The magnitude of condensation induced by cholesterol on the mixtures of sphingomyelin with phosphatidylcholines-Study on ternary and quaternary systems

The studies on the condensing and ordering effect of cholesterol by application of the Langmuir monolayer technique are usually performed on binary lipid/cholesterol systems. The results concerning a quantitative analysis of these effects in multicomponent monolayers are very limited. In this work the condensing and ordering effect of cholesterol in ternary (SM/DSPC/Chol and SM/DOPC/Chol) and quaternary (SM/DSPC/DOPC/Chol) films was investigated. It was evidenced that the systems containing saturated PC (both SM/DSPC and SM/DSPC/Chol) are always more condensed and chain-ordered than the systems containing unsaturated PC (SM/DOPC and SM/DSPC/DOPC and their mixtures with cholesterol). However, the magnitude of condensation provoked by cholesterol at higher surface pressures is stronger on the monolayers containing unsaturated PC. The addition of cholesterol into SM/PC films induces the increase of chain-ordering however, the effectiveness of cholesterol as an ordering agent is determined by the presence/absence of unsaturated phospholipid. The magnitude of the effect of cholesterol on the investigated mixed monolayer was analyzed in the context of the influence of sterol on lipid chains (ordering, straightening and reorientation of chains) as well as the reorientation of polar heads.     

The use of dielectric spectroscopy for the characterisation of polymer-induced flocculation of core-shell particles

This work is aimed at investigating the influence of a plant stanol (β-sitostanol) on Langmuir monolayers from various phospholipids and comparing the effect of phytostanol versus its unsaturated analog--phytosterol (β-sitosterol). The studied phospholipids differed in the structure of polar head (phosphatidylcholine--PC, phosphatidylethanolamine--PE, phosphatidylserine--PS) as well as in the number of monounsaturated chains in PC molecule. It was found that the introduction of stanol into PC monolayers is thermodynamically favorable, contrary to its effect on PE and PS films. The magnitude of condensing and ordering effect of stanol depends both on the number of monounsaturated chains in PC molecule and on the composition of stanol-PC mixture. The analysis of BAM images evidenced phase separation of immiscible components in stanol/DPPS systems. The results of Langmuir monolayer studies for stanol/phospholipids mixtures compared with those for corresponding sterol/phospholipids systems proved quite a similar effect of both compounds on the investigated phospholipid monolayers, despite differences in the structure of tetracyclic ring skeleton.     

Interaction of 3β-amino-5-cholestene with phospholipids in binary and ternary bilayer membranes

3β-Amino-5-cholestene (aminocholesterol) is a synthetic sterol whose properties in bilayer membranes have been examined. In fluid palmitoyl sphingomyelin (PSM) bilayers, aminocholesterol and cholesterol were equally effective in increasing acyl chain order, based on changes in diphenylhexatriene (DPH) anisotropy. In fluid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers, aminocholesterol ordered acyl chains, but slightly less efficiently than cholesterol. Aminocholesterol eliminated the PSM and DPPC gel-to-liquid crystalline phase transition enthalpy linearly with concentration, and the enthalpy approached zero at 30 mol % sterol. Whereas cholesterol was able to increase the thermostability of ordered PSM domains in a fluid bilayer, aminocholesterol under equal conditions failed to do this, suggesting that its interaction with PSM was not as favorable as cholesterols. In ternary mixed bilayers, containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), PSM or DPPC, and cholesterol at proportions to contain a liquid-ordered phase (60:40 by mol of POPC and PSM or DPPC, and 30 mol % cholesterol), the average lifetime of trans-parinaric acid (tPA) was close to 20 ns. When cholesterol was replaced with aminocholesterol in such mixed bilayers, the average lifetime of tPA was only marginally shorter (about 18 ns). This observation, together with acyl chain ordering data, clearly shows that aminocholesterol was able to form a liquid-ordered phase with saturated PSM or DPPC. We conclude that aminocholesterol should be a good sterol replacement in model membrane systems for which a partial positive charge is deemed beneficial.     

Molecular organization of cholesterol in unsaturated phosphatidylethanolamines: X-ray diffraction and solid state 2H NMR reveal differences with phosphatidylcholines

The major mammalian plasma membrane lipids are phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), and cholesterol. Whereas PC-cholesterol interactions are well studied, far less is known about those between PE and cholesterol. Here, we investigated the molecular organization of cholesterol in PEs that vary in their degree of acyl chain unsaturation. For heteroacid sn-1 saturated (palmitoyl), sn-2 unsaturated (various acyl chain) PEs, cholesterol solubility determined by X-ray diffraction was essentially identical with 1 (oleoyl, 51 +/- 3 mol %) and 2 (linoleoyl, 49 +/- 2 mol %) double bonds before decreasing progressively with 4 (arachidonyl, 41 +/- 3 mol %) and 6 (docosahexaenoyl, 31 +/- 3 mol %) double bonds. With 6 double bonds in each chain, cholesterol solubility was further reduced to 8.5 +/- 1 mol %. However, (2)H NMR experiments established that the orientation of cholesterol in the same heteroacid PE membranes was unaffected by the degree of acyl chain unsaturation. A tilt angle of 15 +/- 1 degrees was measured when equimolar [3alpha-(2)H(1)]cholesterol was added, regardless of the number of double bonds in the sn-2 chain. The finding that solubility of cholesterol in sn-1 saturated PEs depends on the amount of polyunsaturation in the sn-2 chain of PE differs from the equivalent PCs that universally incorporate approximately 50 mol % sterol. Unlike PCs, a differential in affinity for cholesterol and tendency to drive lateral segregation is inferred between polyunsaturated PEs. This distinction may have biological implications reflected by the health benefits of dietary polyunsaturated fatty acids that are often taken up into PE > PC.     

Effect of cholesterol and phospholipid on the behavior of dialkyl polyoxyethylene ether surfactant (2C18E12) monolayers and bilayers

Surface pressure-area isotherm, neutron specular reflection, and small-angle neutron scattering studies have been carried out to determine the effects of added cholesterol and distearoylphosphatidylcholine (DSPC), on the molecular structures of monolayers and vesicles containing the dialkyl polyoxyethylene ether surfactant, 1,2-di-O-octadecyl-rac-glyceryl-3-(alpha-dodecaethylene glycol) (2C18E12). Previous neutron reflectivity studies on 2C18E12 monolayers at the air/water interface have shown them to possess a thickness of approximately 24 angstoms and highly disordered structure with significant intermixing of the polymer headgroups and alkyl chains. SANS studies of 2C18E12 vesicles gave a bilayer thickness of approximately 51 angstroms. Addition of cholesterol to 2C18E12 monolayers (1:1 molar ratio), produced a marked condensing effect coupled with an increased the layer thickness of approximately 7 angstroms, and in vesicles, increased bilayer thickness by approximately 16 angstroms. Monolayers consisting of 2C18E12:DSPC:cholesterol (1:1:2 molar ratio), showed a layer thickness of approximately 31 angstroms, whereas in vesicles, three-component bilayer was found to be only approximately 9 angstroms thicker than those possessed by vesicles composed solely of 2C18E12. Mixing between the molecules in three-component monolayers was shown to be ideal through analysis of the neutron reflectivity data. These findings are discussed in relation to increased ordering and decreased headgroup/hydrophobe intermixing within both monolayers and vesicle bilayers containing 2C18E12. The inferred increase in molecular order within vesicles composed of 2C18E12 with additional cholesterol and phospholipid is used as a model for explaining theoretical differences in bilayer permeability.     

Molecular dynamics simulations of bilayers containing mixtures of sphingomyelin with cholesterol and phosphatidylcholine with cholesterol

We performed six molecular dynamics simulations: three on hydrated bilayers containing pure phospholipids and three on hydrated bilayers containing mixtures of these phospholipids with cholesterol. The phospholipids in our simulations were SSM (sphingomyelin containing a saturated 18:0 acyl chain), OSM (sphingomyelin with an unsaturated 18:1 acyl chain), and POPC (palmitoyloleoylphosphatidylcholine containing one saturated and one unsaturated chain). Data from our simulations were used to study systematically the effect of cholesterol on phospholipids that differed in their headgroup and tail composition. In addition to the structural analysis, we performed an energetic analysis and observed that energies of interaction between cholesterol and neighboring SM molecules are similar to the energies of interaction between cholesterol and POPC. We also observed that the interaction energy between cholesterol and neighboring lipids cannot be used for the determination of which lipids are involved in the creation of a complex.     

Phosphatidylcholine vesicle-mediated decomposition of hydrogen peroxide

The decomposition of hydrogen peroxide (H2O2) was examined in aqueous solution (50 mM Tris-HCl buffer, pH 7.4, containing 100 mM NaCl) at 25 degrees C in pure buffer or in the presence of either vesicles or micelles formed from various phosphatidylcholines (PCs). In the absence of PCs, more than 90% of the initially added H2O2 (1.0 mM) remained intact after incubation for 120 h. The effect of the PCs on the decomposition of H2O2 was studied by using different PCs that varied in terms of number of carbon atoms in the two acyl chains n as well as in terms of the degree of unsaturation. PCs with short hydrocarbon chains (n = 4, 6-8) were dissolved in the buffer solution in the form of nonassociated monomers or as micelles in equilibrium with monomers at a fixed PC concentration of 10 mM. The presence of these short-chain PCs slightly enhanced the H2O2 decomposition rate. Micelles formed by non-lipid detergents (sodium cholate, Triton X-100, and sodium dodecylsulfate) had a similar effect. In marked contrast, PCs with long hydrocarbon chains (n > or = 10) dispersed in buffer solution as vesicles (liposomes) significantly enhanced the rate of H2O2 decomposition, with the most effective PC being 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) at 25 degrees C. This indicates that the packing density of the PC molecules influences the reactivity, presumably through the direct interaction of the PC assemblies with H2O2 molecules. Furthermore, in the case of vesicles formed from PCs with unsaturated acyl chains (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC; 1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC), carbon-carbon double bond oxidation did not occur extensively under the conditions used. This indicates that the observed effect of PCs on the decomposition of H2O2 is indeed related to the assembly structure (vesicle vs micelles vs monomers) and is clearly not related to the presence of unsaturated hydrocarbon chains. Fluorescence polarization measurements of two fluorescent probes embedded either in the acyl chain region of the vesicles (DPH, 1,6-diphenyl-1,3,5-hexatriene) or on the surface of the vesicles (TMA-DPH, 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene iodide) show that the presence of H2O2 leads to a decrease in the fluidity of the lipid-water surface and not to a change in the fluidity of the hydrophobic region of the vesicle bilayer. This indicates that the decomposition of H2O2 is triggered through interactions between H2O2 and the polar head group area of PC vesicles.     

Interactions between phosphatidylcholines and cholesterol in monolayers at the air/water interface

Mixtures of cholesterol and synthetic phospholipids, differing in saturation of phosphatidylcholine (PC) acyl chains, such as distearoyl phosphatidylcholine (DSPC), stearoyl-oleoyl phosphatidylcholine (SOPC) and dioleoyl phosphatidylcholine (DOPC) have been studied as floating Langmuir monolayers at the air/water interface. In order to examine the influence of a polar group, distearoyl phosphatidylethanolamine (DSPE) was chosen. The films were spread at room temperature on aqueous subphases and characterized by the surface pressure-area (pi-A) isotherms and compression modulus (C(s)(-1)) values. The interactions were examined by analyzing the mean molecular areas and quantified by the excess free energy of mixing values. The obtained results indicate that the affinity of cholesterol to saturated/unsaturated phosphatidylcholines does not differ significantly, and revealed strong influence of the kind of a polar group on the cholesterol-phospholipid interactions. On the other hand, the apolar group structure was found to modify the stoichiometry of sterol-PC complexes.     

Probing the hydration of lipid bilayers using a solvent isotope effect on phospholipid mixing

Nearest-neighbor recognition experiments, which have been carried out using exchangeable dimers derived from 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine,and 1-palmitoyl-2-oleoyl-sn-glycerophosphoethanolamine, indicate that replacement of H2O by D2O can significantly influence phospholipid mixing, but only in bilayers that are saturated and devoid of cholesterol. These findings, together with those of previous electron spin resonance spin-labeling studies,indicate that mammalian membranes, which are rich in cholesterol and unsaturated phospholipids, are ideal hydrophobic barriers.     

Light scattering of liposomal structure of 1,2-distearoyl-sn-glycero-3-phosphocholine and cholesterol with NaCl

1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and cholesterol are used to prepare liposome. Dynamic light scattering was used to study the dynamics of different concentration of the DSPC on liposomal structure. The results show that with increase of DSPC concentration the diffusion coefficient decreases. The small angle X-Ray scattering (SAXS) experiments show that an increase of the DSPC of 0.5–5% changes the size of liposomal structure from 35 to 112 nm, this is analysed in leaves of hard sphere core shell model. Moreover, the addition of NaCl at 0.001 molar can decrease the size of liposomal structure.     

Structure of docosahexaenoic acid-containing phospholipid bilayers as studied by (2)H NMR and molecular dynamics simulations.

Polyunsaturated phospholipids are known to be important with regard to the biological functions of essential fatty acids, for example, involving neural tissues such as the brain and retina. Here we have employed two complementary structural methods for the study of polyunsaturated bilayer lipids, viz. deuterium ((2)H) NMR spectroscopy and molecular dynamics (MD) computer simulations. Our research constitutes one of the first applications of all-atom MD simulations to polyunsaturated lipids containing docosahexaenoic acid (DHA; 22:6 cis-Delta(4,7,10,13,16,19)). Structural features of the highly unsaturated, mixed-chain phospholipid, 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (PDPC), have been studied in the liquid-crystalline (L(alpha)) state and compared to the less unsaturated homolog, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). The (2)H NMR spectra of polyunsaturated bilayers are dramatically different from those of less unsaturated phospholipid bilayers. We show how use of MD simulations can aid in interpreting the complex (2)H NMR spectra of polyunsaturated bilayers, in conjunction with electron density profiles determined from small-angle X-ray diffraction studies. This work clearly demonstrates preferred helical and angle-iron conformations of the polyunsaturated chains in liquid-crystalline bilayers, which favor chain extension while maintaining bilayer flexibility. The presence of relatively long, extended fatty acyl chains may be important for solvating the hydrophobic surfaces of integral membrane proteins, such as rhodopsin. In addition, the polyallylic DHA chains have a tendency to adopt back-bended (hairpin-like) structures, which increase the interfacial area per lipid. Finally, the material properties have been analyzed in terms of the response of the bilayer to mechanical stress. Simulated bilayers of phospholipids containing docosahexaenoic acid were less sensitive to the applied surface tension than were saturated phospholipids, possibly implying a decrease in membrane elasticity (area elastic modulus, bending rigidity). The above features distinguish DHA-containing lipids from saturated or monounsaturated lipids and may be important for their biological modes of action.     

Identification by gas chromatography and mass spectrometry of lipids from the rat Harderian gland

Lipids from rat Harderian glands were extracted with ethyl acetate, hydrolysed with base and examined by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) as trimethylsilyl (TMS), [2H9]TMS, methyl ester-TMS, picolinyl, nicotinate and nicotinylidene derivatives. The latter three derivatives were used to reveal the structures of the alkyl chains of fatty acids, alcohols and glycerol ethers, respectively. Forty-eight compounds were identified, representing about 97% of the total extracted lipids as measured by GC peak areas. The major constituents were fatty acids with chain lengths from 12 to 22 carbon atoms (mainly C18 and C20) and fatty alcohols (C16 to C26) derived from wax esters. Most of these acids and alcohols were unsaturated in the omega-7 position and were accompanied by smaller amounts of the saturated and omega-5 monounsaturated analogues. glycerol ethers were also identified for the first time in this secretion; the ether chains contained from 14 to 19 carbon atoms (mainly 16) and were straight-chain saturated, unsaturated (omega-5 and omega-7) and branched (iso). The only sterol found was cholesterol amounting to 1.24% of the total extract.     

The impact of sterol structure on the interactions with sphingomyelin in mixed langmuir monolayers

In this work, the Langmuir monolayer technique was applied to study the interactions between sphingomyelin and various sterols differing in the structure of the side chain (cholesterol, beta-sitosterol, stigmasterol). The mean area per molecule and the excess free energy of mixing values were analyzed in the context of sterol-induced condensing effect and interactions between molecules in the mixed monolayers. Moreover, the compression modulus values were calculated and widely discussed from the point of view of the ordering effect of sterols. It was found that all of the sterols investigated form the most stable monolayers with sphingomyelin at 2:1 sphingomyelin:sterol proportion and the strongest interactions exist between molecules in cholesterol-containing films. Moreover, cholesterol provokes the strongest area condensation and reveals the highest ordering properties, while plant sterols were found to differ only slightly with regards to their ordering properties. Additionally, the ordering effect of the sterols on dipalmitoylphosphatidylcholine (DPPC) films was analyzed and compared to that on sphingomyelin films.     

Cholesterol and phytosterols effect on sphingomyelin/phosphatidylcholine model membranes--thermodynamic analysis of the interactions in ternary monolayers

In this work thermodynamic analysis of the interactions between lipids in ternary sphingomyelin/DPPC/sterol Langmuir films were performed to compare the effect of cholesterol, beta-sitosterol and stigmasterol on a model membrane. The condensing effect of the respective sterols and the interactions between molecules in ternary mixtures were analyzed on the basis of the excess area per molecule and the excess free energy of mixing values. The stability of the mixed monolayers was verified with the free energy of mixing values. The conclusions on the ordering effect of sterols were drawn from the analysis of the compression modulus values. It was found that the stoichiometry of the mixed films of the highest thermodynamic stability and of the strongest interactions is the same for all the sterols investigated. The results obtained prove that the mammalian sterol induces the strongest contraction of the area and reveals the strongest stabilizing and ordering effect among the investigated sterol. Stigmasterol was found to condense a model membrane in a weaker extent as compared to beta-sitosterol, however, the differences in ordering properties of both phytosterols are less pronounced. The magnitude of the influence of the investigated sterols on a model membrane was thoroughly discussed from the point of view of the structure of their side chain, which determines the geometry of a sterol molecule.     

Effect of cholesterol or phospholipid incorporation on the chemical stability of the muramyldpeptide derivative B30-MDP in mixed vesicles

Application of the muramyldipeptide derivative B30-MDP to liposomal vaccines will aid in the development of improved high immunogenicity vaccines. To give full play to the effectiveness of B30-MDP as a liposomal vaccine, it is important to evaluate the effect of cholesterol, dimyristoylphosphatidylcholine (DMPC) or distearoylphosphatidylcholine (DSPC) incorporation on the chemical stability of B30-MDP and physicochemical properties of B30-MDP/lipid mixed vesicles from the view point of pharmaceutics.The observed degradation rate constants of B30-MDP by hydrolysis in B30-MDP/cholesterol mixed vesicles were increased with increasing concentration of cholesterol, however, those in B30-MDP/DMPC and B30-MDP/DSPC mixed vesicles were unchanged with increasing concentration of DMPC and DSPC. The degradation behavior of B30-MDP was then compared with physicochemical properties of B30-MDP/lipid mixed vesicles, such as membrane fluidity and particle size. It was apparent that the degradation of B30-MDP in B30-MDP/cholesterol mixed vesicles was influenced by the particle size, but not by the fluidity of the membranes. In the case of B30-MDP/phospholipid mixed vesicles, MDP/phospholipid mixed vesicles, the degradation of B30-MDP was not influenced by either the membranes' fluidity or the particle size of the mixed vesicles.It is considered that the degradation of B30-MDP in the mixed vesicles is dependent on the membrane state, and the addition of cholesterol to B30-MDP vesicle inhibits the mutual interaction of MDP regions, whereas the addition of phospholipids hardly influences the mutual interaction of MDP regions, possibly owing to phase separation between B30-MDP and phospholipids.     

Large influence of cholesterol on solute partitioning into lipid membranes

Cholesterol plays an important role in maintaining the correct fluidity and rigidity of the plasma membrane of all animal cells, and hence, it is present in concentrations ranging from 20 to 50 mol %. Whereas the effect of cholesterol on such mechanical properties has been studied exhaustively over the last decades, the structural basis for cholesterol effects on membrane permeability is still unclear. Here we apply systematic molecular dynamics simulations to study the partitioning of solutes between water and membranes. We derive potentials of mean force for six different solutes permeating across 20 different lipid membranes containing one out of four types of phospholipids plus a cholesterol content varying from 0 to 50 mol %. Surprisingly, cholesterol decreases solute partitioning into the lipid tail region of the membranes much more strongly than expected from experiments on macroscopic membranes, suggesting that a laterally inhomogeneous cholesterol concentration and permeability may be required to explain experimental findings. The simulations indicate that the cost of breaking van der Waals interactions between the lipid tails of cholesterol-containing membranes account for the reduced partitioning rather than the surface area per phospholipid, which has been frequently suggested as a determinant for solute partitioning. The simulations further show that the partitioning is more sensitive to cholesterol (i) for larger solutes, (ii) in membranes with saturated as compared to membranes with unsaturated lipid tails, and (iii) in membranes with smaller lipid head groups.     

Energetics of cholesterol transfer between lipid bilayers

It is believed that natural biological membranes contain domains of lipid ordered phase enriched in cholesterol and sphingomyelin. Although the existence of these domains, called lipid rafts, is still not firmly established for natural membranes, direct microscopic observations and phase diagrams obtained from the study of three-component mixtures containing saturated phospholipids, unsaturated phospholipids, and cholesterol demonstrate the existence of lipid rafts in synthetic membranes. The presence of the domains or rafts in these membranes is often ascribed to the preferential interactions between cholesterol and saturated phospholipids, for example, between cholesterol and sphingomyelin. In this work, we calculate, using molecular dynamics computer simulation technique, the free energy of cholesterol transfer from the bilayer containing unsaturated phosphatidylcholine lipid molecules to the bilayer containing sphingomyelin molecules and find that the affinity of cholesterol to sphingomyelin is higher. Our calculations of the free-energy components, energy and entropy, show that cholesterol transfer is exothermic and promoted by the favorable change in the lipid-lipid interactions near cholesterol and not by the favorable energy of cholesterol-sphingomyelin interaction, as assumed previously.     

Phase transition of a single lipid bilayer measured by sum-frequency vibrational spectroscopy

In this communication, we demonstrate the first use of sum-frequency generation (SFG) vibrational spectroscopy to measure directly the phase transition temperature (Tm) of a single planar supported lipid bilayer (PSLB). Three saturated phospholipids, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-diheptadecanoyl-sn-glycero-3-phosphocholine (DHPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), were studied. Lipid bilayer films were prepared by the the Langmuir-Blodgett method at a surface pressure of 30 nN/m. The symmetric nature of the bilayer was used to determine the Tm of bilayers by measuring the intensity of the symmetric methyl stretch at 2875 cm-1 from the lipid fatty acid chains as a function of temperature. A maximum in the CH3 symmetric stretch transition was observed at the Tm of the lipid film due to the reduction of symmetry in the bilayer. The SFG measured Tm for DPPC, DHPC, and DSPC were 41.0 +/- 0.4, 52.4 +/- 0.7, and 57.9 +/- 0.5 degrees C, respectively. These values correlate well with the literature values of 41.3 +/- 1.8, 49 +/- 3, and 54.5 +/- 1.5 degrees C for DPPC, DHPC, and DSPC, respectively obtained by differential scanning calorimetry (DSC) of lipid vesicles in solution. The high degree of correlation between the SFG spectroscopic measurements and the DSC results suggests the Tm of these lipids is not significantly altered upon immobilization on a surface.     

Properties of β-sitostanol/DPPC monolayers studied with Grazing Incidence X-ray Diffraction (GIXD) and Brewster Angle Microscopy

Although the influence of structurally modified sterols on artificial membranes has been intensively investigated, studies on the properties of stanols, which are saturated analogs of sterols, are very rare. Therefore, we have performed Grazing Incidence X-ray Diffraction (GIXD) experiments aimed at studying in-plane organization of a plant stanol-β-sitostanol monolayer and its mixtures with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine - DPPC at the air/water interface. The collected GIXD data, resulting in-plane parameters and BAM images provide information on molecular organization and in-plane ordering of the investigated films. It was found that the lateral organization of β-sitostanol/DPPC monolayers depends on their composition. The oblique structure of the in-plane lattice of tilted hydrophobic region of molecules, found for DPPC film, is maintained at 10 mol% of stanol in the system. However, at 30 and 90 mol% of stanol in the mixture, the arrangement of molecules is hexagonal and they are oriented perpendicularly to the interface. With the addition of stanol the extend of the in-plane order of the monolayers decreases. Moreover, in mixtures the ordered domains consist of both monolayer's components. Additionally, β-sitostanol film is of similar in-plane organization as the corresponding sterol monolayer (β-sitosterol) and stanol induces condensing effect on DPPC.     

A Cantilever‐based Biosensor for Real‐time Monitoring of Interactions between Amyloid‐β(1–40) and Membranes Comprised of Phosphatidylcholine Lipids with Different Hydrophobic Acyl Chains

Accumulating evidence suggests that interaction between amyloid‐β (Aβ) and cell membrane is crucial to the pathogenesis of Alzheimer's disease (AD), and thus an increasing understanding of the impact of membrane composition on Aβ‐membrane interaction becomes essential for the mechanism elucidation of Aβ‐membrane interaction and the early diagnosis of AD. In this work, electrically neutral phosphatidylcholine (PC) as the most major class of membrane phospholipids, including 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC), 1,2‐distearoyl‐sn‐glycero‐3‐phosphocholine (DSPC), 1‐palmitoyl‐2‐oleoyl‐sn‐glycero‐3‐phosphocholine (POPC), and Aβ(1–40) as the most common amyloid protein were selected as the research subjects, and a developed cantilever‐based biosensor, on which liposomes comprised of PC lipids were immobilized, was applied to characterize in real time the interactions between Aβ(1–40) and membranes comprised of PC lipids with different hydrophobic acyl chains, and to evaluate the effect of cholesterol incorporated in membrane on Aβ‐membrane interaction during the whole process of Aβ(1–40) fibrillization. The results illustrate that the interaction between Aβ(1–40) and PC membrane can be divided into three stages, which are related to the change in molecular states of Aβ. More importantly, it is found that membranes comprised of PC lipids with shorter saturated acyl chains show higher interaction ability with Aβ(1–40), and the incorporation of cholesterol into PC bilayer can remarkably accelerate and strengthen Aβ(1–40)‐membrane interaction. These results confirm that hydrophobicity is the main driving force for the interactions between Aβ(1–40) and PC membranes. In return, the above results enlightened us to apply the current micro‐cantilever immobilized with cholesterol‐containing DPPC liposomes to challenge the detection of low‐concentration Aβ(1–40). This time 50‐nM Aβ(1–40) in aqueous solution has been effectively detected, suggesting that this proposed detection technique would contribute to Aβ detection and early diagnosis of AD in the future.