DPPC Bilayers Research Articles

Properties of Mixed DOTAP−DPPC Bilayer Membranes as Reported by Differential Scanning Calorimetry and Dynamic Light Scattering Measurements

We have investigated the effect of a cationic lipid [DOTAP] on both the thermotropic phase behavior and the structural organization of aqueous dispersions of dipalmitoyl-phosphatidylcholine [DPPC] by means of high-sensitivity differential scanning calorimetry and dynamic light scattering measurements. We find that the incorporation of increasing quantities of DOTAP progressively reduces the temperature and the enthalpy of the gel-to-liquid crystalline transition. We are further showing that, in mixed DOTAP-DPPC systems, the reduction of the phase transition temperature is accompanied by a reduction of the average size of the structures present in the aqueous mixtures, whatever the DOTAP concentration is. These results, which extend a previous investigation by Campbell et al. (Campbell, R. B.; Balasubramanian, S. V.; Straubinger, R. M.; Biochim. Biosphys. Acta 2001, 27, 1512.) limited to a DOTAP concentration below 20 mol %, confirm that the insertion of cationic head groups in zwitterionic phosphatidylcholine bilayers facilitates the formation of stable, relatively small, unilamellar vesicles. This self-assembling restructuring from an aqueous multilamellar structure toward a liposomal phase is favored by decreasing the phospholipid phase transition temperature and by increasing the temperature of the system. This reduction of the average size and the appearance of a stable liposomal phase is also promoted by a heating and cooling thermal treatment.

Molecular dynamics simulation study of interaction between a class IIa bacteriocin and its immunity protein

Molecular dynamics (MD) simulations of carnobacteriocin B2 (CbnB2), a structurally well-characterized class IIa bacteriocin, and its immunity protein (ImB2) in lipid bilayer environment have been conducted to explore the interaction between them. Six 30-ns simulations were conducted in DPPC or POPG bilayer systems. In these simulations, ImB2 was placed in the aqueous layer with different orientations facing CbnB2 to sample all the faces of ImB2. The MD results indicate that (i) while CbnB2 remained embedded in the bilayer, it tends to move toward the interface, and (ii) the presence of CbnB2 in the DPPC bilayer attracts ImB2 toward the bilayer. In one of the orientations in DPPC bilayer system (simulation 1), ImB2 penetrates the bilayer and interacts with CbnB2 by ion-pair interaction. At several instances toward the later half of the simulation (15–30 ns), ImB2 and CbnB2 were found to form salt-bridge between Arg95 of ImB2 and Glu24 of CbnB2. Simulation in POPG bilayer displayed strong interaction between the positively charged ImB2 and the negatively charged polar head groups of the POPG molecules at the lipid–water interface. However, ImB2 was not able to penetrate the bilayer thereby preventing any interaction between ImB2 and CbnB2.

Time-resolved grazing-incidence small-angle X-ray scattering studies of lipid multibilayers with the insertion of amyloid peptide during the swelling process

The β-amyloid peptide (Aβ) (1–40) is one of the major components that form Alzheimer's amyloid deposits. Studies of the membrane insertion of amyloid showed that amyloid is surface active and can insert into lipid monolayers [Ji et al. (2002). Biochemistry (Moscow), 67, 1283–1288; Ege & Lee (2004). Biophys. J. 87, 1732–1740]. The interaction between the peptide and a lipid monolayer or bilayer is critical to the understanding of the formation of amyloid peptide deposits on the membrane. In this paper, we have studied the structural transition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multibilayers with the insertion of amyloid peptide 1–40 by grazing-incidence small-angle X-ray scattering at different bilayer hydration levels (changing the relatively humidity). We mixed the DPPC and amyloid peptide in an organic solvent at a 10 to 1 weight ratio, then cast it onto a silicon wafer to form the mixed lipid multibilayer film. In this study of the pure DPPC multibilayer film and the DPPC multibilayer film inserted with amyloid peptide, it was found that the hydration process was bimodal with a better-hydrated top layer and a less-hydrated bottom layer. The gel-to-ripple phase transition suffers a strong confinement effect due to the presence of the solid substrate. With the insertion of the amyloid peptide, the ripple phase of the membrane bilayers was suppressed at high humidity and the whole film can be swollen more uniformly at lower incubation time than the pure DPPC film supported on a silicon wafer. This means water vapor can penetrate more easily into the DPPC bilayers inserted with Aβ than into the pure DPPC bilayers. Amyloid peptides were found to form clusters in the bilayer and possess in-plane correlation. From analyzing the diffuse scattering around the Bragg peak in the lateral direction, the amyloid peptides are found to form clusters in the bilayer with a radius of about 9 nm. It is also estimated that the number of Aβ molecules in one cluster is about 12 and on average each Aβ molecule occupies an interface area of about 22 nm2. As the relative humidity exceeds about 94%, the Aβ clusters seem to develop an ordered structure with a spacing of about 300 Å.

Characterization of the conformational and orientational dynamics of ganglioside GM1 in a dipalmitoylphosphatidylcholine bilayer by molecular dynamics simulations

The structure and dynamics of a single GM1 (Gal5-β1,3-GalNAc4-β1,4-(NeuAc3-α2,3)-Gal2-β1,4-Glc1-β1,1-Cer) embedded in a DPPC bilayer have been studied by MD simulations. Eleven simulations, each of 10 ns productive run, were performed with different initial conformations of GM1. Simulations of GM1-Os in water and of a DPPC bilayer were also performed to delineate the effects of the bilayer and GM1 on the conformational and orientational dynamics of each other. The conformation of the GM1 headgroup observed in the simulations is in agreement with those reported in literature; but the headgroup is restricted when embedded in the bilayer. NeuAc3 is the outermost saccharide towards the water phase. Glc1 and Gal2 prefer a parallel, and NeuAc3, GalNac4 and Gal5 prefer a perpendicular, orientation with respect to the bilayer normal. The overall characteristics of the bilayer are not affected by the presence of GM1; however, GM1 does influence the DPPC molecules in its immediate vicinity. The implications of these observations on the specific recognition and binding of GM1 embedded in a lipid bilayer by exogenous proteins as well as proteins embedded in lipids have been discussed.

Translocation of C60 and Its Derivatives Across a Lipid Bilayer

Obtaining an understanding, at the atomic level, of the interaction of nanomaterials with biological systems has recently become an issue of great research interest. Here we report on the molecular dynamics study of the translocation of fullerene C60 and its derivative C60(OH)20 across a model cell membrane (dipalmitoylphosphatidylcholine or DPPC bilayer). The simulation results indicate that, although a pristine C60 molecule can readily "jump" into the bilayer and translocate the membrane within a few milliseconds, the C60(OH)20 molecule can barely penetrate the bilayer. Indeed, the mean translocation time via diffusion for the C60(OH)20 molecule is several orders of magnitude longer than for the former. It was also determined that the two different forms of fullerenes, when adsorbed into/onto the bilayer, affected the membrane structure differently. This study offers a mechanistic explanation of that difference and for the reduced acute toxicity of functionalized fullerenes.

New GHK hydrophobic derivatives: Interaction with phospholipid bilayers

Three hydrophobic derivatives of GHK peptide containing either N-terminal hexanoyl, decanoyl or myristoyl acyl moieties were synthesized. The binding of these peptidolipids to phospholipid bilayers as well as their hemolytic activity were determined. Moreover, the influence of these peptidolipids on several physicochemical properties of liposomes was studied. Binding experiments indicate a high affinity of these peptidolipids for lipids ordered in liposomes. Nevertheless, this interaction does not promote the release of entrapped carboxyfluorescein. Experiments carried out by the asymmetric membrane method (NBD-PE/dithionite) and quenching studies (PC-pyrene/KI) indicate that this association has a protective effect suggesting that the hydrophobic moiety inserts in the external part of the bilayer and the peptide chain remains protruding from the surface hindering the entrance or the approach of reactants to it. The microviscosity of DPPC bilayers determined using TMA-DPH as fluorescent marker was not affected by the presence of peptidolipids. Besides, results indicate that myristoyl-GHK produces total hemolysis at 2.5 × 10 −4 M but decanoyl and hexanoyl derivatives at 5 × 10 −4 M induce only 10% of hemolysis.

Effect of the Nature of the 3β-Substitution in Manoyl Oxides on the Thermotropic Behavior of DPPC Lipid Bilayer and on DPPC Liposomes

Functionalized manoyl oxide derivatives have been proved over the years to evoke several biological responses. Among them, 3β-hydroxy-manoyl oxide (1) and 3β-acetoxy-manoyl oxide (2) have been shown to exhibit in vitro antimicrobial and cytotoxic activity, while N-imidazole-3 β-thiocarbonyl ester of manoyl oxide (3) was found to exhibit potent cytotoxic effect. Their partitioning into phospholipid bilayers may lead to membrane structure modifications that are crucial in liposome development as they may influence their maintenance and integrity. DSC was used to study the modifications induced in DPPC bilayers by incorporating increasing concentrations of the three manoyl oxide derivatives. All derivatives were found to strongly affect the bilayer structural organization in terms of a decrease of the cooperativity, the fluidization and partially destabilization of the gel phase and the induction of a lateral phase separation in clustering domains. Derivatives 1 and 3 were incorporated into DPPC liposomes and their physicochemical stability was monitored at 4°C. The stability of liposomes was strongly influenced by the presence of 1 and 3 at any molar ratio studied. DPPC/1 liposomes were found to retain its stability for 48 h at low concentration of 10% mol, while at higher concentrations up to 30% mol they collapsed into aggregated material. In all cases DPPC/3 liposomes were found unstable and sticky aggregated structures precipitated from the bulk suspension.

Influence of lipidation of GBV-C/HGV NS3 (513–522) and (505–514) peptide sequences on its interaction with mono and bilayers

Two decapeptide fragments of the non-structural hepatitis G NS3 protein (GBV-C/HGV), 513–522 (RGRTGRGRSG) and 505–514 (SAELSMQRRG), as well as their palmitoylated derivatives were synthesized. The physico-chemical properties of the peptides were analyzed in both the absence and presence of the zwitterionic 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC), the negative 1,2-dipalmitoyl- sn-glycero-3-[phospho- rac-(1-glycerol)] (DPPG) and the positive 1,2-dioeloyl-3-trimethylammonium-propane (DOTAP) lipid monolayers. Based on their high hydrophilic properties, neither parent peptide presented surface activity and their incorporation into lipid monolayers was low. In contrast, their palmitoylated derivatives showed concentration-dependent surface activity and could be inserted into lipid monolayers to varying degrees depending on their sequence. Compression isotherms showed that the presence of palmitoylated peptides in the subphase resulted in a molecular arrangement less condensed than that corresponding to the pure phospholipid. In concordance with the monolayer results, differential scanning calorimetry (DSC) demonstrated that the parent peptides did not have any effect on the thermograms, while the palmitoylated derivatives affected the thermotropic properties of DPPC bilayers.

Calorimetric study on the induction of interdigitated phase in hydrated DPPC bilayers by bioactive labdanes and correlation to their liposome stability: The role of chemical structure

Labd-7,13-dien-15-ol (1), labd-13-ene-8α,15-diol (2), and labd-14-ene-8,13-diol (sclareol) have been found to exhibit cytotoxic and cytostatic effects. Their partitioning into phospholipid bilayers may induce membrane structure modifications, crucial in the development of liposomes. DSC was used to elucidate the profile of modifications induced in DPPC bilayers by incorporating increasing concentrations of the labdanes. Labdanes 1, 2 and sclareol were incorporated into SUV liposomes composed of DPPC their physicochemical stability was monitored (4 °C) and was compared to liposomes incorporating cholesterol. All labdanes strongly affect the bilayer organization in a concentration dependent manner in terms of a decrease of the cooperativity, the fluidization and partially destabilization of the gel phase, the induction of a lateral phase separation and the possible existence of interdigitated domains in the bilayer. The physicochemical stability of liposomes was strongly influenced by the chemical features of the labdanes. The liposomal preparations were found to retain their stability at low labdane concentration (10 mol%), while at higher concentrations up to 30 mol% a profound decrease in intact liposomes occurred, and a possible existence of interdigitated sheets was concluded.

Molecular studies of the gel to liquid-crystalline phase transition for fully hydrated DPPC and DPPE bilayers

Molecular dynamics simulations were used for a comprehensive study of the structural properties of saturated lipid bilayers, DPPC and DPPE, near the main phase transition. Though the chemical structure of DPPC and DPPE are largely similar (they only differ in the choline and ethanolamine groups), their transformation process from a gel to a liquid-crystalline state is contrasting. For DPPC, three distinct structures can be identified relative to the melting temperature ( T m): below T m with “mixed” domains consisting of lipids that are tilted with partial overlap of the lipid tails between leaflet; near T m with a slight increase in the average area per lipid, resulting in a rearrangement of the lipid tails and an increase in the bilayer thickness; and above T m with unhindered lipid tails in random motion resulting in an increase in %gauche formed and increase in the level of interdigitation between lipid leaflets. For DPPE, the structures identified were below T m with “ordered” domains consisting of slightly tilted lipid tails and non-overlapping lipid tails between leaflets, near T m with minimal rearrangement of the lipids as the bilayer thickness reduces slightly with increasing temperature, and above T m with unhindered lipid tails as that for DPPC. For DPPE, most of the lipid tails do not overlap as observed to DPPC, which is due to the tight packing of the DPPE molecules. The non-overlapping behavior of DPPE above T m is confirmed from the density profile of the terminal carbon atoms in each leaflet, which shows a narrow distribution near the center of the bilayer core. This study also demonstrates that atomistic simulations are capable of capturing the phase transition behavior of lipid bilayers, providing a rich set of molecular and structural information at and near the transition state.

The perturbing effect of cholesterol on the interaction between labdanes and DPPC bilayers

Differential scanning calorimetry (DSC) was used to study the effect of cholesterol on the perturbation of DPPC bilayers induced by eight bioactive structurally related labdanes isolated from the resin ‘ladano’ of Cistus creticus subsp. creticus (Cistaceae) or semisynthesized from the mother compounds. Labdanes themselves induced profound modifications in DPPC bilayer organization and thermotropic properties that were altered when cholesterol was incorporated in equimolar amounts to the labdanes. The present work shows that, up to 10 mol% of the equimolar mixture of cholesterol and the labdanes, the modifications evoked on DPPC bilayer organization are in accordance to these induced by the labdanes themselves. When the concentration exceeded 20 mol%, cholesterol influence dominated while the effect of the labdanes was suppressed and their interaction with the bilayer was probably prevented. The degree by which cholesterol modulated the labdane interaction with the bilayer depended on their structural characteristics that determine their localization in the bilayer interior. Polar groups that force the labdanes to localize themselves at the interfacial region broadened the concentration range by which labdanes interacted with the DPPC bilayer even in the presence of high concentration of cholesterol where cholesterol-rich domains are preferentially formed. On the other hand, labdanes possessing functional groups that promote their deeper penetration in the bilayer interior compete with cholesterol in a high extent for the same localization sites resulting in their possible elimination from the bilayer when the concentration of cholesterol present exceeds the 20 mol%.

Comparative Calorimetric and Spectroscopic Studies of the Effects of Lanosterol and Cholesterol on the Thermotropic Phase Behavior and Organization of Dipalmitoylphosphatidylcholine Bilayer Membranes

We carried out comparative DSC and Fourier transform infrared spectroscopic studies of the effects of cholesterol and lanosterol on the thermotropic phase behavior and organization of DPPC bilayers. Lanosterol is the biosynthetic precursor of cholesterol and differs in having three rather than two axial methyl groups projecting from the β-face of the planar steroid ring system and one axial methyl group projecting from the α-face, whereas cholesterol has none. Our DSC studies indicate that the incorporation of lanosterol is more effective than cholesterol is in reducing the enthalpy of the pretransition. Lanosterol is also initially more effective than cholesterol in reducing the enthalpies of both the sharp and broad components of the main phase transition. However, at sterol concentrations of 50 mol %, lanosterol does not abolish the cooperative hydrocarbon chain-melting phase transition as does cholesterol. Moreover, at higher lanosterol concentrations (∼30–50 mol %), both sharp and broad low-temperature endotherms appear in the DSC heating scans, suggestive of the formation of lanosterol crystallites, and of the lateral phase separation of lanosterol-enriched phospholipid domains, respectively, at low temperatures, whereas such behavior is not observed with cholesterol at comparable concentrations. Our Fourier transform infrared spectroscopic studies demonstrate that lanosterol incorporation produces a less tightly packed bilayer than does cholesterol, which is characterized by increased hydration in the glycerol backbone region of the DPPC bilayer. These and other results indicate that lanosterol is less miscible in DPPC bilayers than is cholesterol, but perturbs their organization to a greater extent, probably due primarily to the rougher faces and larger cross-sectional area of the lanosterol molecule and perhaps secondarily to its decreased ability to form hydrogen bonds with adjacent DPPC molecules. Nevertheless, lanosterol does appear to produce a lamellar liquid-ordered phase in DPPC bilayers, although this phase is not as tightly packed as comparable cholesterol/DPPC mixtures.

Phospholipid Bilayer Free Volume Analysis Employing the Thermal Ring-Closing Reaction of Merocyanine Molecular Switches

The free volume properties of phospholipid bilayers have been determined using a new assay that applies the photochromic and solvatochromic properties of merocyanines. The orientation and embedding depth of the merocyanines in the bilayer are controlled using substitution on the merocyanine indole moiety. The free volume changes at the aqueous interface (region 1), the phospholipid headgroup (region 2), and the aliphatic interior (region 3) of the bilayer are compared by analyzing the rate constants for the merocyanine ring-closing reaction. Free volume variations during the P(beta)(')(gel) <--> L(alpha)(liquid) phase transition are observed in region 1, in accordance with large structural rearrangements between the gel and the liquid phases in this region. The largest free volume is found in region 3, and the smallest is found in region 2. This distribution of free volume in the bilayer agrees with computational studies of these systems. Comparison of the free volume in region 2 of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipids shows that this method is sensitive to small structural differences between lipids. In region 2, the free volume is found to be approximately 2 times larger in DPPC bilayers, which could be related to different merocyanine interactions with the two phosphatidylcholines. Free volume properties determined on picosecond and second time scales are compared based on an analysis of merocyanine formation and decoloration reactions.

Coarse-grained molecular dynamics simulations of membrane proteins and peptides

Molecular dynamics (MD) simulations provide a valuable approach to the dynamics, structure, and stability of membrane–protein systems. Coarse-grained (CG) models, in which small groups of atoms are treated as single particles, enable extended (>100 ns) timescales to be addressed. In this study, we explore how CG–MD methods that have been developed for detergents and lipids may be extended to membrane proteins. In particular, CG–MD simulations of a number of membrane peptides and proteins are used to characterize their interactions with lipid bilayers. CG–MD is used to simulate the insertion of synthetic model membrane peptides (WALPs and LS3) into a lipid (PC) bilayer. WALP peptides insert in a transmembrane orientation, whilst the LS3 peptide adopts an interfacial location, both in agreement with experimental biophysical data. This approach is extended to a transmembrane fragment of the Vpu protein from HIV-1, and to the coat protein from fd phage. Again, simulated protein/membrane interactions are in good agreement with solid state NMR data for these proteins. CG–MD has also been applied to an M3–M4 fragment from the CFTR protein. Simulations of CFTR M3–M4 in a detergent micelle reveal formation of an α-helical hairpin, consistent with a variety of biophysical data. In an I231D mutant, the M3–M4 hairpin is additionally stabilized via an inter-helix Q207/D231 interaction. Finally, CG–MD simulations are extended to a more complex membrane protein, the bacterial sugar transporter LacY. Comparison of a 200 ns CG–MD simulation of LacY in a DPPC bilayer with a 50 ns atomistic simulation of the same protein in a DMPC bilayer shows that the two methods yield comparable predictions of lipid–protein interactions. Taken together, these results demonstrate the utility of CG–MD simulations for studies of membrane/protein interactions.

Interaction of Fusidic Acid with Lipid Membranes: Implications to the Mechanism of Antibiotic Activity

We have studied the effects of cholesterol and steroid-based antibiotic fusidic acid (FA) on the behavior of lipid bilayers using a variety of experimental techniques together with atomic-scale molecular dynamics simulations. Capillary electrophoretic measurements showed that FA was incorporated into fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine membranes. Differential scanning calorimetry in turn showed that FA only slightly altered the thermodynamic properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, whereas cholesterol abolished all endotherms when the mole fraction of cholesterol (Xchol) was >0.20. Fluorescence spectroscopy was then used to further characterize the influence of these two steroids on DPPC large unilamellar vesicles. In the case of FA, our result strongly suggested that FA was organized into lateral microdomains with increased water penetration into the membrane. For cholesterol/DPPC mixtures, fluorescence spectroscopy results were compatible with the formation of the liquid-ordered phase. A comparison of FA and cholesterol-induced effects on DPPC bilayers through atomistic molecular dynamics simulations showed that both FA and cholesterol tend to order neighboring lipid chains. However, the ordering effect of FA was slightly weaker than that of cholesterol, and especially for deprotonated FA the difference was significant. Summarizing, our results show that FA is readily incorporated into the lipid bilayer where it is likely to be enriched into lateral microdomains. These domains could facilitate the association of elongation factor-G into lipid rafts in living bacteria, enhancing markedly the antibiotic efficacy of FA.

Closer Look at Structure of Fully Hydrated Fluid Phase DPPC Bilayers

X-ray data are presented for the benchmark dipalmitoylphosphatidylcholine lipid bilayer in the most biologically relevant state in which the bilayers are fully hydrated and in the fluid (liquid-crystalline) phase. Form factors F( q z) are obtained from a combination of two sample preparations, oriented stacks of bilayers for q z extending to 0.85 Å −1 and unilamellar vesicles for smaller q z. Modeling obtains the electron density profile and values for the area per molecule, for the locations of the component groups, and for the different types of thicknesses of the bilayer, such as the hydrocarbon thickness and the steric thickness.

Phase Transition of a DPPC Bilayer Induced by an External Surface Pressure: From Bilayer to Monolayer Behavior. A Molecular Dynamics Simulation Study

Understanding the lipid phase transition of lipid bilayers is of great interest from biophysical, physicochemical, and technological points of view. With the aim of elucidating the structural changes that take place in a DPPC phospholipid bilayer induced by an external isotropic surface pressure, five computer simulations were carried out in a range from 0.1 to 40 mN/m. Molecular dynamics simulations provided insight into the structural changes that took place in the lipid structure. It was seen that low pressures ranging from 0.1 to 1 mN/m had hardly any effect on the structure, electrical properties, or hydration of the lipid bilayer. However, for pressures above 40 mN/m, there was a sharp change in the lipid-lipid interactions, hydrocarbon lipid fluidity, and electrostatic potential, corresponding to the mesomorphic transition from a liquid crystalline state (L(alpha)) to its gel state (P'(beta)). The head lipid orientation remained almost unaltered, parallel to the lipid layer, as the surface pressure was increased, although a noticeable change in its angular distribution function was evident with the phase transition.

Size and Orientation of the Lipid II Headgroup As Revealed by AFM Imaging

In this study, we investigated the size and orientation of the bacterial Lipid II (L II) headgroup when the L II molecule is present in liquid-crystalline domains of DOPC in a supported DPPC bilayer. Using atomic force microscopy, we detected that L II causes the appearance of a 1.9 nm thick layer, situated over the DOPC headgroup region. With an increased scanning force, this layer can be penetrated by the AFM tip down to the level of the DOPC bilayer. Using different L II precursor molecules, we demonstrated that the detected layer consists of the headgroups of L II and that the MurNAc-pentapeptide unit of the headgroup is responsible for the measured 1.9 nm height of that layer. Monolayer experiments provided information about the in-plane dimensions of the L II headgroup. On the basis of these results and considerations of the molecular dimensions of L II headgroup constituents, we propose a model for the orientation of the L II headgroup in the membrane. In this model, the pentapeptide of the L II headgroup is rather extended and points away from the bilayer surface, which could be important for biological processes, in which L II is involved.

Dynamics of bolaamphiphilic fluorescent polyenes in lipid bilayers from polarization emission spectroscopy

The rotational motions of the biamphiphilic polyenes (bolapolyenes) dimethyl all-( E)-octacosa-10,12,14,16,18-pentaenedioate (DE28:5) and dimethyl all-( E)-tetratriaconta-13,15,17,19,21-pentaenedioate (DE34:5), with head-to-head distances of 34 and 42 Å, respectively, have been examined by fluorescence anisotropy methods. The membrane-spanning bolapolyenes, which contain a central emitting pentaene group tethered to two methoxycarbonyl opposite polar heads by symmetric C 8 (DE28:5) and C 11 (DE34:5) polymethylene chains, were dispersed in lipid bilayers of DPPC or DMPC, and the stationary and picosecond-resolved emission was recorded as a function of temperature. In fluid-phase DMPC bilayers, three relaxation times could be determined, assigned to fast (0.2 and 2 ns) single-bond isomerization processes localized on the alkyl chains, and to whole-molecule oscillations (∼11 ns), respectively. The anisotropy decay parameters were further analyzed in terms of a diffusive model for wobbling in a Gaussian ordering potential, to assess the anchoring effect of the symmetric polar heads. In this way, the average rotational diffusion constant of the bolapolyenes, D ⊥, could be estimated as 0.022–0.026 rad 2 ns − 1 (DMPC bilayers, 35 °C), a value that is only 1 / 3 of that corresponding to the related pentaene fatty acid spanning a single membrane monolayer. In contrast, the amplitude of the equilibrium orientational distribution ( θ half-cone∼50°) is very similar for both the transmembrane and the single-headed polyenes. The reorientational oscillations of the central emitting group in the bolapolyenes necessarily would produce large-amplitude (2–5 Å) and very fast (ns) translational motions of the polar heads.

Molecular investigation of the interactions of trehalose with lipid bilayers of DPPC, DPPE and their mixture

Molecular dynamics simulations were performed to study structural and dynamic properties of fully hydrated pure and mixed bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) in the presence of trehalose (5 wt%). Simulations were performed for 50 ns at 350 K and 1 bar in the liquid-crystalline state of the lipid bilayers. At the concentration considered, the effect of trehalose on pure and mixed DPPC/DPPE structure are minimal, with the area per headgroup and lipid tail order parameter unchanged compared to systems without trehalose. Density profiles indicate a larger concentration of trehalose near the interface, suggesting preferential binding of trehalose with the bilayer. Hydrogen bond analysis between trehalose and the bilayers shows that the largest number of interactions occurs with DPPC lipids, whereas the fewest interactions occur in a mixed 1:1 DPPC/DPPE bilayer. The latter is a result of the inter and intramolecular binding of the amine group in DPPE, thus preventing trehalose from hydrogen bonding to the bilayer. For the pure DPPE bilayer, an excess of hydrogen-donors (amine groups) create a competitive hydrogen bonding environment that weakens lipid–lipid interactions and favors hydration of the amine groups, which enhances the binding of trehalose with the bilayer.