Highly robust lipid membranes on crystalline S-layer supports investigated by electrochemical impedance spectroscopy

In the present work, S-layer supported lipid membranes formed by a modified Langmuir-Blodgett technique were investigated by electrochemical impedance spectroscopy (EIS). Basically two intermediate hydrophilic supports for phospholipid- (DPhyPC) and bipolar tetraetherlipid- (MPL from Thermoplasma acidophilum) membranes have been applied: first, the S-layer protein SbpA isolated from Bacillus sphaericus CCM 2177 recrystallized onto a gold electrode; and second, as a reference support, an S-layer ultrafiltration membrane (SUM), which consists of a microfiltration membrane (MFM) with deposited S-layer carrying cell wall fragments. The electrochemical properties and the stability of DPhyPC and MPL membranes were found to depend on the used support. The specific capacitances were 0.53 and 0.69 microF/cm(2) for DPhyPC bilayers and 0.75 and 0.77 microF/cm(2) for MPL monolayers resting on SbpA and SUM, respectively. Membrane resistances of up to 80 mega Ohm cm(2) were observed for DPhyPC bilayers on SbpA. In addition, membranes supported by SbpA exhibited a remarkable long-term robustness of up to 2 days. The membrane functionality could be demonstrated by reconstitution of membrane-active peptides such as valinomycin and alamethicin. The present results recommend S-layer-supported lipid membranes as promising structures for membrane protein-based biosensor technology.     

Polymerization in black lipid membranes

A variety of different lipids containing dienoyl groups in the side chains were tested for membrane formation using the planar lipid bilayer approach. One of these lipids formed stable bilayers which could be polymerized using UV-illumination. The influence of the polymerization was studied in monolayers, lipid vesicles and planar bilayers. The stability of the lipid bilayer membranes was increased by polymerization. Thus, the lifetime of the membranes increased from about 1 h to 4–5 h or longer. Furthermore, the specific conductance of unmodified membranes and of carrier-mediated transport is reduced. The transport of lipophilic ions was investigated as a function of polymerization using the charge-pulse method. The absorption of dipicrylamine (DPA^-) is not affected. The translocation of this compound and of tetraphenylborate (B(Ph) ₄ ^- ) showed a strong decrease with polymerization time. The influence of polymerization on the membrane structure may be explained on the basis of a strong viscosity increase in the lipid bilayer membrane.     

Biomimetic design of nanopatterned membranes

The biomimetic approach copying the supramolecular building principle of many archaeal cell envelopes (i.e., a plasma membrane with associated S-layer proteins) has resulted in stable lipid membranes with excellent reconstitution properties for transmembrane proteins. This is a particular challenge as one-third of all proteins in an organism are membrane proteins like pores, ion channels, or receptors. At S-layer supported lipid membranes, spatial well-defined domains on the S-layer protein interact noncovalently with lipid head groups within the lipid membrane resulting in a nanopatterning of a few anchored and scores of diffusional free-lipid molecules. In addition, no impact on the hydrophobic core region and on the function of reconstituted integral proteins has been determined. Among others, particularly S-layer stabilized membranes can be used for structure-function studies on reconstituted integral proteins and also in the membrane protein-based molecular nanotechnology, e.g., in the design of biosensing devices (e.g., lipid chip or lab-on-a-chip), or for receptor or ion channel-based high-throughput screening.     

Composite S-layer lipid structures

Designing and utilization of biomimetic membrane systems generated by bottom-up processes is a rapidly growing scientific and engineering field. Elucidation of the supramolecular construction principle of archaeal cell envelopes composed of S-layer stabilized lipid membranes led to new strategies for generating highly stable functional lipid membranes at meso- and macroscopic scale. In this review, we provide a state of the art survey how S-layer proteins, lipids, and polysaccharides may be used as basic building blocks for the assembly of S-layer supported lipid membranes. These biomimetic membrane systems are distinguished by a nanopatterned fluidity, enhanced stability and longevity and thus, provide a dedicated reconstitution matrix for membrane-active peptides and transmembrane proteins. Exciting areas for application of composite S-layer membrane systems concern sensor systems involving specific membrane functions.     

Interaction of lipopolysaccharide and phospholipid in mixed membranes: solid-state 31P-NMR spectroscopic and microscopic investigations

Lipopolysaccharide (LPS), which constitutes the outermost layer of Gram-negative bacterial cells as a typical component essential for their life, induces the first line defense system of innate immunity of higher animals. To understand the basic mode of interaction between bacterial LPS and phospholipid cell membranes, distribution patterns were studied by various physical methods of deep rough mutant LPS (ReLPS) of Escherichia coli incorporated in phospholipid bilayers as simple models of cell membranes. Solid-state ³¹P-NMR spectroscopic analysis suggested that a substantial part of ReLPS is incorporated into 1,2-dimyristoyl-sn-glycero-3-phosphocholine lipid bilayers when multilamellar vesicles were prepared from mixtures of these. In egg L-α-phosphatidylcholine (egg-PC)-rich membranes, ReLPS undergoes micellization. In phosphatidylethanolamine-rich membranes, however, micellization was not observed. We studied by microscopic techniques the location of ReLPS in membranes of ReLPS/egg-PC (1:10 M/M) and ReLPS/egg-PC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) (1:9:1 M/M/M). The influence of ReLPS on the physicochemical properties of the membranes was studied as well. Microscopic images of both giant unilamellar vesicles and supported planar lipid bilayers showed that LPS was uniformly incorporated in the egg-PC lipid bilayers. In the egg-PC/POPG (9:1 M/M) lipid bilayers, however, ReLPS is only partially incorporated and becomes a part of the membrane in a form of aggregates (or as mixed aggregates with the lipids) on the bilayer surface. The lipid lateral diffusion coefficient measurements at various molar ratios of ReLPS/egg-PC/POPG indicated that the incorporated ReLPS reduces the diffusion coefficients of the phospholipids in the membrane. The retardation of diffusion became more significant with increasing POPG concentrations in the membrane at high ReLPS/phospholipid ratios. This work demonstrated that the phospholipid composition has critical influence on the distribution of added ReLPS in the respective lipid membranes and also on the morphology and physicochemical property of the resulting membranes. A putative major factor causing these phenomena is reasoned to be the miscibility between ReLPS and individual phospholipid compositions.     

Porous membranes for reconstitution of ion channels

Functional biological synthetic composite (BSC) membranes were made using phospholipids, biological membrane proteins and permeable synthetic supports or membranes. Lipid bilayers were formed on porous polycarbonate (PC), polyethylene terephthalate (PETE) and poly (l-lactic acid) (PLLA) membranes and in 10-100 μm laser-drilled pores in a 96-well plastic plate as measured by increased resistance or decreased currents. Bilayers in 50 μm and smaller pores were stable for up to 4 h as measured by resistance changes or a current after gramicidin D reconstitution. Biological membrane transport reconstitution was then carried out. Using vesicles containing Kv1.5 K⁺ channels, K⁺ currents and decreased resistance were measured across bilayers in 50 μm pores in the plastic plate and PLLA membranes, respectively, which were inhibited by compound B, a Kv1.5 K⁺ channel inhibitor. Functional reconstitution of Kv1.5 K⁺ channels was successful. Incorporation of membrane proteins in functional form in stable permeable membrane-supported lipid bilayers is a simple technology to create BSC membranes that mimic biological function which is readily adaptable for high throughput screening.     

The molecular organization of bimolecular lipid membranes. A study of the low frequency Maxwell-Wagner impedance dispersion

Theoretical considerations show that the presence of the polar group regions in bimolecular lipid membranes will produce a small (2–3%) dispersion of the bimolecular lipid membrane capacitance at low frequencies (0.1–100 Hz).A dispersion in conductance will also result. Calculations are given of the resolution of phase angle and impendance amplitude required to detect this dispersion and a new measuring technique is described which can achieve this. From the experimental result presented for lecithin bimolecular lipid membranes a determination was made of the individual capacitances and conductances of both the hydrocarbon and polar groups regions. The polar group conductance was found to vary from 700 μΩ^?1 · cm^?2 (in 1 mM KCl) to 2000 μΩ^?1 · cm^?2 (in 1 M KCl).The polar group capacitances were found to be approx.30 μF · cm^?2 and not systematically dependent on the concentration of the external electrolyte.     

Formation and Characterization of Supported Lipid Bilayers Composed of Hydrogenated and Deuterated Escherichia coli Lipids

Supported lipid bilayers are widely used for sensing and deciphering biomolecular interactions with model cell membranes. In this paper, we present a method to form supported lipid bilayers from total lipid extracts of Escherichia coli by vesicle fusion. We show the validity of this method for different types of extracts including those from deuterated biomass using a combination of complementary surface sensitive techniques; quartz crystal microbalance, neutron reflection and atomic force microscopy. We find that the head group composition of the deuterated and the hydrogenated lipid extracts is similar (approximately 75% phosphatidylethanolamine, 13% phosphatidylglycerol and 12% cardiolipin) and that both samples can be used to reconstitute high-coverage supported lipid bilayers with a total thickness of 41 ± 3 Å, common for fluid membranes. The formation of supported lipid bilayers composed of natural extracts of Escherichia coli allow for following biomolecular interactions, thus advancing the field towards bacterial-specific membrane biomimics.     

Antimicrobial activity and membrane selective interactions of a synthetic lipopeptide MSI-843

Lipopeptide MSI-843 consisting of the nonstandard amino acid ornithine (Oct-OOLLOOLOOL-NH₂) was designed with an objective towards generating non-lytic short antimicrobial peptides, which can have significant pharmaceutical applications. Octanoic acid was coupled to the N-terminus of the peptide to increase the overall hydrophobicity of the peptide. MSI-843 shows activity against bacteria and fungi at micromolar concentrations. It permeabilizes the outer membrane of Gram-negative bacterium and a model membrane mimicking bacterial inner membrane. Circular dichroism investigations demonstrate that the peptide adopts α-helical conformation upon binding to lipid membranes. Isothermal titration calorimetry studies suggest that the peptide binding to membranes results in exothermic heat of reaction, which arises from helix formation and membrane insertion of the peptide. ²H NMR of deuterated-POPC multilamellar vesicles shows the peptide-induced disorder in the hydrophobic core of bilayers. ³¹P NMR data indicate changes in the lipid head group orientation of POPC, POPG and Escherichia colitotal lipid bilayers upon peptide binding. Results from ³¹P NMR and dye leakage experiments suggest that the peptide selectively interacts with anionic bilayers at low concentrations (up to 5 mol%). Differential scanning calorimetry experiments on DiPOPE bilayers and ³¹P NMR data from E.coli total lipid multilamellar vesicles indicate that MSI-843 increases the fluid lamellar to inverted hexagonal phase transition temperature of bilayers by inducing positive curvature strain. Combination of all these data suggests the formation of a lipid-peptide complex resulting in a transient pore as a plausible mechanism for the membrane permeabilization and antimicrobial activity of the lipopeptide MSI-843.     

Evidence of pores and thinned lipid bilayers induced in oriented lipid membranes interacting with the antimicrobial peptides, magainin-2 and aurein-3.3

Dynamic structures of supramolecular lipid assemblies, such as toroidal pores and thinned bilayers induced in oriented lipid membranes, which are interacting with membrane-acting antimicrobial peptides (AMPs), magainin-2 and aurein-3.3, were explored by ³¹P and ²H solid-state NMR (ssNMR) spectroscopy. Various types of phospholipid systems, such as POPC-d₃₁, POPC-d₃₁/POPG, and POPC-d₃₁/cholesterol, were investigated to understand the membrane disruption mechanisms of magainin-2 and aurein-3.3 peptides at various peptide-to-lipid (P:L) ratios. The experimental lineshapes of anisotropic ³¹P and ²H ssNMR spectra measured on these peptide-lipid systems were simulated reasonably well by assuming the presence of supramolecular lipid assemblies, such as toroidal pores and thinned bilayers, in membranes. Furthermore, the observed decrease in the anisotropic frequency span of either ³¹P or ²H ssNMR spectra of oriented lipid bilayers, particularly when anionic POPG lipids are interacting with AMPs at high P:L ratios, can directly be explained by a thinned membrane surface model with fast lateral diffusive motions of lipids. The spectral analysis protocol we developed enables extraction of the lateral diffusion coefficients of lipids distributed on the curved surfaces of pores and thinned bilayers on a few nanometers scale.     

Electrical Properties of Structural Components of the Crystalline Lens

The electrical properties of the crystalline lens of the frog eye are measured with stochastic currents applied with a microelectrode near the center of the preparation and potential recorded just under the surface. The stochastic signals are decomposed by Fourier analysis into sinusoidal components, and the impedance is determined from the ratio of mean cross power to input power. The data are fit by an electrical model that includes two paths for current flow: one through the cytoplasm, gap junctions, and outer membrane; the other through inner membranes and the extracellular space between lens fibers. The electrical properties of the structures of the lens which appear as circuit components in the model are determined by the fit to the data. The resistivity of the extracellular space within the lens is comparable to the resistivity of Ringer. The outer membrane has a normal resistance of 5 kohm · cm² but large capacitance of 10 μF/cm², probably because it represents the properties of several layers of fibers. The inner membranes have properties reminiscent of artificial lipid bilayers: they have high membrane resistance, 2.2 megohm · cm², and low specific capacitance, 0.8 μF/cm². There is so much membrane within the lens, however, that the sum of the current flow across all the inner membranes is comparable to that across the outer surface.     

Display of a peptide mimotope on a crystalline bacterial cell surface layer (S-layer) lattice for diagnosis of Epstein-Barr virus infection

Fusion proteins based on the crystalline bacterial cell surface layer (S-layer) proteins SbpA from Bacillus sphaericus CCM 2177 and SbsB from Geobacillus stearothermophilus PV72/p2 and a peptide mimotope F1 that mimics an immunodominant epitope of Epstein-Barr virus (EBV) were designed and overexpressed in Escherichia coli. Constructs were designed such that the peptide mimotope was presented either at the C-terminus (SbpA/F1) or at the N-terminus (SbsB/F1) of the respective S-layer proteins. The resulting S-layer fusion proteins, SbpA/F1 and SbsB/F1, fully retained the intrinsic self-assembly capability of the S-layer moiety into monomolecular lattices. As determined by immunodot assays and ELISAs using monoclonal antibodies, the F1 mimotope was well-presented on the outer surface of the S-layer lattices and accessible for antibody binding. Further comparison of the two S-layer fusion proteins showed that the S-layer fusion protein SbpA/F1 had a higher antibody binding capacity than SbsB/F1 in aqueous solution and in immune sera, illustrating the importance of epitope orientation on the performance of solid-phase immunoassays. To assess the diagnostic values of S-layer mimotope fusion protein SbpA/F1, we screened a panel of 83 individual EBV IgM-positive, EBV negative, and potential cross-reactive sera for their reactivities. This resulted in 98.2% specificity and 89.3% sensitivity, and furthermore no cross-reactivity with related viral disease states including rheumatoid factor was observed. This study shows the potential of S-layer fusion proteins as a matrix for site-directed immobilization of small ligands in solid-phase immunoassays using EBV diagnostics as a model system.     

Molecular Characterization of the S-Layer Gene, sbpA, of Bacillus sphaericus CCM 2177 and Production of a Functional S-Layer Fusion Protein with the Ability To Recrystallize in a Defined Orientation while Presenting the Fused Allergen

The nucleotide sequence encoding the crystalline bacterial cell surface (S-layer) protein SbpA of Bacillus sphaericus CCM 2177 was determined by a PCR-based technique using four overlapping fragments. The entire sbpA sequence indicated one open reading frame of 3,804 bp encoding a protein of 1,268 amino acids with a theoretical molecular mass of 132,062 Da and a calculated isoelectric point of 4.69. The N-terminal part of SbpA, which is involved in anchoring the S-layer subunits via a distinct type of secondary cell wall polymer to the rigid cell wall layer, comprises three S-layer-homologous motifs. For screening of amino acid positions located on the outer surface of the square S-layer lattice, the sequence encoding Strep-tag I, showing affinity to streptavidin, was linked to the 5′ end of the sequence encoding the recombinant S-layer protein (rSbpA) or a C-terminally truncated form (rSbpA_31-1068). The deletion of 200 C-terminal amino acids did not interfere with the self-assembly properties of the S-layer protein but significantly increased the accessibility of Strep-tag I. Thus, the sequence encoding the major birch pollen allergen (Bet v1) was fused via a short linker to the sequence encoding the C-terminally truncated form rSpbA_31-1068. Labeling of the square S-layer lattice formed by recrystallization of rSbpA_31-1068/Bet v1 on peptidoglycan-containing sacculi with a Bet v1-specific monoclonal mouse antibody demonstrated the functionality of the fused protein sequence and its location on the outer surface of the S-layer lattice. The specific interactions between the N-terminal part of SbpA and the secondary cell wall polymer will be exploited for an oriented binding of the S-layer fusion protein on solid supports to generate regularly structured functional protein lattices.     

Can proton pumping by SERCA enhance the regulatory role of phospholamban and sarcolipin?

The effect of the incorporation of phosphorylated phospholamban (pPLN) and sarcolipin (SLN) in mercury-supported self-assembled lipid monolayers and in lipid bilayers tethered to mercury via a hydrophilic spacer was investigated by voltammetric techniques and electrochemical impedance spectroscopy. It was shown that pPLN and SLN do not permeabilize lipid bilayers toward ions at physiological pH. However, they exert a permeabilizing action toward inorganic monovalent cations such as K⁺ and Tl⁺, but not toward divalent cations such as Ca²⁺ and Cd^2 +, following a small decrease in pH. This behavior can be associated with their regulatory action on the Ca-ATPase of the sarcoplasmic reticulum (SERCA). SERCA pumps two Ca²⁺ ions from the cytosol to the lumen of the sarcoplasmic reticulum (SR) and two protons in the opposite direction, causing a transient decrease of pH in the immediate vicinity of its cytoplasmic domain. This decrease is expected to activate the liberated pPLN molecules and SLN to make the SR membrane leakier toward K⁺ and Na⁺ and the SLN ion channel to translocate small inorganic anions, such as Cl⁻. The effect of pPLN and SLN, which becomes synergic when they are both present in the SR membrane, is expected to favor a rapid equilibration of ions on both sides of the membrane.     

Resolving the kinetics of lipid,protein and peptide diffusion in membranes

Abstract

Recent developments in the understanding of molecular diffusion phenomena in membranes are reviewed. Both model bilayers and biological membranes are considered in respect of lateral diffusion, rotational diffusion and transverse diffusion (flip-flop). For model systems, particular attention is paid to recent data obtained using surface-specific techniques such as sum frequency generation vibrational spectroscopy on supported lipid bilayers, and fluorescence correlation spectroscopy on giant unilamellar vesicles, both of which have yielded new insights into the intrinsic rates of diffusion and the energetic barriers to processes such as lipid flip-flop. Advances in single-molecule and many-molecule fluorescence methodologies have enabled the observation of processes such as anomalous diffusion for some membrane species in biological membranes. These are discussed in terms of new models for the role of membrane interactions with the cytoskeleton, the effects of molecular crowding in membranes, and the formation of lipid rafts. The diffusion of peptides, proteins and lipids is considered, particularly in relation to the means by which antimicrobial peptide activity may be rationalized in terms of membrane poration and lipid flip-flop.     

The membrane insertion of helical antimicrobial peptides from the N-terminus of Helicobacter pylori ribosomal protein L1

The interaction of two helical antimicrobial peptides, HPA3 and HPA3P with planar supported lipid membranes was quantitatively analysed using two complementary optical biosensors. The peptides are analogues of Hp(2-20) derived from the N-terminus of Helicobacter pylori ribosomal protein L1 (RpL1). The binding of these two peptide analogues to zwitterionic dimyristoyl-phosphatidylcholine (DMPC) and negatively charged membranes composed of DMPC/dimyristoylphosphatidylglycerol (DMPG) (4:1) was determined using surface plasmon resonance (SPR) and dual polarisation interferometry (DPI). Using SPR analysis, it was shown that the proline substitution in HPA3P resulted in much lower binding for both zwitterionic and anionic membranes than HPA3. Structural changes in the planar DMPC and DMPC/DMPG (4:1) bilayers induced by the binding of both Hp(2-20) analogues were then resolved in real-time with DPI. The overall process of peptide-induced changes in membrane structure was analysed by the real-time changes in bound peptide mass as a function of bilayer birefringence. The insertion of both HPA3 and HPA3P into the supported lipid bilayers resulted in a decrease in birefringence with increasing amounts of bound peptide which reflects a decrease in the order of the bilayer. The binding of HPA3 to each membrane was associated with a higher level of bound peptide and greater membrane lipid disordering and a faster and higher degree of insertion into the membrane than HPA3P. Furthermore, the binding of both HPA3 and HPA3P to negatively charged DMPC/DMPG bilayers also leads to a greater disruption of the lipid ordering. These results demonstrate the geometrical changes in the membrane upon peptide insertion and the extent of membrane structural changes can be obtained quantitatively. Moreover, monitoring the effect of peptides on a structurally characterised bilayer has provided further insight into the role of membrane structure changes in the molecular basis of peptide selectivity and activity and may assist in defining the mode of antimicrobial action.     

PrP106-126 peptide disrupts lipid membranes: Influence of C-terminal amidation

PrP106-126 is located within the important domain concerning membrane related conformational conversion of human Prion protein (from cellular isoform PrP^C to scrapie isoform PrP^Sc). Recent advances reveal that the pathological and physicochemical properties of PrP106-126 peptide are very sensitive to its N-terminal amidation, however, the detailed mechanism remains unclear. In this work, we studied the interactions of the PrP106-126 isoforms (PrP106-126_CONH2 and PrP106-126_COOH) with the neutral lipid bilayers by atomic force microscopy, surface plasmon resonance and fluorescence spectroscopy. The membrane structures were disturbed by the two isoforms in a similarly stepwise process. The distinct morphological changes of the membrane were characterized by formation of semi-penetrated defects and sigmoidal growth of flat high-rise domains on the supported lipid bilayers. However, PrP106-126_COOH displayed a higher peptide-lipid binding affinity than PrP106-126_CONH2 (∼2.9 times) and facilitated the peptide-lipid interactions by shortening the lag time. These results indicate that the C-terminal amidation may influence the pathological actions of PrP106-126 by lowering the interaction potentials with lipid membranes.     

α-Adrenergically mediated changes in membrane lipid fluidity and Ca2+ binding in isolated rat liver plasma membranes

Noradrenaline (0.1–5 μM, in the presence of 5 μM propranolol to block β-receptors), ATP (100 μM) and angiotensin II (0.1 μM), which are thought to increase cytosolic Ca²⁺ concentration by mobilizing Ca²⁺ from internal stores, increased the lipid fluidity as measured by diphenylhexatriene fluorescence polarization in plasma membranes isolated from rat liver. The effect of noradrenaline was dose-dependent and blocked by the α-antagonists phenoxybenzamine (50 μM) and phentolamine (1 μM). The response to a maximal dose of noradrenaline (5 μM) and that to ATP (100 μM) were not cumulative, suggesting that both agents use a common mechanism to alter the membrane lipid fluidity. In contrast, the addition of noradrenaline (5 μM) along with the foreign amphiphile Na⁺-oleate (1–30 μM) resulted in an increase in membrane lipid fluidity which was equivalent to the sum of individual responses to the two agents. In the absence of Mg²⁺, reducing free Ca²⁺ concentration by adding EGTA increased membrane lipid fluidity and abolished the effect of noradrenaline, suggesting that Ca²⁺ is involved in the mechanism by which the hormone exerts its effect on plasma membranes. Noradrenaline (5 μM) and angiotensin II (0.1 μM) also promoted a small release of ⁴⁵Ca²⁺ (16 pmol/mg membrane proteins) from prelabelled plasma membranes. The effect of noradrenaline was suppressed by the α-antagonist phentolamine (5 μM). It is proposed that noradrenaline, via α-adrenergic receptors and other Ca²⁺-mobilizing hormones, increases membrane lipid fluidity by displacing a small pool of Ca²⁺ bound to phospholipids, removing thus the mechanical constraints brought about by this ion.     

Structure and water permeability of fully hydrated diphytanoylPC

Diphytanoylphosphatidylcholine (DPhyPC) is a branched chain lipid often used for model membrane studies, including peptide/lipid interactions, ion channels and lipid rafts. This work reports results of volume measurements, water permeability measurements P_f, X-ray scattering from oriented samples, and X-ray and neutron scattering from unilamellar vesicles at T = 30 °C. We measured the volume/lipid V_L = 1426 ± 1 Å³. The area/lipid was found to be 80.5 ± 1.5 Å² when both X-ray and neutron data were combined with the SDP model analysis (Ku?erka, N., Nagle, J.F., Sachs, J.N., Feller, S.E., Pencer, J., Jackson, A., Katsaras, J., 2008. Lipid bilayer structure determined by the simultaneous analysis of neutron and X-ray scattering data. Biophys. J. 95, 2356–2367); this is substantially larger than the area of DOPC which has the largest area of the common linear chain lipids. P_f was measured to be (7.0 ± 1.0) × 10^?3 cm/s; this is considerably smaller than predicted by the recently proposed 3-slab model (Nagle, J.F., Mathai, J.C., Zeidel, M.L., Tristram-Nagle, S., 2008. Theory of passive permeability through lipid bilayers. J. Gen. Physiol. 131, 77–85). This disagreement can be understood if there is a diminished diffusion coefficient in the hydrocarbon core of DPhyPC and that is supported by previous molecular dynamics simulations (Shinoda, W., Mikami, M., Baba, T., Hato, M., 2004. Molecular dynamics study on the effects of chain branching on the physical properties of lipid bilayers. 2. Permeability. J. Phys. Chem. B 108, 9346–9356). While the DPhyPC head–head thickness (D_HH = 36.4 Å), and Hamaker parameter (H = 4.5 × 10^?21 J) were similar to the linear chain lipid DOPC, the bending modulus (K_C = 5.2 ± 0.5 × 10^?21 J) was 30% smaller. Our results suggest that, from the biophysical perspective, DPhyPC belongs to a different family of lipids than phosphatidylcholines that have linear chain hydrocarbon chains.     

Bounding fluid viscosity and translational diffusion in a fluid lipid bilayer

Fluorescence recovery after photobleaching was used to investigate the translational diffusion of a fluorescent derivative of a membrane-spanning lipid in L phase multibilayers of 1-palmitoyl-2-oleoylphosphatidylcholine prepared in water and in glycerol. The translational diffusion coefficient in hydrated bilayers (D _w) ranged between 2 and 5x10^–8 cm²/s and in glycerinated bilayers (D _g) the range was between 3 and 24×10^–10 cm²/s between 10° and 40°C. These results are discussed in terms of models for diffusion in membranes.