Calcium-Lipid Interactions Observed with Isotope-Edited Infrared Spectroscopy

Calcium ions bind to lipid membranes containing anionic lipids; however, characterizing the specific ion-lipid interactions in multicomponent membranes has remained challenging because it requires nonperturbative lipid-specific probes. Here, using a combination of isotope-edited infrared spectroscopy and molecular dynamics simulations, we characterize the effects of a physiologically relevant (2 mM) Ca²⁺ concentration on zwitterionic phosphatidylcholine and anionic phosphatidylserine lipids in mixed lipid membranes. We show that Ca²⁺ alters hydrogen bonding between water and lipid headgroups by forming a coordination complex involving the lipid headgroups and water. These interactions distort interfacial water orientations and prevent hydrogen bonding with lipid ester carbonyls. We demonstrate, experimentally, that these effects are more pronounced for the anionic phosphatidylserine lipids than for zwitterionic phosphatidylcholine lipids in the same membrane.     

Dynamics of the Kv1.2 voltage-gated K+ channel in a membrane environment

All-atom molecular dynamics simulations are used to better understand the dynamic environment experienced by the Kv1.2 channel in a lipid membrane. The structure of the channel is stable during the trajectories. The pore domain keeps a well-defined conformation, whereas the voltage-sensing domains undergo important lateral fluctuations, consistent with their modular nature. A channel-like region at the center of the S1-S4 helical bundle fills rapidly with water, reminiscent of the concept of high-dielectric aqueous crevices. The first two arginines along S4 (R294 and R297) adopt an interfacial position where they interact favorably with water and the lipid headgroups. The following two arginines (R300 and R303) interact predominantly with water and E226 in S2. Despite the absence of a structurally permanent gating pore formed by protein residues and surrounding the S4 helix, as traditionally pictured, the charged residues are located in a favorable environment and are not extensively exposed to the membrane nonpolar region. Continuum electrostatic computations indicate that the transmembrane potential sensed by the charged residues in the voltage sensor varies abruptly over the outer half of the membrane in the arginine-rich region of S4; thus, the voltage gradient or membrane electric field is "focused". Interactions of basic residues with the lipid headgroups at the intracellular membrane-solution interface reduce the membrane thickness near the channel, resulting in an increased transmembrane field.     

Transmembrane domains of viral ion channel proteins: a molecular dynamics simulation study

Nanosecond molecular dynamics simulations in a fully solvated phospholipid bilayer have been performed on single transmembrane alpha-helices from three putative ion channel proteins encoded by viruses: NB (from influenza B), CM2 (from influenza C), and Vpu (from HIV-1). alpha-Helix stability is maintained within a core region of ca. 28 residues for each protein. Helix perturbations are due either to unfavorable interactions of hydrophobic residues with the lipid headgroups or to the need of the termini of short helices to extend into the surrounding interfacial environment in order to form H-bonds. The requirement of both ends of a helix to form favorable interactions with lipid headgroups and/or water may also lead to tilting and/or kinking of a transmembrane alpha-helix. Residues that are generally viewed as poor helix formers in aqueous solution (e.g., Gly, Ile, Val) do not destabilize helices, if located within a helix that spans a lipid bilayer. However, helix/bilayer mismatch such that a helix ends abruptly within the bilayer core destabilizes the end of the helix, especially in the presence of Gly and Ala residues. Hydrogen bonding of polar side-chains with the peptide backbone and with one another occurs when such residues are present within the bilayer core, thus minimizing the energetic cost of burying such side-chains.     

Influence of stigmastanol and stigmastanyl-phosphorylcholine, two plasma cholesterol lowering substances, on synthetic phospholipid membranes. A 2H- and 31P-NMR study.

Cholesterol, stigmastanol, and stigmastanyl-phosphorylcholine (ST-PC) were incorporated into model membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). POPC and ST-PC were deuterated at the lipid headgroup, DOPC at the cis-double bonds. The influence of the three sterols on the motion and conformation of the lipid headgroups and the hydrocarbon chains was monitored with 2H- and 31P-NMR. All three sterols were freely miscible with the lipid matrix in concentrations of up to 50 mol% without inducing phase separations or nonbilayer structures. However, the molecules exert quite different effects on the phospholipid bilayer. Cholesterol and stigmastanol are largely buried in the hydrocarbon part of the membrane, distinctly restricting the flexing motions of the fatty acyl chains whereas the conformation of the phospholipid headgroups is little affected. In contrast, ST-PC is anchored with its headgroup in the layer of phospholipid dipoles, preventing an extensive penetration of the sterol ring into the hydrocarbon layer. Hence ST-PC has almost no effect on the hydrocarbon chains but induces a characteristic conformational change of the phospholipid headgroups. The 2H- and 31P-NMR spectra of mixed phospholipid/ST-PC membranes further demonstrate that the PC headgroup of ST-PC has a similar orientation as the surrounding phosphatidylcholine headgroups. For both types of molecules the -P-N+ dipole is essentially parallel to the membrane surface. Addition of ST-PC induces a small rotation of the POPC headgroup towards the water phase.     

Effect of nonbilayer lipids on membrane binding and insertion of the catalytic domain of leader peptidase

Biological membranes contain a substantial amount of "nonbilayer lipids", which have a tendency to form nonlamellar phases. In this study the hypothesis was tested that the presence of nonbilayer lipids in a membrane, due to their overall small headgroup, results in a lower packing density in the headgroup region, which might facilitate the interfacial insertion of proteins. Using the catalytic domain of leader peptidase (delta2-75) from Escherichia coli as a model protein, we studied the lipid class dependence of its insertion and binding. In both lipid monolayers and vesicles, the membrane binding of (catalytically active) delta2-75 was much higher for the nonbilayer lipid DOPE compared to the bilayer lipid DOPC. For the nonbilayer lipids DOG and MGDG a similar effect was observed as for DOPE, strongly suggesting that no specific interactions are involved but that the small headgroups create hydrophobic interfacial insertion sites. On the basis of the results of the monolayer experiments, calculations were performed to estimate the space between the lipid headgroups accessible to the protein. We estimate a maximal size of the insertion sites of 15 +/- 7 A2/lipid molecule for DOPE, relative to DOPC. The size of the insertion sites decreases with an increase in headgroup size. These results show that nonbilayer lipids stimulate the membrane insertion of delta2-75 and support the idea that such lipids create insertion sites by reducing the packing density at the membrane-water interface. It is suggested that PE in the bacterial membrane facilitates membrane insertion of the catalytic domain of leader peptidase, allowing the protein to reach the cleavage site in preproteins.     

Design of Peptide-Membrane Interactions to Modulate Single-File Water Transport through Modified Gramicidin Channels

Water permeability through single-file channels is affected by intrinsic factors such as their size and polarity and by external determinants like their lipid environment in the membrane. Previous computational studies revealed that the obstruction of the channel by lipid headgroups can be long-lived, in the range of nanoseconds, and that pore-length-matching membrane mimetics could speed up water permeability. To test the hypothesis of lipid-channel interactions modulating channel permeability, we designed different gramicidin A derivatives with attached acyl chains. By combining extensive molecular-dynamics simulations and single-channel water permeation measurements, we show that by tuning lipid-channel interactions, these modifications reduce the presence of lipid headgroups in the pore, which leads to a clear and selective increase in their water permeability.     

Interaction of Zn2+ with phospholipid membranes

To characterize the specificity of zinc binding to phospholipid membranes in terms of headgroup structure, hydration and phase behavior we studied the zwitterionic lipid 1-palmitoyl-2-oleoyl-phosphatidylcholine as a function of hydration at 30 degreesC in the presence and absence of ZnCl2. Zinc forms a 2:1-1:1 complex with the lipid, and in particular with the negatively charged phosphate groups. Zn2(+)-bridges between neighboring lipid molecules stabilize the gel phase of the lipid relative to the liquid-crystalline state. Upon Zn2+ binding the C-O-P-O-C- backbone of the lipid headgroup changes from a gauche/gauche into the trans/trans conformation and it loses roughly 50% of the hydration shell. The ability of the Zn2(+)-bound phosphate groups to take up water is distinctly reduced, meaning that the headgroups have become less hydrophilic. The energetic cost (on the scale of Gibbs free energy) for completely dehydrating the lipid headgroups is decreased by approximately 10 kJ/mole in the presence of Zn2+. The interaction of phospholipid headgroups with Zn2+ is conveniently described by a hydrated zinc-phosphate complex the key energy contribution of which is more covalent than electrostatic in nature. Dehydration of phospholipid headgroups due to complexation with zinc cations is suggested to increase fusogenic potency of lipid membranes. Zinc appears to be one of the most potent divalent cation in inducing membrane fusion.     

Synthesis of phospholipids containing photoactivatable carbene precursors in the headgroups and their crosslinking with membrane proteins

Many integral membrane enzymes require for their activity interactions with the polar headgroups of phospholipids, in addition to the hydrophobic interactions within the lipid bilayer. The interactions with the polar headgroups may have preferential or absolute specificity. To study such interactions, phospholipids have been synthesized which carry photoactivable moieties in their headgroups. Three types of phospholipids, PL-I, PL-II and PL-III, were synthesized. The synthetic phospholipids, PL-I and PL-II were able to reconstitute enzymatic activity of the membrane proteins which were studied. Covalent crosslinking between these phospholipids and the membrane proteins was demonstrated after photolysis of the reconstituted phospholipid-protein complexes.     

Algorithm to catalogue topologies of dynamic lipid hydrogen-bond networks

Lipid membrane interfaces host reactions essential for the functioning of cells. The hydrogen-bonding environment at the membrane interface is particularly important for binding of proteins, drug molecules, and ions. We present here the implementation and applications of a depth-first search algorithm that analyzes dynamic lipid interaction networks. Lipid hydrogen-bond networks sampled transiently during simulations of lipid bilayers are clustered according to main types of topologies that characterize three-dimensional arrangements of lipids connected to each other via short water bridges. We characterize the dynamics of hydrogen-bonded lipid clusters in simulations of model POPE and POPE:POPG membranes that are often used for bacterial membrane proteins, in a model of the Escherichia coli membrane with six different lipid types, and in POPS membranes. We find that all lipids sample dynamic hydrogen-bonded networks with linear, star, or circular arrangements of the lipid headgroups, and larger networks with combinations of these three types of topologies. Overall, linear lipid-water bridges tend to be short. Water-mediated lipid clusters in all membranes with PE lipids tend to be somewhat small, with about four lipids in all membranes studied here. POPS membranes allow circular arrangements of three POPS lipids to be sampled frequently, and complex arrangements of linear, star, and circular paths may also be sampled. These findings suggest a molecular picture of the membrane interface whereby lipid molecules transiently connect in clusters with somewhat small spatial extension.     

Structure and dynamics of model pore insertion into a membrane

A cylindrical transmembrane molecule is constructed by linking hydrophobic sites selected from a coarse grain model. The resulting hollow tube assembly serves as a representation of a transmembrane channel, pore, or a carbon nanotube. The interactions of a coarse grain di-myristoyl-phosphatidyl-choline hydrated bilayer with both a purely hydrophobic tube and a tube with hydrophilic caps are studied. The hydrophobic tube rotates in the membrane and becomes blocked by lipid tails after a few tens of nanoseconds. The hydrophilic sites of the capped tube stabilize it by anchoring the tube in the lipid headgroup/water interfacial region of each membrane leaflet. The capped tube remains free of lipid tails. The capped tube spontaneously conducts coarse grain water sites; the free-energy profile of this process is calculated using three different methods and is compared to the barrier for water permeation through the lipid bilayer. Spontaneous tube insertion into an undisturbed lipid bilayer is also studied, which we reported briefly in a previous publication. The hydrophobic tube submerges into the membrane core in a carpetlike manner. The capped tube laterally fuses with the closest leaflet, and then, after plunging into the membrane interior, rotates to assume a transbilayer orientation. Two lipids become trapped at the end of the tube as it penetrates the membrane. The hydrophilic headgroups of these lipids associate with the lower tube cap and assist the tube in crossing the interior of the membrane. When the rotation is complete these lipids detach from the tube caps and fuse with the lower leaflet lipids.     

Rotational decoupling between the hydrophilic and hydrophobic regions in lipid membranes

Cells use homeostatic mechanisms to ensure an optimal composition of distinct types of lipids in cellular membranes. The hydrophilic region of biological lipid membranes is mainly composed of several types of phospholipid headgroups that interact with incoming molecules, nanoparticles, and viruses, whereas the hydrophobic region consists of a distribution of acyl chains and sterols affecting membrane fluidity/rigidity related properties and forming an environment for membrane-bound molecules such as transmembrane proteins. A fundamental open question is to what extent the motions of these regions are coupled and, consequently, how strongly the interactions of phospholipid headgroups with other molecules depend on the properties and composition of the membrane hydrophobic core. We combine advanced solid-state nuclear magnetic resonance spectroscopy with high-fidelity molecular dynamics simulations to demonstrate how the rotational dynamics of choline headgroups remain nearly unchanged (slightly faster) with incorporation of cholesterol into a phospholipid membrane, contrasting the well-known extreme slowdown of the other phospholipid segments. Notably, our results suggest a new paradigm in which phospholipid dipole headgroups interact as quasi-freely rotating flexible dipoles at the interface, independent of the properties in the hydrophobic region.     

Freezing and melting water in lamellar structures.

The manner in which ice forms in lamellar suspensions of dielaidoylphosphatidylethanolamine, dielaidoylphosphatidylcholine, and dioleoylphosphatidylcholine in water depends strongly on the water fraction. For weight fractions between 15 and 9%, the freezing and melting temperatures are significantly depressed below 0 degree C. The ice exhibits a continuous melting transition spanning as much as 20 degrees C. When the water weight fraction is below 9%, ice never forms at temperatures as low as -40 degrees C. We show that when water contained in a lamellar lipid suspension freezes, the ice is not found between the bilayers; it exists as pools of crystalline ice in equilibrium with the bound water associated with the polar lipid headgroups. We have used this effect, together with the known chemical potential of ice, to measure hydration forces between lipid bilayers. We find exponentially decaying hydration repulsion when the bilayers are less than about 7 A apart. For larger separations, we find significant deviations from single exponential decay.     

Noncontact dipole effects on channel permeation. III. Anomalous proton conductance effects in gramicidin

Proton transport on water wires, of interest for many problems in membrane biology, is analyzed in side-chain analogs of gramicidin A channels. In symmetrical 0.1 N HCl solutions, fluorination of channel Trp(11), Trp-(13), or Trp(15) side chains is found to inhibit proton transport, and replacement of one or more Trps with Phe enhances proton transport, the opposite of the effects on K(+) transport in lecithin bilayers. The current-voltage relations are superlinear, indicating that some membrane field-dependent process is rate limiting. The interfacial dipole effects are usually assumed to affect the rate of cation translocation across the channel. For proton conductance, however, water reorientation after proton translocation is anticipated to be rate limiting. We propose that the findings reported here are most readily interpreted as the result of dipole-dipole interactions between channel waters and polar side chains or lipid headgroups. In particular, if reorientation of the water column begins with the water nearest the channel exit, this hypothesis explains the negative impact of fluorination and the positive impact of headgroup dipole on proton conductance.     

Molecular dynamics study of peptide-bilayer adsorption

Two 6-ns simulations of the somatostatin analog sandostatin and a 1-palmityl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer are presented. In the first simulation, the peptide was placed in a region of bulk water density and allowed to spontaneously move toward and bind to the bilayer surface. An attractive force between the peptide and bilayer drove the binding process, which was opposed by a significant frictional force caused by the solvent (water). During the approach of the peptide toward the bilayer the area of the interacting surface between the species was inversely proportional to the distance between them, supporting the application of such a relationship in continuum calculations of peptide-bilayer binding free energies. In the second simulation, the N-terminus of the surface-bound peptide was deprotonated. Consistent with experiment, this strengthened interactions between the peptide and the bilayer. Details of both peptide-bilayer complexes, including the orientation, percent buried surface area, and orientation of the lipid headgroups are in good agreement with those obtained from experiment. The location of the different side chains in the bilayer is in direct correlation with an interfacial hydrophobicity scale developed using model peptides. The aromatic side chains of the Phe and Trp residues all lie flat with respect to the bilayer surface in both complexes. Changes in lipid and water ordering due to peptide binding suggest a possible domination of lipophobic over hydrophobic effects, as proposed by other workers. Where appropriate, peptide and lipid properties in the bound states are compared with separate simulations of sandostatin and the bilayer in water, respectively, so as to monitor the response of the system to the binding process.     

Cationic DMPC/DMTAP lipid bilayers: molecular dynamics study

Cationic lipid membranes are known to form compact complexes with DNA and to be effective as gene delivery agents both in vitro and in vivo. Here we employ molecular dynamics simulations for a detailed atomistic study of lipid bilayers consisting of a mixture of cationic dimyristoyltrimethylammonium propane (DMTAP) and zwitterionic dimyristoylphosphatidylcholine (DMPC). Our main objective is to examine how the composition of the DMPC/DMTAP bilayers affects their structural and electrostatic properties in the liquid-crystalline phase. By varying the mole fraction of DMTAP, we have found that the area per lipid has a pronounced nonmonotonic dependence on the DMTAP concentration, with a minimum around the point of equimolar DMPC/DMTAP mixture. We show that this behavior has an electrostatic origin and is driven by the interplay between positively charged TAP headgroups and the zwitterionic phosphatidylcholine (PC) heads. This interplay leads to considerable reorientation of PC headgroups for an increasing DMTAP concentration, and gives rise to major changes in the electrostatic properties of the lipid bilayer, including a significant increase of total dipole potential across the bilayer and prominent changes in the ordering of water in the vicinity of the membrane. Moreover, chloride counterions are bound mostly to PC nitrogens implying stronger screening of PC heads by Cl ions compared to TAP headgroups. The implications of these findings are briefly discussed.     

Local Partition Coefficients Govern Solute Permeability of Cholesterol-Containing Membranes

The permeability of lipid membranes for metabolic molecules or drugs is routinely estimated from the solute’s oil/water partition coefficient. However, the molecular determinants that modulate the permeability in different lipid compositions have remained unclear. Here, we combine scanning electrochemical microscopy and molecular-dynamics simulations to study the effect of cholesterol on membrane permeability, because cholesterol is abundant in all animal membranes. The permeability of membranes from natural lipid mixtures to both hydrophilic and hydrophobic solutes monotonously decreases with cholesterol concentration [Chol]. The same is true for hydrophilic solutes and planar bilayers composed of dioleoyl-phosphatidylcholine or dioleoyl-phosphatidyl-ethanolamine. However, these synthetic lipids give rise to a bell-shaped dependence of membrane permeability on [Chol] for very hydrophobic solutes. The simulations indicate that cholesterol does not affect the diffusion constant inside the membrane. Instead, local partition coefficients at the lipid headgroups and at the lipid tails are modulated oppositely by cholesterol, explaining the experimental findings. Structurally, these modulations are induced by looser packing at the lipid headgroups and tighter packing at the tails upon the addition of cholesterol.     

Incorporation of alpha-chymotrypsin into the 3D channels of bicontinuous cubic lipid mesophases

The effects of protein entrapment on the structure and phase behavior of periodically curved lipid mesostructures have been examined by synchrotron small-angle X-ray diffraction and FT-IR spectroscopy. The study was directed towards a better understanding of the effect of confinement in a lipid environment on the stability and unfolding behavior of alpha-chymotrypsin, and, vice versa, the effect of the entrapped protein on the lipid's mesophase structure and temperature- and pressure-dependent phase behavior. We compare the interaction of protein molecules of two different sizes (cytochrome c, 12.4 kDa, and alpha-chymotrypsin, 25.8 kDa) with the cubic Ia3d phase of monoolein (MO), which forms spontaneously in water. The cubic structure changes significantly when cyt c is incorporated: above a protein concentration of 0.2 wt.%, the interaction between the positively charged protein and the lipid headgroups leads to an increase in interfacial curvature which promotes the formation of a new micellar cubic phase, presumably of crystallographic space group P4(3)32, which the lipid system does not form on its own. The larger alpha-chymotrypsin leads to a different scenario. On the basis of an examination of the calculated geometric parameters and water volume fractions, it is concluded that the alpha-chymotrypsin molecules cannot be located exclusively in the water channels of the cubic Ia3d or P4(3)32 phases, but rather form new, less ordered (presumably cubic Pn3m) structures. The new structure disappears above the unfolding temperature of chymotrypsin and exhibits a pressure stability, which-- in contrast to cyt c in MO-- decreases with increasing chymotrypsin concentration in the system. While the secondary structure of cyt c remains unaffected in the confining lipid environment, the structure of alpha-chymotrypsin gets destabilized slightly, and the protein tends to aggregate even at relatively low concentrations.     

Local Partition Coefficients Govern Solute Permeability of Cholesterol-Containing Membranes

The permeability of lipid membranes for metabolic molecules or drugs is routinely estimated from the solute’s oil/water partition coefficient. However, the molecular determinants that modulate the permeability in different lipid compositions have remained unclear. Here, we combine scanning electrochemical microscopy and molecular-dynamics simulations to study the effect of cholesterol on membrane permeability, because cholesterol is abundant in all animal membranes. The permeability of membranes from natural lipid mixtures to both hydrophilic and hydrophobic solutes monotonously decreases with cholesterol concentration [Chol]. The same is true for hydrophilic solutes and planar bilayers composed of dioleoyl-phosphatidylcholine or dioleoyl-phosphatidyl-ethanolamine. However, these synthetic lipids give rise to a bell-shaped dependence of membrane permeability on [Chol] for very hydrophobic solutes. The simulations indicate that cholesterol does not affect the diffusion constant inside the membrane. Instead, local partition coefficients at the lipid headgroups and at the lipid tails are modulated oppositely by cholesterol, explaining the experimental findings. Structurally, these modulations are induced by looser packing at the lipid headgroups and tighter packing at the tails upon the addition of cholesterol.     

Constant-pressure molecular dynamics investigation of cholesterol effects in a dipalmitoylphosphatidylcholine bilayer.

We report a 1.4-ns constant-pressure molecular dynamics simulation of cholesterol at 12.5 mol% in a dipalmitoylphosphatidylcholine (DPPC) bilayer at 50 degrees C and compare the results to our previous simulation of a pure DPPC bilayer. The interlamellar spacing was increased by 2.5 A in the cholesterol-containing bilayer, consistent with x-ray diffraction results, whereas the bilayer thickness was increased by only 1 A. The bilayer/water interface was more abrupt because the lipid headgroups lie flatter to fill spaces left by the cholesterol molecules. This leads to less compensation by the lipid headgroups of the oriented water contribution to the membrane dipole potential and could explain the experimentally observed increase in the magnitude of the dipole potential by cholesterol. Our calculations suggested that 12.5 mol% cholesterol does not significantly affect the conformations and packing of the hydrocarbon chains and produces only a slight reduction in the empty free volume. However, cholesterol has a significant influence on the subnanosecond time scale lipid dynamics: the diffusion constant for the center-of-mass "rattling" motion was reduced by a factor of 3, and the reorientational motion of the methylene groups was slowed along the entire length of the hydrocarbon chains.     

Hydration of the dienic lipid dioctadecadienoylphosphatidylcholine in the lamellar phase--an infrared linear dichroism and x-ray study on headgroup orientation, water ordering, and bilayer dimensions.

In the phospholipid 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) in each of the fatty acid chains, a rigid diene group is inserted in a position near the polar/apolar boundary that is exceptionally sensitive for membrane stability. DODPC transforms upon gradual dehydration from the liquid-crystalline to a metastable gel state, which rearranges into two subgel phases at low and intermediate degrees of hydration. The molecular dimensions of the respective bilayers were determined by means of x-ray diffraction. Infrared linear dichroism of selected vibrations of the phosphate and trimethylammonium groups and of the nu13(OH) band of water adsorbed onto the lipid was used to study the molecular order in the polar part of the bilayers in macroscopically oriented samples. The dense packing of the tilted acyl chains in the subgel causes the in-plane orientation of the phosphatidylcholine headgroups with direct interactions between the phosphate and trimethylammonium groups, and a strong orientation of adsorbed water molecules. In the more disordered gel, the thickness of the polar part of the bilayer increases and the lateral interactions between the lipid headgroups weaken. The higher order in the headgroup region of the subgels correlates with shorter decay lengths of the repulsive forces acting between opposite membrane surfaces. This result can be understood if the work to dehydrate the lipid is determined to a certain degree by the work to break up the lipid-water interactions without compensation by adequate lipid-lipid contacts. Almost similar area compressibility moduli are found in the liquid-crystalline and solid phases. Obviously, the lipid avoids lateral stress by the structural rearrangement.