Hydrocarbon Interior Research Articles

The Location of the Hydrophobic Proteins SP-B and SP-C in Fluid-Phase Bilayers

Pulmonary surfactant is a mixture of lipids and proteins that lowers surface tension in the lungs. The material acts as a surfactant, forming a film on the liquid layer that lines the alveoli. Two hydrophobic proteins, the homodimeric SP-B and monomeric SP-C, are essential for surfactant function. SP-B, which accelerates adsorption of the surfactant lipids to form the interfacial film, belongs to the Saposin-like family of proteins, which adopt a saposin fold in the open or closed configuration. A model membrane containing a single lipid, dioleoyl phosphatidylcholine, allowed molecular dynamics simulation of the bilayer with each protein. Simulations considered SP-B in both the open and closed configuration, and SP-C as transmembrane or deeply buried in the hydrophobic region. X-ray diffuse scattering (XDS) provided experimental structural information. The experimental and simulated form factors agreed best with SP-B in the closed configuration, and SP-C in the hydrocarbon interior. XDS also provided structural information for bilayers of extracted calf surfactant, which contains the physiological mixture of lipids and proteins. The location of the proteins, determined by analyzing the data with the Scattering Density Profile program, again showed SP-B in the headgroup region of the bilayer and SP-C in the hydrocarbon interior. Electron density profiles with component groups, as well as dynamic snapshots from simulations, yielded a visual representation of where the proteins are located.

Uniaxial Diffusional Narrowing of NMR Lineshapes for Membrane Proteins Reconstituted in Magnetically Aligned Bicelles and Macrodiscs

Structure and dynamics of membrane proteins can be effectively studied by oriented-sample solid-state nuclear magnetic resonance (NMR) techniques when the lipid bilayers are macroscopically aligned with respect to the main magnetic field. Magnetic alignment of the protein-containing membrane bilayer results from the negative susceptibility anisotropy of the lipid hydrocarbon interior yielding perpendicular sample alignment. At this orientation, while the uniformity of alignment represents an essential prerequisite for obtaining high-quality NMR spectra, further line narrowing is obtained by uniaxial motional averaging of the azimuthal parts of the chemical shift anisotropies and dipolar couplings. The motional averaging is brought about by uniaxial rotational diffusion of the protein molecules about the normal to the membrane surface, which is perpendicular to the magnetic field. Uniaxial averaging is efficient when the motion about the axis of alignment becomes sufficiently fast (on the timescale of the dipolar couplings and chemical shift anisotropies). Line narrowing under uniaxial rotation can be theoretically modeled using the stochastic Liouville equation. In this mini-review, we illustrate the method of uniaxial averaging for the relatively small Pf1 coat protein which exhibits excellent resolution in magnetically aligned bicelles due to its fast uniaxial diffusion and even superior resolution in large (30 nm) nanodiscs (macrodiscs) stabilized by a belt peptide. Spectra of Pf1 coat protein in polymer-stabilized macrodiscs, an alternative and more robust alignment media, are presented. We also report on preliminary spectra of a much larger protein—uniformly 15N labeled M1-M4 domain for the human acetylcholine receptor. While some spectral resolution is apparent, significantly broader linewidths emphasize the need for creating fast rotating discoidal membrane mimetics.

Selective Interaction of Colistin with Lipid Model Membranes

Although colistin's clinical use is limited due to nephrotoxicity, colistin is considered an antibiotic of last resort since it is used to treat patients infected with multi-drug resistant bacteria. In an effort to provide molecular details about colistin's ability to kill Gram-negative (G(-)) but not Gram-positive (G(+)) bacteria, we investigated the biophysics of interaction between colistin and lipid mixtures mimicking the cytoplasmic membrane of G(+), G(-) bacteria as well as eukaryotic cells. Two different models of G(-) outer membrane (OM) were assayed: lipid A with two deoxy-manno-octulosonyl sugar residues (KDO2) and Escherichia coli lipopolysaccharide (LPS) mixed with dilaurylphosphatidylglycerol. We used circular dichroism and X-ray diffuse scattering at low and wide angle in stacked multilayered samples, and neutron reflectivity (NR) of single, tethered bilayers mixed with colistin. We found no differences in secondary structure when colistin was bound to G(-) vs G(+) membrane mimics, ruling out a protein conformational change as the cause of this difference. However, bending modulus KC perturbation was quite irregular for G(-) IM, where colistin produced a softening of membranes at intermediate lipid:peptide molar ratio but stiffening at lower and higher peptide concentrations, while in G(+) and eukaryotic mimics there was only a slight softening. Acyl chain order in G(-) was perturbed similarly to KC. In G(+) there was only a slight softening and disordering effect, while in OM mimics, there was a slight stiffening and ordering of both membranes with increasing colistin. X-ray and NR structural results reveal colistin partitions deepest into the hydrocarbon interior in G(-) membranes, but remains in the headgroup region in G(+), OM and eukaryotic mimics. It is possible that domain formation is responsible for the erratic response of G(-) inner membranes to colistin and for its deeper penetration, which could increase membrane permeability.

Selective Interaction of Colistin with Lipid Model Membranes

Although colistin’s clinical use is limited due to its nephrotoxicity, colistin is considered to be an antibiotic of last resort because it is used to treat patients infected with multidrug-resistant bacteria. In an effort to provide molecular details about colistin’s ability to kill Gram-negative (G(−)) but not Gram-positive (G(+)) bacteria, we investigated the biophysics of the interaction between colistin and lipid mixtures mimicking the cytoplasmic membrane of G(+), G(−) bacteria as well as eukaryotic cells. Two different models of the G(−) outer membrane (OM) were assayed: lipid A with two deoxy-manno-octulosonyl sugar residues, and Escherichia coli lipopolysaccharide mixed with dilaurylphosphatidylglycerol. We used circular dichroism and x-ray diffuse scattering at low and wide angle in stacked multilayered samples, and neutron reflectivity of single, tethered bilayers mixed with colistin. We found no differences in secondary structure when colistin was bound to G(−) versus G(+) membrane mimics, ruling out a protein conformational change as the cause of this difference. However, bending modulus KC perturbation was quite irregular for the G(−) inner membrane, where colistin produced a softening of the membranes at an intermediate lipid/peptide molar ratio but stiffening at lower and higher peptide concentrations, whereas in G(+) and eukaryotic mimics there was only a slight softening. Acyl chain order in G(−) was perturbed similarly to KC. In G(+), there was only a slight softening and disordering effect, whereas in OM mimics, there was a slight stiffening and ordering of both membranes with increasing colistin. X-ray and neutron reflectivity structural results reveal colistin partitions deepest to reach the hydrocarbon interior in G(−) membranes, but remains in the headgroup region in G(+), OM, and eukaryotic mimics. It is possible that domain formation is responsible for the erratic response of G(−) inner membranes to colistin and for its deeper penetration, which could increase membrane permeability.

HIV-1 Matrix-31 Membrane Binding Peptide Interacts Differently with Membranes Containing PS vs. PI(4,5)P2

Efficient assembly of HIV-1 at the plasma membrane (PM) of the T-cell specifically requires PI(4,5)P2. It was previously shown that a highly basic region (HBR) of the matrix protein (MA) on the Gag precursor polyprotein Pr55Gag is required for membrane association. MA is N-terminally myristoylated, which enhances its affinity to membranes. In this work we used X-ray scattering and neutron reflectivity to determine how the physical properties and structure of lipid bilayers respond to the addition of binding domain peptides, either in the myristoylated form (MA31myr) or without the myristoyl group (MA31). Neutron reflectivity measurements showed the peptides predominantly located in the hydrocarbon interior. Diffuse X-ray scattering showed differences in membrane properties upon addition of peptides and the direction of the changes depended on lipid composition. The PI(4,5)P2-containing bilayers softened, thinned and became less ordered as peptide concentration increased. In contrast, POPS-containing bilayers with equivalent net charge first stiffened, thickened and became more ordered with increasing peptide concentration. As softening the host cell's PM upon contact with the protein lowers the free energy for membrane restructuring, thereby potentially facilitating budding of viral particles, our results suggest that the role of PI(4,5)P2 in viral assembly goes beyond specific stereochemical membrane binding. These studies reinforce the importance of lipids in virology.

HIV-1 matrix-31 membrane binding peptide interacts differently with membranes containing PS vs. PI(4,5)P2

Efficient assembly of HIV-1 at the plasma membrane (PM) of the T-cell specifically requires PI(4,5)P2. It was previously shown that a highly basic region (HBR) of the matrix protein (MA) on the Gag precursor polyprotein Pr55Gag is required for membrane association. MA is N-terminally myristoylated, which enhances its affinity to membranes. In this work we used X-ray scattering and neutron reflectivity to determine how the physical properties and structure of lipid bilayers respond to the addition of binding domain peptides, either in the myristoylated form (MA31myr) or without the myristoyl group (MA31). Neutron reflectivity measurements showed the peptides predominantly located in the hydrocarbon interior. Diffuse X-ray scattering showed differences in membrane properties upon addition of peptides and the direction of the changes depended on lipid composition. The PI(4,5)P2-containing bilayers softened, thinned and became less ordered as peptide concentration increased. In contrast, POPS-containing bilayers with equivalent net charge first stiffened, thickened and became more ordered with increasing peptide concentration. As softening the host cell's PM upon contact with the protein lowers the free energy for membrane restructuring, thereby potentially facilitating budding of viral particles, our results suggest that the role of PI(4,5)P2 in viral assembly goes beyond specific stereochemical membrane binding. These studies reinforce the importance of lipids in virology.

Methanol Perturbing Modeling Cell Membranes Investigated using Linear and Nonlinear Vibrational Spectroscopy

Cell membranes play a crucial role in many biological functions of cells. A small change in the composition of cell membranes can strongly influence the functions of membrane-associated proteins, such as ion and water channels, and thus mediate the chemical and physical balance in cells. Such composition change could originate from the introduction of short-chain alcohols, or other anesthetics into membranes. In this work, we have applied sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), to investigate interaction between methanol and 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (d54-DMPC) lipid bilayers. Lipid's hydrocarbon interior is deuterated while its head group is hydrogenated. At the same time, CH3 symmetric stretch from methanol and lipid head amine group has different frequency, thus we can distinguish the behaviors of methanol, lipid head amine group, and lipid hydrocarbon interior. Based on the spectral feature of the bending mode of the water molecules replaced by methanol, we determined that the methanol molecules are intercalated into the region between amine and phosphate groups at the lipid hydrophilic head. The dipole of CH3 groups of methanol and lipid head, and the water O-H all adopt the same orientation directions. The introduction of methanol into the lipid hydrophilic head group can strongly perturb the entire length of the alkyl chains, resulting that the signals of CD2 and CD3 groups from both leaflets can not cancel each other.

Permeation of polystyrene nanoparticles across model lipid bilayer membranes

An understanding of the relationship between the physiochemical properties of nanoparticles and their uptake into cells is crucial for realising the potential of nanoparticles for biomedical applications and for developing strategies to minimise human and environmental nanotoxicity. In this work we studied the permeation of lipid bilayer membranes by uncharged hydrophobic polystyrene nanoparticles (PSNPs) as a function of nanoparticle size by means of coarse-grained molecular dynamics simulations. We also investigated the effect of changing the membrane environment by adding cholesterol to the bilayer. Free energy calculations reveal that overall it is energetically favourable for PSNPs to insert into the hydrocarbon interior of dipalmitoylphosphatidylcholine (DPPC) bilayers; however PSNPs with a diameter greater than the bilayer thickness are less readily accommodated than smaller PSNPs. As a PSNP approaches the bilayer there is an increase in the curvature of the bilayer as it bends and partially engulfs the PSNP. The energetic cost of bending is compensated for by the removal of unfavourable PSNP–water contacts. Despite cholesterol increasing the order and rigidity of the bilayer, the interior of the DPPC–cholesterol bilayer appears to be a more favourable environment for PSNPs, possibly due to stronger intermolecular interactions between polystyrene and the model cholesterol molecule than between polystyrene and DPPC.

Colony Organization in the Green Alga Botryococcus braunii (Race B) Is Specified by a Complex Extracellular Matrix

Botryococcus braunii is a colonial green alga whose cells associate via a complex extracellular matrix (ECM) and produce prodigious amounts of liquid hydrocarbons that can be readily converted into conventional combustion engine fuels. We used quick-freeze deep-etch electron microscopy and biochemical/histochemical analysis to elucidate many new features of B. braunii cell/colony organization and composition. Intracellular lipid bodies associate with the chloroplast and endoplasmic reticulum (ER) but show no evidence of being secreted. The ER displays striking fenestrations and forms a continuous subcortical system in direct contact with the cell membrane. The ECM has three distinct components. (i) Each cell is surrounded by a fibrous β-1, 4- and/or β-1, 3-glucan-containing cell wall. (ii) The intracolonial ECM space is filled with a cross-linked hydrocarbon network permeated with liquid hydrocarbons. (iii) Colonies are enclosed in a retaining wall festooned with a fibrillar sheath dominated by arabinose-galactose polysaccharides, which sequesters ECM liquid hydrocarbons. Each cell apex associates with the retaining wall and contributes to its synthesis. Retaining-wall domains also form "drapes" between cells, with some folding in on themselves and penetrating the hydrocarbon interior of a mother colony, partitioning it into daughter colonies. We propose that retaining-wall components are synthesized in the apical Golgi apparatus, delivered to apical ER fenestrations, and assembled on the surfaces of apical cell walls, where a proteinaceous granular layer apparently participates in fibril morphogenesis. We further propose that hydrocarbons are produced by the nonapical ER, directly delivered to the contiguous cell membrane, and pass across the nonapical cell wall into the hydrocarbon-based ECM.

Amphiphilic Effects of Chlorhexidine Digluconate on Rotational and Lateral Mobilities of Bacterial Outer and Model Membrane Lipid Bilayers

Intramolecular excimer formation of 1,3-di(1-pyrenyl)propane (Py-3-Py), and fluorescence polarization of 1,6-diphenyl1,3,5-hexatriene(DPH) were used to investigate the effect of chlorhexidine digulconate (CHX) on the bulk fluidity of outer membranes (OPG) isolated from cultured Porphyromonas gingivalis and multilamellar vesicles were prepared with total lipids (OPGTL) extracted from OPG and prepared with mixture (PL) of DPPE and DPPC. The fluorescence polarization of n-(9-anthroyloxy)stearic acid (n-AS) were used to examined the effect of CHX on the rotational mobility of the surface and interior region of membrane lipid bilayers. CHX decreased the mobility of the surface region of bilayers but the drug increased the mobility of the interior region of membrane lipid bilayers.These indicate that CHX increased both the lateral and rotational mobilities of probes in OPG, OPGTL and PL bilayers. Selective quenching of Py-3-Py and DPH by TNBS was utilized to examine transbilayer fluidity asymmetry of OPG, OPGTL and PL lipid bilayers. CHX had a greater fluidizing effect on the inner monolayers as compared to the outer monolayer of OPG, OPGTL and PL lipid bilayers. The sensitivities to the increasing effect of fluidity differed according to these membrane lipid bilayers in the descending order of OPG, PL and OPGTL. CHX increased the rotational mobility of the hydrocarbon interior of OPG, OPGTL and PL but decreased the mobility of membrane interface of OPG, OPGTL and PL. The sensitivities to the increasing effects of CHX on the rotational mobility were in proportion to the located depths of the probes in descending order, as follows: 16-AP, 12-AS, 9-AS and 6-AS. The disordering or ordering effects of CHX on the membrane lipids might be responsible for some, but not all of its bateriostatic and bactericidal actions.

Spectroscopic probing of location and dynamics of an environment-sensitive intramolecular charge transfer probe within liposome membranes

The present work demonstrates the interaction of an intramolecular charge transfer (ICT) probe 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid methyl ester (DPDAME) with liposome membranes of dimyristoyl-l-α-phosphatidylcholine (DMPC) and dimyristoyl-l-α-phosphatidylglycerol (DMPG) studied by steady-state absorption, emission and time-resolved emission techniques. A huge hypsochromic shift together with remarkable enhancement of fluorescence quantum yield of the polarity sensitive ICT emission of DPDAME upon interaction with the lipids has been rationalized in terms of incorporation of the probe into hydrophobic interior of the lipids. Compelling evidences for penetration of the probe into the hydrocarbon interior of the lipids have been deduced from intertwining different experimental results e.g., micropolarity in the immediate vicinity of the probe in lipid environments, steady-state anisotropy, red-edge excitation shift (REES), fluorescence quenching experiments and time-resolved measurements. The rotational relaxation dynamics study of the membrane-bound probe unveils the impartation of high degree of motional rigidity. Wavelength-selective emission behaviour paves way for monitoring of solvent-relaxation in the membranes. Overall, the ICT probe DPDAME displays its commendable sensitivity in deciphering the microheterogeneous environments of liposomal membranes of DMPC and DMPG and promises a new membrane-polarity sensitizing probe.

Dioxygen Transmembrane Distributions and Partitioning Thermodynamics in Lipid Bilayers and Micelles

Cellular respiration, mediated by the passive diffusion of oxygen across lipid membranes, is key to many basic cellular processes. In this work, we report the detailed distribution of oxygen across lipid bilayers and examine the thermodynamics of oxygen partitioning via NMR studies of lipids in a small unilamellar vesicle (SUV) morphology. Dissolved oxygen gives rise to paramagnetic chemical shift perturbations and relaxation rate enhancements, both of which report on local oxygen concentration. From SUVs containing the phospholipid sn-2-perdeuterio-1-myristelaidoyl, 2-myristoyl-sn-glycero-3-phosphocholine (MLMPC), an analogue of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), we deduced the complete trans-bilayer oxygen distribution by measuring (13)C paramagnetic chemical shifts perturbations for 18 different sites on MLMPC arising from oxygen at a partial pressure of 30 bar. The overall oxygen solubility at 45 °C spans a factor of 7 between the bulk water (23.7 mM) and the bilayer center (170 mM) and is lowest in the vicinity of the phosphocholine headgroup, suggesting that oxygen diffusion across the glycerol backbone should be the rate-limiting step in diffusion-mediated passive transport of oxygen across the lipid bilayer. Lowering of the temperature from 45 to 25 °C gave rise to a slight decrease of the oxygen solubility within the hydrocarbon interior of the membrane. An analysis of the temperature dependence of the oxygen solubility profile, as measured by (1)H paramagnetic relaxation rate enhancements, reveals that oxygen partitioning into the bilayer is entropically favored (ΔS° = 54 ± 3 J K(-1) mol(-1)) and must overcome an enthalpic barrier (ΔH° = 12.0 ± 0.9 kJ mol(-1)).

The pH-dependence of chloroquine uptake by phosphatidylcholine vesicles.

The pH-dependence of chloroquine uptake by phosphatidylcholine bilayered vesicles was investigated to examine the relative affinity of ionized and un-ionized chloroquine species to membrane bilayers. The results suggest that the neutral species and monocation are taken up by the bilayers but only the un-ionized species interacts significantly with the hydrocarbon interior.

Integration of β-carotene molecules in small liposomes

The most typical feature of carotenoids is the long polyene chain with conjugated double bonds suggesting that they can serve as conductors of electrons, acting as ''molecular wires'', important elements in the molecular electronic devices. Carotenoids are essential components of photosynthetic systems, performing different functions as light harvesting, photoprotection and electron transfer. They act also as natural antioxidants. In addition they perform structural role stabilizing the three-dimensional organization of photosynthetic membranes. Carotenoids contribute to the stability of the lipid phase, preserving the membrane integrity under potentially harmful environmental conditions. Carotenoids can be easily integrated into model membranes, facilitating the investigation of their functional roles. In carotenoid-egg phosphatidylcholine (EPC) liposomes ß-carotene is randomly distributed in the hydrocarbon interior of the bilayer, without any preferred, well defined orientation and retains a substantial degree of mobility. Here we investigate the degree of integration of ß-carotene in small unilamellar EPC liposomes and the changes in ß-carotene absorption and Raman spectra due to the lipid-pigment interaction. All observed changes in ß-carotene absorption and Raman spectra may be regarded as a result of the lipid-pigment interactions leading to the polyene geometry distortion and increasing of the environment heterogenety in the liposomes as compared to the solutions.

ChemInform Abstract: 2H NMR in Liquid Crystals and Membranes

Abstract Deuterium NMR spectroscopy is widely applicable to studies of the structure and dynamics of molecular solids, liquid crystals, and thin films of membrane lipids. The properties of soft nanomaterials are also accessible on the mesoscopic length scale intermediate between the molecular and bulk dimensions. For membrane lipids in the liquid-crystalline state, rapid axial averaging occurs about the director axis (the membrane normal). One can then relate the profiles of the order parameters | S CD | of the individual C– 2 H labeled segments to average bilayer properties. These include the mean area per molecule and projected acyl chain length, the area compressibility modulus, and the radius of curvature for reverse hexagonal ( H II ) phase nanotubes. In addition, measurements of the relaxation rates for Zeeman order, R 1Z , and quadrupolar order, R 1Q , enable one to investigate the mean-squared amplitudes and time-scales of the fluctuations that underlie the thermodynamic properties. A unified interpretation is provided by a composite membrane deformation model, which fits simultaneously the frequency dependence and the angular anisotropy of the R 1Z and R 1Q relaxation rates. The results suggest the bilayer dynamics in the MHz regime can be modeled in terms of nematic-like deformations of the membrane hydrocarbon interior, together with axial rotations of the lipid acyl chains. A small contribution from internal segmental motions is found, which implies the bilayer microviscosity is comparable to that of a liquid hydrocarbon. Finally, the 2 H-NMR relaxation rates of lipid bilayers containing cholesterol in the liquid-ordered phase suggest a dynamically more rigid bilayer, involving fast axial lipid rotations together with a reduction in collective bilayer deformations. Possible future applications include studies of liquid crystals and thin films of membrane lipids and surfactants, as well as lipid-protein systems.

The effect of dimiristoylphosphatidylethanol on the rotational mobility of n-(9-Anthroyloxy) stearic acid in neuronal and model membranes

The aim of this study was to provide a basis for examining the molecular mechanism for the pharmacological action of ethanol. We studied dinotmyristoylnotphosphatidylethanol (DMPEt)’s effects on specific locations of n-(9-anthroyloxy) palmitic acid or stearic acid (n-AS) within phosnotpholipids of synaptosomal plasma membrane vesicles isolated from bovine cerebral cortex (SPMV) and liposomes of total lipids (SPMVTL) and phospholipids (SPMVPL) extracted from SPMV. DMPEt increased rotational mobility (increased disordering) of hydrocarbon interior, but it decreased mobility (increased ordering) of membrane interface, in native and model membranes. The degree of rotational mobility in accordance with the carbon atom numbers of phospholipids comprising neuronal membranes was in the order at the 16, 12, 9, 6 and 2 position of aliphatic chain present in phospholipids. The sensitivity of increasing or decreasing effect of rotational mobility of hydrocarbon interior or surface region by DMPEt differed depending on the neuronal and model membranes in the descending order of SPMV, SPMVPL and SPMVTL.

Daunomycin Binding to Detergent Micelles: A Model System for Evaluating the Hydrophobic Contribution to Drug−DNA Interactions

The interaction of daunomycin with sodium dodecyl sulfate and Triton X-100 micelles was investigated as a model for the hydrophobic contribution to the free energy of DNA intercalation reactions. Measurements of visible absorbance, fluorescence lifetime, steady-state fluorescence emission intensity, and fluorescence anisotropy indicate that the anthraquinone ring partitions into the hydrophobic micelle interior. Fluorescence quenching experiments using both steady-state and lifetime measurements demonstrate reduced accessibility of daunomycin in sodium dodecyl sulfate micelles to the anionic quencher iodide and to the neutral quencher acrylamide. Quenching of daunomycin fluorescence by iodide in Triton X-100 micelles was similar to that seen with free daunomycin. Studies of the energetics of the interaction of daunomycin with micelles by fluorescence and absorbance titration methods and by isothermal titration calorimetry in the presence of excess micelles revealed that association with sodium dodecyl sulfate and Triton X-100 micelles is driven by a large negative enthalpy. Association of the drug with both types of micelles also has a favorable entropic contribution, which is larger in magnitude for Triton X-100 micelles than for sodium dodecyl sulfate micelles. The thermodynamic profile for the interaction of daunomycin with both types of micelles is characteristic of the "nonclassical" hydrophobic effect. The enthalpy for the interaction of daunomycin with sodium dodecyl sulfate micelles increases nonlinearly with temperature, indicating a positive (and temperature dependent) heat capacity change. The binding isotherm for daunomycin association with sodium dodecyl sulfate micelles was cooperative, with a Hill coefficient of 1.6. The cooperative behavior and the positive heat capacity change suggest that the drug alters micelle size or imposes order on the hydrocarbon interior of the micelle.

The role of hydrophobic interactions in positioning of peripheral proteins in membranes

BackgroundThree-dimensional (3D) structures of numerous peripheral membrane proteins have been determined. Biological activity, stability, and conformations of these proteins depend on their spatial positions with respect to the lipid bilayer. However, these positions are usually undetermined.ResultsWe report the first large-scale computational study of monotopic/peripheral proteins with known 3D structures. The optimal translational and rotational positions of 476 proteins are determined by minimizing energy of protein transfer from water to the lipid bilayer, which is approximated by a hydrocarbon slab with a decadiene-like polarity and interfacial regions characterized by water-permeation profiles. Predicted membrane-binding sites, protein tilt angles and membrane penetration depths are consistent with spin-labeling, chemical modification, fluorescence, NMR, mutagenesis, and other experimental studies of 53 peripheral proteins and peptides. Experimental membrane binding affinities of peripheral proteins were reproduced in cases that did not involve a helix-coil transition, specific binding of lipids, or a predominantly electrostatic association. Coordinates of all examined peripheral proteins and peptides with the calculated hydrophobic membrane boundaries, subcellular localization, topology, structural classification, and experimental references are available through the Orientations of Proteins in Membranes (OPM) database.ConclusionPositions of diverse peripheral proteins and peptides in the lipid bilayer can be accurately predicted using their 3D structures that represent a proper membrane-bound conformation and oligomeric state, and have membrane binding elements present. The success of the implicit solvation model suggests that hydrophobic interactions are usually sufficient to determine the spatial position of a protein in the membrane, even when electrostatic interactions or specific binding of lipids are substantial. Our results demonstrate that most peripheral proteins not only interact with the membrane surface, but penetrate through the interfacial region and reach the hydrocarbon interior, which is consistent with published experimental studies.

Amphiphilic effects of dibucaine·HCl on rotational mobility of n-(9-anthroyloxy)stearic acid in neuronal and model membranes

We studied dibucaine's effects on specific locations of n-(9-anthroyloxy)palmitic acid or stearic acid ( n-AS) within phospholipids of synaptosomal plasma membrane vesicles isolated from bovine cerebral cortex (SPMV) and model membranes. Giant unilamellar vesicles (GUVs) were prepared with total lipids (SPMVTL) and mixture of several phospholipids (SPMVPL) extracted from SPMV. Dibucaine·HCl increased rotational mobility (increased disordering) of hydrocarbon interior, but it decreased mobility (increased ordering) of membrane interface, in both native and model membranes. The degree of rotational mobility in accordance with the carbon atom numbers of phospholipids comprising neuronal and model membranes was in the order at the 16, 12, 9, 6 and 2 position of aliphatic chain present in phospholipids. The sensitivity of increasing or decreasing effect of rotational mobility of hydrocarbon interior or surface region by dibucaine·HCl differed depending on the neuronal and model membranes in the descending order of SPMV, SPMVPL and SPMVTL.

Sodium Ion Internalized within Phospholipid Membranes

Seven phospholipids, modified with ester groups in their hydrophobic chains, were synthesized and examined for their ability to promote sodium ion flux across vesicular membranes. It was found by 23Na NMR that only the phospholipids having short chain segments beyond their terminal ester groups catalyze sodium ion transfer by up to 2 orders of magnitude relative to a conventional phospholipid, POPC. The rates increase with the concentration of the ester-phospholipid admixed with POPC in the bilayer. More surprisingly, the rates increase with the time allowed for the vesicles to age. This was attributed to ester-phospholipid migrating in the bilayers to form domains that solubilize the sodium ion within the hydrocarbon interior of the membrane. Such membrane domains explain why shift reagent-modified NMR spectra display three 23Na signals representing sodium outside the vesicles, sodium within the vesicular water pools, and sodium within the membranes themselves.