Temperature change of the ripple structure in fully hydrated dimyristoylphosphatidylcholine/cholesterol multibilayers

Molecular interactions of hesperidin with DMPC/cholesterol bilayers

Since cell membranes are complex systems, the use of model lipid bilayers is quite important for the study of their interactions with bioactive molecules. Mammalian cell membranes require cholesterol (CHOL) for their structure and function. For this reason, the mixtures of phospholipid and cholesterol are necessary to use in model membrane studies to better simulate the real systems. In the present study, we investigated the effect of the incorporation of hesperidin in model membranes consisting of dimyristoylphosphatidylcholine (DMPC) and CHOL by using differential scanning calorimetry (DSC), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and atomic force microscopy (AFM). ATR-FTIR results demonstrated that hesperidin increases the fluidity of the DMPC/CHOL binary system. DSC findings indicated that the presence of 5 mol% hesperidin induces a broadening of the main phase transition consisting of three overlapping components. AFM experiments showed that hesperidin increases the thickness of DMPC/CHOL lipid bilayer model membranes. In addition to experimental results, molecular docking studies were conducted with hesperidin and human lanosterol synthase (LS), which is an enzyme found in the final step of cholesterol synthesis, to characterize hesperidin's interactions with its surrounding via its hydroxyl and oxygen groups. Then, hesperidin's ADME/Tox (absorption, distribution, metabolism, excretion and toxicity) profile was computed to see the potential impact on living system. In conclusion, considering the data obtained from experimental studies, this work ensures molecular insights in the interaction between a flavonoid, as an antioxidant drug model, and lipids mimicking those found in mammalian membranes. Moreover, computational studies demonstrated that hesperidin may be a great potential for use as a therapeutic agent for hypercholesterolemia due to its antioxidant property.

Phase behavior of palmitoyl and egg sphingomyelin

Despite the biological significance of sphingomyelins (SMs), there is far less structural information available for SMs compared to glycerophospholipids. Considerable confusion exists in the literature regarding even the phase behavior of SM bilayers. This work studies both palmitoyl (PSM) and egg sphingomyelin (ESM) in the temperature regime from 3 °C to 55 °C using X-ray diffraction and X-ray diffuse scattering on hydrated, oriented thick bilayer stacks. We observe clear evidence for a ripple phase for ESM in a large temperature range from 3 °C to the main phase transition temperature (T_M) of ∼38 °C. This unusual stability of the ripple phase was not observed for PSM, which was in a gel phase at 3 °C, with a gel-to-ripple transition at ∼24 °C and a ripple-to-fluid transition at ∼41 °C. We also report structural results for all phases. In the gel phase at 3 °C, PSM has chains tilted by ∼30° with an area/lipid ∼45 Ų as determined by wide angle X-ray scattering. The ripple phases for both PSM and ESM have temperature dependent ripple wavelengths that are ∼145 Å near 30 °C. In the fluid phase, our electron density profiles combined with volume measurements allow calculation of area/lipid to be ∼64 Ų for both PSM and ESM, which is larger than that from most of the previous molecular dynamics simulations and experimental studies. Our study demonstrates that oriented lipid films are particularly well-suited to characterize ripple phases since the scattering pattern is much better resolved than in unoriented samples.

Integration of Quercetin-Iron Complexes into Phosphatidylcholine or Phosphatidylethanolamine Liposomes

It is well known that flavonoids can chelate transition metals. Flavonoid-metal complexes exhibit a high antioxidative and therapeutic potential. However, the complexes are frequently hydrophobic ones and low soluble in water, which restricts their medical applications. Integration of these complexes into liposomes may increase their bioavailability and therapeutic effect. Here, we studied the interaction of quercetin-iron complexes with dimyristoylphosphatidylcholine (DMPC) or palmitoyl-oleoyl phosphatidylethanolamine (POPE) multilamellar liposomes. Differential scanning calorimetry (DSC) and freeze-fracture electron microscopy revealed that quercetin-iron complexes did not interact with liposomes. Quercetin however could penetrate lipid bilayers, when added to liposomes at a temperature above lipid melting. Iron cations added later penetrated into the lipid bilayers and produced complexes with quercetin in the liposomes. The quercetin-iron entry in POPE liposomes was improved when the suspension was heated above the temperature of the bilayer-hexagonal HII phase transition of the lipid. The approach proposed facilitates the integration of quercetin-iron complexes into liposomes and may promote their use in medicine.

Structure of sphingomyelin bilayers and complexes with cholesterol forming membrane rafts

Sphingomyelin and cholesterol are of interest to biologists because they interact to form condensed structures said to be responsible for a variety of functions that membranes perform. Synchrotron X-ray diffraction methods have been used to investigate the structure of bilayers of D-erythro palmitoyl-sphingomyelin and complexes formed by palmitoyl- and egg-sphingomyelin with cholesterol in aqueous multibilayer dispersions. D-erythro palmitoyl sphingomyelin bilayers exist in two conformers that are distinguished by their lamellar repeat spacing, bilayer thickness, and polar group hydration. The distinction is attributed to hydrogen bonding to water or to intermolecular hydrogen bonds that are disrupted by the formation of ripple structure. The coexisting bilayer structures of pure palmitoyl sphingomyelin are observed in the presence of cholesterol-rich bilayers that are characterized by different bilayer parameters. The presence of cholesterol preferentially affects the conformer of D-erythro sphingomyelin with thicker, more hydrated bilayers. Coexisting bilayers of sphingomyelin and complexes with cholesterol are in register and remain coupled at temperatures at least up to 50 °C. Cholesterol forms a complex of 1.8 mols of sphingomyelin per cholesterol at 37 °C that coexists with bilayers of pure sphingomyelin up to 50 °C. Redistribution of the two lipids takes place on cooling below the fluid- to gel-phase transition temperature, resulting in the withdrawal of sphingomyelin into gel phase and the formation of coexisting bilayers of equimolar proportions of the two lipids. Cholesterol-rich bilayers fit a stripe model at temperatures less than 37 °C characterized by alternating rows of sphingomyelin and cholesterol molecules. A quasicrystalline array models the arrangement at higher temperatures in which each cholesterol molecule is surrounded by seven hydrocarbon chains, each of which is in contact with two cholesterol molecules. The thickness of bilayer complexes of sphingomyelin and cholesterol is less than that of coexisting bilayers of pure sphingomyelin. The implications for protein sorting theories based on bilayer thickness are discussed.

Arbutin blocks defects in the ripple phase of DMPC bilayers by changing carbonyl organization

The effect of arbutin, a 4-hydroxyphenyl-beta-glucopyranoside, on dimyristoylphosphatidylcholine (DMPC) bilayers was studied by turbidimetry, EPR and FTIR spectroscopies. The disruption of DMPC multilamellar vesicles (MLV's) with monomyristoylphosphatidylcholine (lysoPC), a product of hydrolysis of phospholipase A(2) (PLA(2)), is more efficient at 18 degrees C, where DMPC MLV's are known to be in the ripple P(beta') phase, than at 10 degrees C (L(beta') flat gel phase). Disruption at 18 degrees C was inhibited by increasing concentrations of arbutin in the solution. This inhibition was correlated with the disappearance of the ripple phase in MLV's when arbutin is present. Shifts in FTIR carbonyl bands caused by arbutin or by temperature changes allow us to propose a model. It is interpreted that the changes in the water-hydrocarbon interface caused by arbutin, forcing a reaccommodation of the carbonyl groups, eliminate the topological defects in the lattice due to mismatches among regions with different area per lipid where lysoPC can insert.

Phase diagram of stigmasterol-dipalmitoylphosphatidylcholine mixtures dispersed in excess water

As a simple model of rafts in plant cells, the effect of stigmasterol, one of the predominant sterols in plant plasma membranes, on the phase behavior of dipalmitoylphosphatidylcholine (DPPC) multilayers has been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and freeze-fracture electron microscopy (FFEM) techniques. A partial phase diagram of the binary system has been constructed. Particularly, the stigmasterol concentrations of the "left endpoint" and "right endpoint" of the three-phase line have been determined using the newly developed linear and nonlinear fitting method. They are 6.2 and 23.7 mol%, respectively. Furthermore, the resemblance and difference of phase diagrams of DPPC/stigmasterol, DPPC/cholesterol, and DPPC/ergosterol have been compared and the efficiency of these sterols in promoting the formation of the liquid-ordered domains (rafts) have also been discussed.

Structural calorimetry of main transition of supported DMPC bilayers by temperature-controlled AFM

Atomic force microscopy at high temperature resolution (DeltaT < or approximately 0.1 K) provided a quantitative structural calorimetry of the transition from the fluid (Lalpha)- to the gel (Pbeta')-phase of supported dimyristoylphosphatidylcholine bilayers. Besides a determination of the main transition temperature (T0) and the van't Hoff transition enthalpy (DeltaHvH), the structural analysis in the nm-scale at T close to T0 of the ripple phase allowed an experimental estimation of the area of cooperative units from small lipid domains. Thereby, the corresponding transition enthalpy (DeltaH) of single molecules could be determined. The lipid organization and the corresponding parameters T0 and DeltaHvH (DeltaH) were modulated by heptanol or external Ca2+ and compared with physiological findings. The size of the cooperative unit was not significantly affected by the presence of 1 mM heptanol. The observed linear relationship of DeltaHvH and T0 was discussed in terms of a change in heat capacity.

Temperature-controlled structure and kinetics of ripple phases in one- and two-component supported lipid bilayers

Temperature-controlled atomic force microscopy (AFM) has been used to visualize and study the structure and kinetics of ripple phases in one-component dipalmitoylphosphatidylcholine (DPPC) and two-component dimyristoylphosphatidylcholine-distearoylphosphatidylcholine (DMPC-DSPC) lipid bilayers. The lipid bilayers are mica-supported double bilayers in which ripple-phase formation occurs in the top bilayer. In one-component DPPC lipid bilayers, the stable and metastable ripple phases were observed. In addition, a third ripple structure with approximately twice the wavelength of the metastable ripples was seen. From height profiles of the AFM images, estimates of the amplitudes of the different ripple phases are reported. To elucidate the processes of ripple formation and disappearance, a ripple-phase DPPC lipid bilayer was taken through the pretransition in the cooling and the heating direction and the disappearance and formation of ripples was visualized. It was found that both the disappearance and formation of ripples take place virtually one ripple at a time, thereby demonstrating the highly anisotropic nature of the ripple phase. Furthermore, when a two-component DMPC-DSPC mixture was heated from the ripple phase and into the ripple-phase/fluid-phase coexistence temperature region, the AFM images revealed that several dynamic properties of the ripple phase are important for the melting behavior of the lipid mixture. Onset of melting is observed at grain boundaries between different ripple types and different ripple orientations, and the longer-wavelength metastable ripple phase melts before the shorter-wavelength stable ripple phase. Moreover, it was observed that the ripple phase favors domain growth along the ripple direction and is responsible for creating straight-edged domains with 60 degrees and 120 degrees angles, as reported previously.

Phospholipid-cholesterol bilayers under osmotic stress

Isothermal (27 degrees C) phase behavior of dimyristoyl phosphatidyl choline-cholesterol mixtures at various osmotic pressures and cholesterol contents was investigated by means of isothermal sorption microcalorimetry and (2)H-nuclear magnetic resonance. The calorimetric method allows for simultaneous measurement of the partial molar enthalpy and the chemical potential (the osmotic pressure) of water, thus providing an almost complete thermodynamic description of the sorption process. From the experimental results, the Pi(osm) - X(chol) and the ternary composition phase diagrams are constructed. We note that there are strong similarities between the Pi(osm) - X(chol) phase diagram and the previously reported T - X(chol) phase diagram at excess water. At high cholesterol contents a single liquid ordered (L(alpha)(o)) phase is present over the whole range of water contents, implying that this phase has a remarkable stability not only at decreasing temperature but also at increasing osmotic pressure. At low cholesterol contents, the microcalorimetric experiments confirm the extraordinary property of cholesterol not to cause any substantial melting point depression. One important conclusion in the present study is that the P(beta) phase can dissolve cholesterol more readily than the L(beta) phase and that the addition of cholesterol induces the P(beta) phase. Finally, the putative P(beta) - L(alpha)(o) periodic modulated structure is discussed.

Ripple phases induced by alpha-tocopherol in saturated diacylphosphatidylcholines

The effect of alpha-tocopherol on the structure and phase behavior of dilauroyl-, dimyristoyl-, dipalmitoyl-, and distearoyl-phosphatidylcholines was examined using X-ray diffraction and freeze-fracture electron microscopic methods. A ripple phase was observed in all of the mixtures at temperatures well below the pretransition temperature of the corresponding pure phospholipid. Freeze-fracture studies indicated that with proportion of alpha-tocopherol less than 5 mol% a ripple phase with large periodicity (50-150 nm) predominated and with about 10 mol% alpha-tocopherol a ripple phase of periodicity about 16 nm was formed. With more than 10 mol% alpha-tocopherol planar bilayers tended to be formed. Partial phase diagrams of mixed aqueous dispersions of saturated phosphatidylcholines and alpha-tocopherol over temperature ranges about the gel to liquid-crystal phase boundary have been constructed. Alpha-tocopherol-enriched domains form ripple phases that coexist with regions of lamellar gel phase of the pure phospholipid in mixtures containing less than 10 mol% alpha-tocopherol. The presence of increasing amounts of alpha-tocopherol in the phospholipid causes an increase in the proportion of ripple phase at the expense of pure phospholipid bilayer indicating that the alpha-tocopherol-enriched domains might possess a defined stoichiometry of the two constituents.

A model for the lipid pretransition: coupling of ripple formation with the chain-melting transition

Below the thermotropic chain-melting transition, lipid membrane c(P) traces display a transition of low enthalpy called the lipid pretransition. It is linked to the formation of periodic membrane ripples. In the literature, these two transitions are usually regarded as independent events. Here, we present a model that is based on the assumption that both pretransition and main transition are caused by the same physical effect, namely chain melting. The splitting of the melting process into two peaks is found to be a consequence of the coupling of structural changes and chain-melting events. On the basis of this concept, we performed Monte Carlo simulations using two coupled monolayer lattices. In this calculation, ripples are considered to be one-dimensional defects of fluid lipid molecules. Because lipids change their area by approximately 24% upon melting, line defects are the only ones that are topologically possible in a triangular lattice. The formation of a fluid line defect on one monolayer leads to a local bending of the membrane. Geometric constraints result in the formation of periodic patterns of gel and fluid domains. This model, for the first time, is able to predict heat capacity profiles, which are comparable to the experimental c(P) traces that we obtained using calorimetry. The basic assumptions are in agreement with a large number of experimental observations.

Trehalose-induced destabilization of interdigitated gel phase in dihexadecylphosphatidylcholine

Trehalose is believed to have the ability to protect some organisms against low temperatures. To clarify the cryoprotective mechanism of trehalose, the structure and the phase behavior of fully hydrated dihexadecylphosphatidylcholine (DHPC) membranes in the presence of various concentrations of trehalose were studied by means of differential scanning calorimetry (DSC), static x-ray diffraction, and simultaneous x-ray diffraction and DSC measurements. The temperature of the interdigitated gel (Lbeta(i))-to-ripple (Pbeta') phase transition of DHPC decreases with a rise in trehalose concentration up to approximately 1.0 M. Above a trehalose concentration of approximately 1.0 M, no Lbeta(i) phase is observed. In this connection, the electron density profile calculated from the lamellar diffraction data in the presence of 1.6 M trehalose indicates that DHPC forms noninterdigitated bilayers below the P beta' phase. It was concluded that trehalose destabilizes the Lbeta(i) phase of DHPC bilayers. This suggests that trehalose reduces the area at the interface between the lipid and water. The relation between this effect of trehalose and a low temperature tolerance was discussed from the viewpoint of cold-induced denaturation of proteins.

The effect of sterols on structures formed in the gel/subgel phase state of dipalmitoylphosphatidylcholine bilayers

The effect of cholesterol and lanosterol on the formation of structures in the gel/subgel phase of 1,2-dipalmitoylphosphatidylcholine was investigated using freeze-fracture electron microscopy and X-ray diffraction. Mixtures with up to 25 mol% sterol were analysed after annealing for between several days and some months at 4 degrees C. Bilayers of DPPC with 5 or 10 mol% sterol showed a domain structure in the gel state. There are rounded or lens-like and often inclined plates within less smooth either plane or striped bilayer areas. The stripes are formed by parallel lines separated by a distance of 30-60 nm. Parallel lines can be induced also in the less smooth but plane areas by warming up to 25 degrees C. X-ray diffraction showed two lamellar repeat spacings at 6.45 and 8.3 nm in both 5 and 10 mol% samples. The plates are interpreted as domains of (nearly) pure DPPC within the sterol containing bilayer. Stripes are present if the concentration of cholesterol is below a critical value (approx. 15 mol%). With time of incubation at 4 degrees C curved deformations appear in parts of the bilayers. Two main types are formed. The small type has a repeat distance of about 100 nm and the large type of about 400 nm. The curved deformations were progressively flattened by warming up to 25-32 degrees C with an accompanying reappearance of stripes but no plates. After prolonged annealing at 4 degrees C there is also the formation of regular ripples. It is concluded that in presence of 5 and 10 mol% sterol in bilayers of 1,2-dipalmitoy-phosphatidylcholine the immiscibility of gel phase and subgel phase changes during prolonged annealing at 4 degrees C. We assume a rearrangement of the molecules into a homogeneous phase state with liquid-ordered properties.

High-resolution scanning tunneling microscopy of fully hydrated ripple-phase bilayers

A modified freeze-fracture replication technique for use with the scanning tunneling microscope (STM) has provided a quantitative, high-resolution description of the waveform and amplitude of rippled bilayers in the P beta' phase of dimyristoylphosphatidylcholine (DMPC) in excess water. The ripples are uniaxial and asymmetrical, with a temperature-dependent amplitude of 2.4 nm near the chain melting temperature that decreases to zero at the chain crystallization temperature. The wavelength of 11 nm does not change with temperature. The observed ripple shape and the temperature-induced structural changes are not predicted by any current theory. Calibration and reproducibility of the STM/replica technique were tested with replicas of well-characterized bilayers of cadmium arachidate on mica that provide regular 5.5-nm steps. STM images were analyzed using a cross-correlation averaging program to eliminate the effects of noise and the finite size and shapes of the metal grains that make up the replica. The correlation averaging allowed us to develop a composite ripple profile averaged over hundreds of individual ripples measured on different samples with different STM tips. The STM/replica technique avoids many of the previous artifacts of biological STM imaging and can be used to examine a variety of periodic hydrated lipid and protein samples at a lateral resolution of about 1 nm and a vertical resolution of about 0.3 nm. This resolution is superior to conventional and tapping mode AFM to soft biological materials; the technique is substrate-free, and the conductive and chemically uniform replicas make image interpretation simple and direct.

Pretransition-ripples in bilayers of dipalmitoylphosphatidylcholine: undulation or periodic segments? A freeze-fracture study

Freeze-fracture analysis of ripple structures of 1,2-dipalmitoylphosphatidylcholine bilayers leads to the conclusion that the asymmetric ripple is the basic structure formed by periodic segments with different tilt direction. The molecules are tilted by about 30 degrees from the bilayer normal but arranged in two positions. Symmetric ripples are also formed by an alternation in tilt direction of the segments but the succession is more complex. A ridge in their valleys or a cleft at their crests may indicate structures formed or deformed during preparation (replication, etching). The freeze-fracture method reveals transition structures in ripple formation which are helpful in interpretation, but does not support a model consisting of an undulation of the bilayer by periodic fluid-like and gel-like domains.

Characterisation of clusters of alpha-tocopherol in gel and fluid phases of dipalmitoylglycerophosphocholine

The effect of alpha-tocopherol on the phase behaviour of aqueous dispersions of dipalmitoylglycerophosphocholine has been examined by differential scanning calorimetry, freeze-fracture electron microscopy, and real-time X-ray diffraction methods. The presence of alpha-tocopherol in proportions 2.5, 5, 7.5, and 10 mol/100 mol results in a progressive decrease in the temperature of the gel to liquid-crystalline phase transition from 41.5 degrees C to 36 degrees C and a reduction in transition enthalpy from 35 kJ.mol-1 to 15 kJ.mol-1 phospholipid. The thermal data indicated that the pretransition of the phospholipid is eliminated even in mixtures containing 2.5 mol/100 mol alpha-tocopherol. Real-time X-ray diffraction measurements using synchrotron radiation performed under identical conditions to the thermal studies showed clear transition sequences of L beta-->P beta-->L alpha for all mixtures. The sequence was reversible with hysteresis of 2-3 degrees C on cooling. Low-angle X-ray scattering from mixtures in the gel phase showed three lamellar repeat spacings of 6.35, 7.5, and 8.4 nm. The spacing at 6.35 nm was assigned to pure phospholipid from which alpha-tocopherol has been phase separated into enriched domains giving lamellar repeat spacings of 7.5 nm and 8.4 nm. Low-angle diffraction patterns of mixtures in the fluid phase were characterised by two lamellar repeat spacings. The longer spacing of about 6.6 nm was assigned to pure phospholipid and the shorter spacing at about 6.1 nm to an alpha-tocopherol-enriched phase. Electron microscopy of freeze-fracture replicas of mixtures of phospholipid containing 10 mol/100 mol alpha-tocopherol thermally quenched from 10 degrees C and 60 degrees C, showed evidence of domain structures within the bilayer plane that appeared to be correlated between successive bilayers in multilamellar dispersions. Calculations of the stoichiometry of phospholipid: alpha-tocopherol in the alpha-tocopherol-enriched domains based on enthalpy data and integrated X-ray scattering intensity gave values of 9.6:1 for the fluid phase and 9.2:1 for the gel phase. This was consistent with a clustering of alpha-tocopherol molecules in both gel and liquid-crystal phases of dipalmitoyl-glycerophospholcholine in approximately the same stoichiometry.

New aspects of the interaction of cholesterol with dipalmitoylphosphatidylcholine bilayers as revealed by high-sensitivity differential scanning calorimetry

We have investigated the effects of cholesterol on the thermotropic phase behavior of annealed and unannealed aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) using high-sensitivity differential scanning calorimetry (DSC), concentrating particularly on the cholesterol concentration range from 0 to 20 mol%. We find that the incorporation of cholesterol into low-temperature annealed DPPC bilayers decreases the enthalpy of the subtransition without affecting the transition temperature, such that the subtransition is abolished by 20 mol% cholesterol. Similarly, the incorporation of cholesterol progressively decreases the temperature and enthalpy of the pretransition and abolishes it entirely at cholesterol concentrations above 5 mol%. The incorporation of increasing quantities of cholesterol also alters the main or chain-melting phase transition. At cholesterol concentrations of 2 to 20 mol% cholesterol, the DSC endotherm arising from the main transition consists of superimposed sharp and broad components, the former due to the melting of cholesterol-poor and the latter to the melting of the cholesterol-rich DPPC domains. The temperature and cooperativity of the sharp component decreases slightly with increasing cholesterol concentration whereas the enthalpy decreases markedly, becoming zero at 20-25 mol% cholesterol. In contrast, the temperature and enthalpy of the broad component increases, and the cooperativity decreases markedly over this same range of cholesterol concentrations. An apparent increase in cooperativity of the overall DPPC endotherm at 7 mol% cholesterol is shown to arise because of a convergence in the transition temperatures of the sharp and broad components of the DSC endotherms. Some of our experimental findings, particularly the absence of any evidence for the existence of a triple point near 7.5 mol% cholesterol, do not accord with a recently proposed DPPC/cholesterol phase diagram derived from DSC and 2H-NMR data (see Vist, M.R. and Davis, J.H. (1990) Biochemistry 29, 451-464). In addition, we examined the effect of cholesterol on phosphatidylcholines (PCs) of different chain lengths and confirm that a eutectic point does not exist for any of these PC/cholesterol mixtures. We then propose a new, more complete DPPC/cholesterol phase diagram based on our high-sensitivity DSC data as well as some recent spectroscopic data on PC/cholesterol mixtures and explore some of its biological implications.