced; the observed effects amount to a downward shift in temperature (and reduction in range) of the liquid-crystalline portion of the bicelle phase diagram with increasing dopant concentration. Other model dopants (e.g., tetrahydrofuran and benzene) yielded smaller effects. Additionally, the variable bicelle alignment permitted the characterization of the ordering of chloroform molecules within the lipid phase.

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.

Investigating fatty acids inserted into magnetically aligned phospholipid bilayers using EPR and solid-state NMR spectroscopy

This is the first time (2)H solid-state NMR spectroscopy and spin-labeled EPR spectroscopy have been utilized to probe the structural orientation and dynamics of a stearic acid incorporated into magnetically aligned phospholipid bilayers or bicelles. The data gleaned from the two different techniques provide a more complete description of the bilayer membrane system. Both methods provided similar qualitative information on the phospholipid bilayer, high order, and low motion for the hydrocarbon segment close to the carboxyl groups of the stearic acid and less order and more rapid motion at the end towards the terminal methyl groups. However, the segmental order parameters differed markedly due to the different orientations that the nitroxide and C-D bond axes transform with the various stearic acid acyl chain conformations, and because of the difference in dynamic sensitivity between NMR and EPR over the timescales examined. 5-, 7-, 12-, and 16-doxylstearic acids spin-labels were used in the EPR experiments and stearic acid-d(35) was used in the solid-state NMR experiments. The influence of the addition of cholesterol and the variation of temperature on the fatty acid hydrocarbon chain ordering in the DMPC/DHPC phospholipid bilayers was also studied. Cholesterol increased the degree of ordering of the hydrocarbon chains. Conversely, as the temperature of the magnetically aligned phospholipid bilayers increased, the order parameters decreased due to the higher random motion of the acyl chain of the stearic acid. The results indicate that magnetically aligned phospholipid bilayers are an excellent model membrane system and can be used for both NMR and EPR studies.

Diffusion and dynamics of penetratin in different membrane mimicking media

The interaction between the cell-penetrating peptide (CPP) penetratin and different membrane mimetic environments has been investigated by two different NMR methods: 15N spin relaxation and translational diffusion. Diffusion coefficients were measured for penetratin in neutral and in negatively charged bicelles of different size, in sodium dodecyl sulfate micelles (SDS), and in aqueous solution. The diffusion coefficients were used to estimate the amount of free and bicelle/micelle-bound penetratin and the results revealed that penetratin binds almost fully to all studied membrane mimetics. 15N relaxation data for three sites in penetratin were interpreted with the model-free approach to obtain overall and local dynamics. Overall correlation times for penetratin were in agreement with findings for other peptides of similar size in the same solvents. Large differences in order parameters were observed for penetratin in the different membrane mimetics. Negatively charged surfaces were seen to restrict motional flexibility, while a more neutral membrane mimetic did not. This indicates that although the peptide binds to both bicelles and SDS micelles, the interaction between penetratin and the various membrane mimetics is different.

Membrane Mimetic Environments Alter the Conformation of the Outer Membrane Protein BtuB

Membrane mimetic environments formed from detergents or short-chain phospholipids are widely utilized in structural studies of membrane proteins. Using site-directed spin labeling (SDSL), we show that micelle and isotropic bicellar environments alter the N-terminal region of BtuB, the outer membrane vitamin B12 transporter found in Escherichia coli. These membrane mimetic systems promote an unfolding of the N-terminus of the protein that does not occur when the protein is in either native or reconstituted bilayers. The N-terminal Ton box of BtuB has been shown to exist in two conformations, depending upon the presence or absence of substrate. However, the detergent-destabilized conformation is different from either the substrate-free or the substrate-bound form of this transporter. This example demonstrates that membrane mimetic systems will not always substitute for the lamellar bilayer environment provided by a biological membrane.

Investigating magnetically aligned phospholipid bilayers with various lanthanide ions for X-band spin-label EPR studies

This paper reports the EPR spectroscopic characterization of a model membrane system that magnetically aligns with a variety of different lanthanide ions in the applied magnetic field (<1 T) of an X-band EPR spectrometer. The ability to align phospholipid bilayer systems is valuable because the anisotropic spectra provide a more detailed and complete description of the structural and motional properties of the membrane-associated spin label when compared to randomly dispersed EPR spectra. The nitroxide spin probe 3beta-doxyl-5alpha-cholestane (cholestane or CLS) was inserted into the bilayer discs to demonstrate the effects of macroscopic bilayer alignment through the measurement of orientational dependent hyperfine splittings. The effects of different lanthanide ions with varying degrees of magnetic susceptibility anisotropy and relaxation properties were examined. For X-band EPR studies, the minimal amounts of the Tm(3+), Yb(3+), and Dy(3+) lanthanide ions needed to align the phospholipid bilayers were determined. Power saturation EPR experiments indicate that for the sample compositions described here, the spin-lattice relaxation rate of the CLS spin label was increased by varying amounts in the presence of different lanthanide (Gd(3+), Dy(3+), Er(3+), Yb(3+), and Tm(3+)) ions, and in the presence of molecular oxygen. The addition of Gd(3+) caused a significant increase in the spin-lattice relaxation rate of CLS when compared to the other lanthanide ions tested.

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).

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.

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

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

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.

Bicelle crystallization: a new method for crystallizing membrane proteins yields a monomeric bacteriorhodopsin structure

Obtaining crystals of membrane proteins that diffract to high resolution remains a major stumbling block in structure determination. Here we present a new method for crystallizing membrane proteins from a bicelle forming lipid/detergent mixture. The method is flexible and simple to use. As a test case, bacteriorhodopsin (bR) from Halobacterium salinarum was crystallized from a bicellar solution, yielding a new bR crystal form. The crystals belong to space group P2(1) with unit cell dimensions of a=45.0 A, b=108.9 A, c=55.9 A, beta=113.58 degrees and a dimeric asymmetric unit. The structure was solved by molecular replacement and refined at 2.0 A resolution. In all previous bR structures the protein is organized as a parallel trimer, but in the crystals grown from bicelles, the individual bR subunits are arranged in an antiparallel fashion.

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.

Morphology of fast-tumbling bicelles: a small angle neutron scattering and NMR study

Bilayered micelles, or bicelles, which consist of a mixture of long- and short-chain phospholipids, are a popular model membrane system. Depending on composition, concentration, and temperature, bicelle mixtures may adopt an isotropic phase or form an aligned phase in magnetic fields. Well-resolved (1)H NMR spectra are observed in the isotropic or so-called fast-tumbling bicelle phase, over the range of temperatures investigated (10-40 degrees C), for molar ratios of long-chain lipid to short-chain lipid between 0.20 and 1.0. Small angle neutron scattering data of this phase are consistent with the model in which bicelles were proposed to be disk-shaped. The experimentally determined dimensions are roughly consistent with the predictions of R.R. Vold and R.S. Prosser (J. Magn. Reson. B 113 (1996)). Differential paramagnetic shifts of head group resonances of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC), induced by the addition of Eu(3+), are also consistent with the bicelle model in which DHPC is believed to be primarily sequestered to bicelle rims. Selective irradiation of the DHPC aliphatic methyl resonances results in no detectable magnetization transfer to the corresponding DMPC methyl resonances (and vice versa) in bicelles, which also suggests that DHPC and DMPC are largely sequestered in the bicelle. Finally, (1)H spectra of the antibacterial peptide indolicidin (ILPWKWPWWPWRR-NH(2)) are compared, in a DPC micellar phase and the above fast-tumbling bicellar phases for a variety of compositions. The spectra exhibit adequate resolution and improved dispersion of amide and aromatic resonances in certain bicelle mixtures.

Investigating magnetically aligned phospholipid bilayers with EPR spectroscopy at 94 GHz

In this paper, we report our initial results on studying magnetically aligned phospholipid bilayers (bicelles) at high magnetic fields (approximately 3.4 T) with electron paramagnetic resonance (EPR) spectroscopy at 95 GHz (W-band). In order to characterize this system for W-band EPR studies, we have utilized the nitroxide spin probe 3beta-doxyl-5alpha-cholestane to demonstrate the effects of macroscopic bilayer alignment. At W-band due to the increase in magnetic field strength (when compared to X-band studies at 9.5 GHz) (S. M. Garber et al., J. Am. Chem. Soc. 121, 3240-3241 (1999)), we were able to examine magnetically aligned phospholipid bilayers at two orientations with the bilayer normal oriented either perpendicular or parallel (upon addition of YbCl3) with respect to the direction of the static magnetic field. Additionally, at a magnetic field of 3.4 T (g=2 resonance at W-band), we were able to study the parallel alignment with a lower concentration of Yb3+, thereby eliminating the possible unwanted effects associated with lanthanide-protein interactions and paramagnetic shifts and/or line broadening induced by the lanthanide ions. The development of this new spin label alignment technique will open up a whole new area of investigation for phospholipid bilayer systems and membrane protein EPR studies at high magnetic fields.

Development of magnetically aligned phospholipid bilayers in mixtures of palmitoylstearoylphosphatidylcholine and dihexanoylphosphatidylcholine by solid-state NMR spectroscopy

This study reports the solid-state NMR spectroscopic characterization of a long chain phospholipid bilayer system which spontaneously aligns in a static magnetic field. Magnetically aligned phospholipid bilayers or bicelles are model systems which mimic biological membranes for magnetic resonance studies. The oriented membrane system is composed of a mixture of the bilayer forming phospholipid palmitoylstearoylphosphatidylcholine (PSPC) and the short chain phospholipid dihexanoylphosphatidylcholine (DHPC) that breaks up the extended bilayers into bilayered micelles or bicelles that are highly hydrated (approx. 75% aqueous). Traditionally, the shorter 14 carbon chain phospholipid dimyristoylphosphatidylcholine (DMPC) has been utilized as the bilayer forming phospholipid in bicelle studies. Alignment (perpendicular) was observed with a PSPC/DHPC q ratio between 1.6 and 2.0 slightly above T(m) at 50 degrees C with (2)H and (31)P NMR spectroscopy. Paramagnetic lanthanide ions (Yb(3+)) were added to flip the bilayer discs such that the bilayer normal was parallel with the static magnetic field. The approx. 1.8 (PSPC/DHPC) molar ratio yields a thicker membrane due to the differences in the chain lengths of the DMPC and PSPC phospholipids. The phosphate-to-phosphate thickness of magnetically aligned PSPC/DHPC phospholipid bilayers in the L(alpha) phase may enhance the activity and/or incorporation of different types of integral membrane proteins for solid-state NMR spectroscopic studies.

Determination of membrane immersion depth with O(2): a high-pressure (19)F NMR study

Oxygen is known to partition with an increasing concentration gradient toward the hydrophobic membrane interior. At partial pressures (P(O2)) of 100 Atm or more, this concentration gradient is sufficient to induce paramagnetic effects that depend sensitively on membrane immersion depth. This effect is demonstrated for the fluorine nucleus by depth-dependent paramagnetic shifts and spin-lattice relaxation rates, using a fluorinated detergent, CF3(CF(2))(5)C(2)H(4)-O-maltose (TFOM), reconstituted into a lipid bilayer model membrane system. To interpret the spin-lattice relaxation rates (R) in terms of a precise immersion depth, two specifically fluorinated cholesterol species (6-fluorocholesterol and 25-fluorocholesterol), whose membrane immersion depths were independently estimated, were studied by (19)F NMR. The paramagnetic relaxation rates, R, of the cholesterol species were then used to parameterize a Gaussian profile that directly relates R to immersion depth z. This same Gaussian curve could then be used to determine the membrane immersion depth of all six fluorinated chain positions of TFOM and of two adjacent residues of specifically fluorinated analogs of the antibacterial peptide indolicidin. The potential of this method for determination of immersion depth and topology of membrane proteins is discussed.

Structure determination of membrane-associated proteins from nuclear magnetic resonance data

This Review covers the delineation and optimization of protein-lipid systems for study using solution-state NMR spectroscopy. The first half presents the necessary background for a membrane protein biochemist to initiate collaboration with an NMR spectroscopist. The second half provides guidelines for the spectroscopist on data collection, analysis for obtaining conformational information, and structure generation and assessment. Although the emphasis is on the study of peptides in detergent micelles, methods are outlined for larger membrane-associated proteins and for use of other solubilizing agents.

Customizing model membranes and samples for NMR spectroscopic studies of complex membrane proteins

Both solution and solid state nuclear magnetic resonance (NMR) techniques for structural determination are advancing rapidly such that it is possible to contemplate bringing these techniques to bear upon integral membrane proteins having multiple transmembrane segments. This review outlines existing and emerging options for model membrane media for use in such studies and surveys the special considerations which must be taken into account when preparing larger membrane proteins for NMR spectroscopic studies.

Functionality of a membrane protein in bicelles

Bicelles are bilayered discoidal lipid-detergent assemblies which are useful as model membranes. To date, there has been no direct demonstration of functional viability for an integral membrane protein reconstituted into bicelles. In this contribution, the catalytic activity of diacylglycerol kinase (DAGK) was measured following reconstitution into several different bicelle systems and compared to activities measured in traditional mixed micelles and vesicles. For the most optimal bicelle systems tested, DAGK activities approached those observed in mixed micelles or vesicles. For some other bicellar mixtures tested, activities were much lower, with steady-state kinetic data indicating reduced V(max) rather than perturbations in substrate K(m). Catalytically, DAGK showed a strong preference for bicelles containing 3-(cholamidopropyl)dimethylammonio-2-hydroxy-1-propanesulfonate (CHAPSO) as the detergentcomponent relative to short-chained phosphatidylcholine.DAGK also exhibited a preference for dimyristoylphosphatidylcholine or dipalmitoylphosphatidylcholine bicelles relative to those of dilauroylphosphatidylcholine.

Using O2 to probe membrane immersion depth by 19F NMR

A fluorinated detergent, CF(3)(CF(2))(5)C(2)H(4)-O-maltose, was reconstituted into a lipid bilayer model membrane system to demonstrate the feasibility of determining solvent accessibility and membrane immersion depth of each fluorinated group by (19)F NMR. Apolar oxygen, which is known to partition with an increasing concentration gradient toward the hydrophobic membrane interior, exhibits a range of paramagnetic relaxation effects on (19)F nuclei, depending on its depth in the membrane. This effect, which is predominately associated with spin-lattice relaxation rates (R(1)) and chemical shifts, can be amplified greatly with minimal line broadening by increasing the partial pressure of O(2) at least 100-fold (i.e., P(O(2)) greater than 20 bar). The differences of longitudinal relaxation rates at 20 bar of oxygen pressure to those under ambient pressure (R(1)(20bar) - R(1)(0)) are largest for those fluorine groups expected to be most deeply buried in the membrane bilayer. This result contrasts with the reverse trend, which is observed on addition of a membrane surface-associated paramagnetic species, 4-(N,N-dimethyl-N-hexadecyl) ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (CAT-16) at ambient pressures. Thus, differential relaxation rates may be observed in (19)F-labeled membrane-associated molecules resulting from the addition of apolar oxygen under high pressure. The results demonstrate that the degree of solvent accessibility and membrane immersion depth of specific fluorinated species in membrane-associated macromolecules can be probed by (19)F NMR.

Pegylation of magnetically oriented lipid bilayers

We report NMR data for magnetically oriented phospholipid bilayers which have been doped with a lipid derivatized with a polyethylene glycol polymer headgroup to stabilize samples against aggregation. (13)C, (31)P, and (2)H NMR data indicate that the incorporation of PEG2000-PE (1% molar to DMPC) does not interfere with the orientation properties of bicelles prepared at 25% w/v with or without the presence of lanthanide. Bicelles prepared at 10% w/v are also shown to orient when PEG2000-PE is added. The addition of PEG2000-PE to cholesterol-containing, lanthanide-flipped bicelles is shown to inhibit sample phase separation and improve spectral quality. Furthermore, the addition of PEG2000-PE to high w/v bicelles (40% w/v) is demonstrated to lead to an increase in overall sample order.

The structure, dynamics and orientation of antimicrobial peptides in membranes by multidimensional solid-state NMR spectroscopy

Linear peptide antibiotics have been isolated from amphibians, insects and humans and used as templates to design cheaper and more potent analogues for medical applications. Peptides such as cecropins or magainins are < or = 40 amino acids in length. Many of them have been prepared by solid-phase peptide synthesis with isotopic labels incorporated at selected sites. Structural analysis by solid-state NMR spectroscopy and other biophysical techniques indicates that these peptide antibiotics strongly interact with lipid membranes. In bilayer environments they exhibit amphipathic alpha-helical conformations and alignments of the helix axis parallel to the membrane surface. This contrasts the transmembrane orientations observed for alamethicin or gramicidin A. Models that have been proposed to explain the antibiotic and pore-forming activities of membrane-associated peptides, as well as other experimental results, include transmembrane helical bundles, wormholes, carpets, detergent-like effects or the in-plane diffusion of peptide-induced bilayer instabilities.