The influence of cholesterol on interactions and dynamics of ibuprofen in a lipid bilayer

In this work, molecular dynamics (MD) simulations with atomistic details were performed to examine the influence of the cholesterol on the interactions and the partitioning of the hydrophobic drug ibuprofen in a fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer. Analysis of MD simulations indicated that ibuprofen molecules prefer to be located in the hydrophobic acyl chain region of DMPC/cholesterol bilayers. This distribution decreases the lateral motion of lipid molecules. The presence of ibuprofen molecules in the bilayers with 0 and 25 mol% cholesterol increases the ordering of hydrocarbon tails of lipids whereas for the bilayers with 50 mol% cholesterol, ibuprofen molecules perturb the flexible chains of DMPC lipids which leads to the reduction of the acyl chain order parameter. The potential of the mean force (PMF) method was used to calculate the free energy profile for the transferring of an ibuprofen molecule from the bulk water into the DMPC/cholesterol membranes. The PMF studies indicated that the presence of 50 mol% cholesterol in the bilayers increases the free energy barrier and slows down the permeation of the ibuprofen drug across the DMPC bilayer. This can be due to the condensing and ordering effects of the cholesterol on the bilayer.     

Multibilayer structure of dimyristoylphosphatidylcholine dihydrate as determined by energy minimization

Complete energy minimization was carried out on the multibilayer crystal structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine dihydrate (DMPC.2H2O), starting from the X-ray structure determination reported by Pearson and Pascher (1979) Nature 281, 499-501. The asymmetric unit contains two nonidentical DMPC molecules and four water molecules. Minimization removed the acyl chain disorder present in the X-ray structure and caused the carbon planes of the acyl chains to become mutually parallel. Two energy-minimized structures (structures I and II) were found which mainly differed in the hydrogen-bonding arrangement of the waters of hydration. In structure I as in the X-ray structure, one of the water molecules forms a hydrogen-bonded bridge between successive bilayers; but in structure II, all hydrogen bonds are satisfied on the same bilayer. Structure II corresponds to the global energy minimum and is also a suitable structure for single bilayers. The lattice constants and cell volume of the minimized structures are close to the experimental values. The electrostatic force between DMPC bilayers is attractive. The mean hydration energy of the water is -14.2 kcal/mol, which is 2.5 kcal/mol lower than the binding energy of ice.     

Effects of cholesterol concentration on the interaction of cytarabine with lipid membranes: a molecular dynamics simulation study

Liposomal cytarabine, DepoCyt, is a chemotherapy agent which is used in cancer treatment. This form of cytarabine has more efficacy and fewer side effects relative to the other forms. Since DepoCyt contains the cytarabine encapsulated within phosphatidylcholine and the sterol molecules, we modeled dioleoylphosphatidylcholine (DOPC)/cholesterol bilayer membrane as a carrier for cytarabine to study drug–bilayer interactions. For this purpose, we performed a series of united-atom molecular dynamics (MD) simulations for 25?ns to investigate the interactions between cytarabine and cholesterol-containing DOPC lipid bilayers. Only the uncharged form of cytarabine molecule was investigated. In this study, different levels of the cholesterol content (0, 20, and 40%) were used. MD simulations allowed us to determine dynamical and structural properties of the bilayer membrane and to estimate the preferred location and orientation of the cytarabine molecule inside the bilayer membrane. Properties such as membrane thickness, area per lipid, diffusion coefficient, mass density, bilayer packing, order parameters, and intermolecular interactions were examined. The results show that by increasing the cholesterol concentration in the lipid bilayers, the bilayer thickness increases and area per lipid decreases. Moreover, in accordance with the experiments, our calculations show that cholesterol molecules have ordering effect on the hydrocarbon acyl chains. Furthermore, the cytarabine molecule preferentially occupies the polar region of the lipid head groups to form specific interactions (hydrogen bonds). Our results fully support the experimental data. Our finding about drug–bilayer interaction is crucial for the liposomal drug design.     

Comparison of energy-minimized crystal structures of 2,3-dilauroyl-D-glycerol, 3-palmitoyl-DL-glycerol-1-phosphorylethanolamine and 1,2-dilauroyl-DL-phosphatidylethanolamine:acetic acid

Computational procedures have been developed by which the total energy of a lipid multibilayer can be calculated and minimized. The energy is expressed as a sum of non-bonded, electrostatic, hydrogen bonded and torsional energy terms and includes intramolecular and intermolecular components. Calculations were carried out on three lipid crystals for which structural data are available from X-ray diffraction analysis. For each crystal, the energy was minimized as a function of all bond rotations, molecular rotations and translations and the lattice constants. The minimized structures differed by only small amounts from the experimental structures, which confirms the validity of the current set of energy functions and parameters for use with lipids. The intermolecular energy of each crystal is analyzed in terms of lateral interactions, interactions between the two monolayers of the same bilayer and interactions between bilayers. The intermolecular non-bonded energy per CH2 or CH3 group in the acyl chains is also given.     

Interactions of the local anesthetic tetracaine with membranes containing phosphatidylcholine and cholesterol: a 2H NMR study

The interactions of the local anesthetic tetracaine with multilamellar dispersions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol have been investigated by deuterium nuclear magnetic resonance of specifically deuteriated tetracaines, DMPC and cholesterol. Experiments were performed at pH 5.5, when the anesthetic is primarily charged, and at pH 9.5, when it is primarily uncharged. The partition coefficients of the anesthetic in the membrane have been measured at both pH values for phosphatidylcholine bilayers with and without cholesterol. The higher partition coefficients obtained at pH 9.5 reflect the hydrophobic interactions between the uncharged form of the anesthetic and the hydrocarbon region of the bilayer. The lower partition coefficients for the DMPC/cholesterol system at both pH values suggest that cholesterol, which increases the order of the lipid chains, decreases the solubility of tetracaine into the bilayer. For phosphatidylcholine bilayers, it has been proposed [Boulanger, Y., Schreier, S., & Smith, I. C. P. (1981) Biochemistry 20, 6824-6830] that the charged tetracaine at low pH is located mostly at the phospholipid headgroup level while the uncharged tetracaine intercalates more deeply into the bilayer. The present study suggests that the location of tetracaine in the cholesterol-containing system is different from that in pure phosphatidylcholine bilayers: the anesthetic sits higher in the membrane. An increase in temperature results in a deeper penetration of the anesthetic into the bilayer. Moreover, the incorporation of the anesthetic into DMPC bilayers with or without cholesterol results in a reduction of the lipid order parameters both in the plateau and in the tail regions of the acyl chains, this effect being greater with the charged form of the anesthetic.     

Effects of phospholipid unsaturation on the bilayer nonpolar region: a molecular simulation study

Molecular dynamics simulations of two monounsaturated phosphatidylcholine (PC) bilayers made of 1-palmitoyl-2-oleoyl-PC (POPC; cis-unsaturated) and 1-palmitoyl-2-elaidoyl-PC (PEPC; trans-unsaturated) were carried out to investigate the effect of a double bond in the PC beta-chain and its conformation on the bilayer core. Four nanosecond trajectories were used for analyses. A fully saturated 1,2-dimyristoyl-PC (DMPC) bilayer was used as a reference system. In agreement with experimental data, this study shows that properties of the PEPC bilayer are more similar to those of the DMPC than to the POPC bilayer. The differences between POPC and PEPC bilayers may be attributed to the different ranges of angles covered by the torsion angles beta10 and beta12 of the single bonds next to the double bond in the oleoyl (O) and elaidoyl (E) chains. Broader distributions of beta10 and beta12 in the E chain than in the O chain make the E chain more flexible. In effect, the packing of chains in the PEPC bilayer is similar to that in the DMPC bilayer, whereas that in the POPC bilayer is looser than that in the DMPC bilayer. The effect of the cis-double bond on torsions at the beginning of the O chain (beta4 and beta5) is similar to that of cholesterol on these torsions in a myristoyl chain.     

Cholesterol effects on a mixed-chain phosphatidylcholine bilayer: a molecular dynamics simulation study

A molecular dynamics simulation of a mono-cis-unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine bilayer containing approximately 22 mol% of cholesterol (POPC-Chol) was carried out for 15 ns. An 8-ns trajectory was analysed to determine the effects of Chol on the membrane properties and compare it with that on the fully saturated 1,2-dimyristoyl-phosphatidylcholine bilayer containing approximately 22 mol% of Chol (DMPC-Chol). The study suggests that the experimentally observed weaker effect of Chol on the POPC than DMPC bilayer might result from a different vertical localisation of the Chol hydroxyl group (OH-Chol) in both bilayers: in the POPC-Chol bilayer, OH-Chol is placed approximately 3 A higher in the bilayer interface than in the DMPC-Chol bilayer. Because of the rigid cis double bond in the beta-chain of POPC, Chol fits worse to the POPC-Chol membrane environment and is pushed up, in effect all Chol ring atoms are, on average, located above the double bond. Both in mono-cis-unsaturated and fully saturated PC bilayers, Chol induces stronger van der Waals interactions among the chains, whereas its interactions with the chains are weak. In contrast to DMPC, the smooth alpha-face of the Chol ring lowers the order of POPC chains, whereas the rough beta-face increases the order.     

Influence of hydrophobic mismatch on structures and dynamics of gramicidin a and lipid bilayers

Gramicidin A (gA) is a 15-amino-acid antibiotic peptide with an alternating L-D sequence, which forms (dimeric) bilayer-spanning, monovalent cation channels in biological membranes and synthetic bilayers. We performed molecular dynamics simulations of gA dimers and monomers in all-atom, explicit dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dioleoylphosphatidylcholine (DOPC), and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayers. The variation in acyl chain length among these different phospholipids provides a way to alter gA-bilayer interactions by varying the bilayer hydrophobic thickness, and to determine the influence of hydrophobic mismatch on the structure and dynamics of both gA channels (and monomeric subunits) and the host bilayers. The simulations show that the channel structure varied little with changes in hydrophobic mismatch, and that the lipid bilayer adapts to the bilayer-spanning channel to minimize the exposure of hydrophobic residues. The bilayer thickness, however, did not vary monotonically as a function of radial distance from the channel. In all simulations, there was an initial decrease in thickness within 4–5 Å from the channel, which was followed by an increase in DOPC and POPC or a further decrease in DLPC and DMPC bilayers. The bilayer thickness varied little in the monomer simulations—except one of three independent simulations for DMPC and all three DLPC simulations, where the bilayer thinned to allow a single subunit to form a bilayer-spanning water-permeable pore. The radial dependence of local lipid area and bilayer compressibility is also nonmonotonic in the first shell around gA dimers due to gA-phospholipid interactions and the hydrophobic mismatch. Order parameters, acyl chain dynamics, and diffusion constants also differ between the lipids in the first shell and the bulk. The lipid behaviors in the first shell around gA dimers are more complex than predicted from a simple mismatch model, which has implications for understanding the energetics of membrane protein-lipid interactions.     

Cadmium-induced changes in the membrane of human erythrocytes and molecular models

The structural effects of cadmium on cell membranes were studied through the interaction of Cd(2+) ions with human erythrocytes and their isolated unsealed membranes (IUM). Studies were carried out by scanning electron microscopy and fluorescence spectroscopy, respectively. Cd(2+) induced shape changes in erythrocytes, which took the form of echinocytes. According to the bilayer couple hypothesis, this result meant that Cd(2+) ions located in the outer monolayer of the erythrocyte membrane. Fluorescence spectroscopy measurements in IUM indicated a disordering effect at both the polar headgroup and the acyl chain packing arrangements of the membrane phospholipid bilayer. Cd(2+) ions also interacted with molecular models of the erythrocyte membrane consisting in bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), representing classes of phospholipids located in the outer and inner monolayers the erythrocyte membrane, respectively. X-ray diffraction indicated that Cd(2+) ions induced structural perturbation of the polar headgroup and of the hydrophobic acyl regions of DMPC, while the effects of cadmium on DMPE bilayers were much milder. This conclusion is supported by fluorescence spectroscopy measurements on DMPC large unilamellar vesicles (LUV). All these findings point to the important role of phospholipid bilayers in the interaction of cadmium on cell membranes.     

Effects of phospholipid unsaturation on the membrane/water interface: a molecular simulation study.

Molecular dynamics (MD) simulations of fully hydrated bilayers in the liquid-crystalline state made of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) or 1-palmitoyl-2-elaidoyl-phosphatidylcholine (PEPC) were carried out to investigate the effect of the incorporation of a double bond in the phosphatidylcholine (PC) beta-chain (cis or trans) on the membrane/water interface. The bilayers reached thermal equilibrium after 3 and 1 ns of MD simulations, respectively, and productive runs were carried out for 3 ns for each bilayer. As reference systems, the 1,2-dimyristoyl-phosphatidylcholine (DMPC) bilayer (M. Pasenkiewicz-Gierula, Y. Takaoka, H. Miyagawa, K. Kitamura, and A. Kusumi, 1999, Biophys. J. 76:1228-1240) and DMPC-cholesterol (Chol) bilayer containing 22 mol % Chol (M. Pasenkiewicz-Gierula, T. Róg, K. Kitamura, A. and Kusumi, 2000, Biophys. J. 78:1376-1389) were used. The study shows that at the interface of POPC, PEPC, and DMPC-Chol bilayers, average numbers of PC-water and PC-PC interactions are similar and, respectively, greater and smaller than in the DMPC bilayer. The average area/PC in mono-unsaturated bilayers is approximately 4 A(2) larger than in the DMPC bilayer; nevertheless, a strong correlation was found between a single molecular area (SMA) of a PC and the number of interactions this PC makes; i.e., PCs (either saturated or unsaturated) with the same SMA form similar numbers of intermolecular links. The numbers and corresponding SMAs are distributed about averages pertinent to each bilayer. No significant difference between cis and trans bonds was found.     

Bilayer polarity and its thermal dependency in the l(o) and l(d) phases of binary phosphatidylcholine/cholesterol mixtures

Diverse variations in membrane properties are observed in binary phosphatidylcholine/cholesterol mixtures. These mixtures are nonideal, displaying single or phase coexistence, depending on chemical composition and other thermodynamic parameters. When compared with pure phospholipid bilayers, there are changes in water permeability, bilayer thickness and thermomechanical properties, molecular packing and conformational freedom of phospholipid acyl chains, in internal dipolar potential and in lipid lateral diffusion. Based on the phase diagrams for DMPC/cholesterol and DPPC/cholesterol, we compare the equivalent polarity of pure bilayers with specific compositions of these mixtures, by using the Py empirical scale of polarity. Besides the contrast between pure and mixed lipid bilayers, we find that liquid-ordered (l(o)) and liquid-disordered (l(d)) phases display significantly different polarities. Moreover, in the l(o) phase, the polarities of bilayers and their thermal dependences vary with the chemical composition, showing noteworthy differences for cholesterol proportions at 35, 40, and 45 mol%. At 20 degrees C, for DMPC/cholesterol at 35 and 45 mol%, the equivalent dielectric constants are 21.8 and 23.8, respectively. Additionally, we illustrate potential implications of polarity in various membrane-based processes and reactions, proposing that for cholesterol containing bilayers, it may also go along with the occurrence of lateral heterogeneity in biological membranes.     

Transmembrane Peptides Influence the Affinity of Sterols for Phospholipid Bilayers

Cholesterol is distributed unevenly between different cellular membrane compartments, and the cholesterol content increases from the inner bilayers toward the plasma membrane. It has been suggested that this cholesterol gradient is important in the sorting of transmembrane proteins. Cholesterol has also been to shown play an important role in lateral organization of eukaryotic cell membranes. In this study the aim was to determine how transmembrane proteins influence the lateral distribution of cholesterol in phospholipid bilayers. Insight into this can be obtained by studying how cholesterol interacts with bilayer membranes of different composition in the presence of designed peptides that mimic the transmembrane helices of proteins. For this purpose we developed an assay in which the partitioning of the fluorescent cholesterol analog CTL between LUVs and mβCD can be measured. Comparison of how cholesterol and CTL partitioning between mβCD and phospholipid bilayers with different composition suggests that CTL sensed changes in bilayer composition similarly as cholesterol. Therefore, the results obtained with CTL can be used to understand cholesterol distribution in lipid bilayers. The effect of WALP23 on CTL partitioning between DMPC bilayers and mβCD was measured. From the results it was clear that WALP23 increased both the order in the bilayers (as seen from CTL and DPH anisotropy) and the affinity of the sterol for the bilayer in a concentration dependent way. Although WALP23 also increased the order in DLPC and POPC bilayers the effects on CTL partitioning was much smaller with these lipids. This indicates that proteins have the largest effect on sterol interactions with phospholipids that have longer and saturated acyl chains. KALP23 did not significantly affect the acyl chain order in the phospholipid bilayers, and inclusion of KALP23 into DMPC bilayers slightly decreased CTL partitioning into the bilayer. This shows that transmembrane proteins can both decrease and increase the affinity of sterols for the lipid bilayers surrounding proteins. This is likely to affect the sterol distribution within the bilayer and thereby the lateral organization in biomembranes.     

Interaction of a bacterial monorhamnolipid secreted by Pseudomonas aeruginosa MA01 with phosphatidylcholine model membranes

This work presents a biophysical study on the interactions of a monorhamnolipid (monoRL) produced by Pseudomonas aeruginosa MA01 with model phosphatidylcholine membranes. The molecular characterization of the biological activities, including the modulation of phospholipid membranes structure, of this monoRL biosurfactant is of importance for the validation of this particular Pseudomonas aeruginosa strain as a useful biosurfactant producer. The marked amphiphilic structure of monoRL is expected to result in strong interactions with the phospholipid constituents of membrane bilayers. Incorporation of monoRL into DMPC completely abolished the pretransition, and the main gel to liquid-crystalline phase transition was progressively broadened and shifted to lower temperatures, as observed by differential scanning calorimetry. Partial phase diagrams for DPPC and DSPC indicated near-ideal behavior. However, the DMPC diagram indicated fluid phase immiscibility. X-ray diffraction showed and apparent increase in d-value for DPPC containing monoRL, which might be the result of an effective increase in the bilayer thickness, or in the thickness of the hydration layer between bilayers. FTIR indicated that interaction of monoRL with the phospholipid acyl chains did not result in a large additional disordering of the acyl chain region of the fluid bilayer. Analysis of the CO stretching band of DPPC indicated an important effect of monoRL on the interfacial region of phosphatidylcholine bilayers, which might contribute to explain some of the biological activities of this glycolipid.     

Comparative molecular dynamics study of lipid membranes containing cholesterol and ergosterol

Sterol molecules are essential for maintaining the proper structure and function of eukaryotic cell membranes. The influence of cholesterol (the principal sterol of higher animals) on the lipid bilayer properties was extensively studied by both experimental and simulation methods. In contrast, the effect of ergosterol (the principal fungal sterol) on the membrane structure and dynamics is much less recognized. This work presents the results of comparative molecular dynamics simulation of the hydrated dimyristoylphosphatidylcholine bilayer containing approximately 25 mol % of cholesterol or ergosterol. A detailed analysis of the molecular properties (e.g., bilayer thickness, lipid order, diffusion, intermolecular interactions, etc.) of both sterol-induced liquid-ordered membrane phases is presented. Presence of sterols in the membrane significantly changes its property, especially fluidity and molecular packing. Moreover, in accordance with the experiments, our calculations show that, compared to cholesterol, ergosterol has higher ordering effect on the phospholipid acyl chains. This different influence on the properties of the lipid bilayer stems from differences in conformational freedom of sterol side chains. Additionally, obtained models of lipid membranes containing human and fungal sterols, constituting the result of our work, can be also utilized in other chemotherapeutic studies on interaction of selected ligands (e.g., antifungal compounds) with membranes.     

Small-angle X-ray scattering study of the interaction of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymers with lipid bilayers

The relationship between molecular architecture and the nature of interactions with lipid bilayers has been studied for a series of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers using small-angle X-ray scattering (SAXS) and thermal analysis (differential scanning calorimetry, DSC). The number of molecular repeat units in the hydrophobic poly(propylene oxide), PPO, block has been found to be a critical determinant of the nature of triblock copolymer-lipid bilayer association. For dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-based biomembrane structures, polymers possessing a PPO chain length commensurate with the acyl chain dimensions of the lipid bilayer yield highly ordered, swollen lamellar structures consistent with well-integrated (into the lipid bilayer) PPO blocks. Triblock copolymers of lesser PPO chain length yield materials with structural characteristics similar to a simple dispersion of DMPC in water. Increasing the concentration (from 4 to 12 mol %) of well-integrated triblock copolymers enhances the structural ordering of the lamellar phase, while concentrations exceeding 16 mol % result in the formation of a hexagonal phase. Examination of temperature-induced changes in the structure of these mesophases (complex fluids) reveals that if the temperature is reduced sufficiently, all compositions exclude polymer and thus exhibit the characteristic SAXS pattern for hydrated DMPC bilayers. Increasing the temperature promotes better insertion of the polymers possessing PPO chain lengths sufficient for membrane insertion. No temperature-induced structural changes are observed in compositions prepared with PEO-PPO-PEO polymers that feature PPO length insufficient to permit full incorporation into the lipid bilayer.     

Bilayer polarity and its thermal dependency in the ?o and ?d phases of binary phosphatidylcholine/cholesterol mixtures

Diverse variations in membrane properties are observed in binary phosphatidylcholine/cholesterol mixtures. These mixtures are nonideal, displaying single or phase coexistence, depending on chemical composition and other thermodynamic parameters. When compared with pure phospholipid bilayers, there are changes in water permeability, bilayer thickness and thermomechanical properties, molecular packing and conformational freedom of phospholipid acyl chains, in internal dipolar potential and in lipid lateral diffusion. Based on the phase diagrams for DMPC/cholesterol and DPPC/cholesterol, we compare the equivalent polarity of pure bilayers with specific compositions of these mixtures, by using the Py empirical scale of polarity. Besides the contrast between pure and mixed lipid bilayers, we find that liquid-ordered (?_o) and liquid-disordered (?_d) phases display significantly different polarities. Moreover, in the ?_o phase, the polarities of bilayers and their thermal dependences vary with the chemical composition, showing noteworthy differences for cholesterol proportions at 35, 40, and 45 mol%. At 20 °C, for DMPC/cholesterol at 35 and 45 mol%, the equivalent dielectric constants are 21.8 and 23.8, respectively. Additionally, we illustrate potential implications of polarity in various membrane-based processes and reactions, proposing that for cholesterol containing bilayers, it may also go along with the occurrence of lateral heterogeneity in biological membranes.     

Molecular packing in 1-hexanol-DMPC bilayers studied by molecular dynamics simulation

The structure and molecular packing density of a "mismatched" solute, 1-hexanol, in lipid membranes of dimyristoyl phosphatidylcholine (DMPC) was studied by molecular dynamics simulations. We found that the average location and orientation of the hexanol molecules matched earlier experimental data on comparable systems. The local density or molecular packing in DMPC-hexanol was elucidated through the average Voronoi volumes of all heavy (non-hydrogen) atoms. Analogous analysis was conducted on trajectories from simulations of pure 1-hexanol and pure (hydrated) DMPC bilayers. The results suggested a positive volume change, DeltaV(m), of 4 cm(3) mol(-1) hexanol partitioned at 310 K in good accordance with experimental values. Analysis of the apparent volumes of each component in the pure and mixed states further showed that DeltaV(m) reflects a balance between a substantial increase in the packing density of the alcohol upon partitioning and an even stronger loosening in the packing of the lipid. Furthermore, analysis of Voronoi volumes along the membrane normal identifies a distinctive depth dependence of the changes in molecular packing. The outer (interfacial) part of the lipid acyl chains (up to C8) is stretched by about 4%. Concomitantly, the average lateral area per chain decreases and these two effects compensate so that the overall packing density in the outer region, where the hexanol molecules are located, remains practically constant. The core of the bilayer (C9-C13) is slightly thinned. The average lateral area per chain in this region expands, resulting in a looser packing density. The net effect in the core is a 2-3% decrease in density corresponding to a total volume increase of approximately 14 cm(3) mol(-1) hexanol partitioned.     

Molecular packing and intermolecular interactions in two structural polymorphs of N-palmitoylethanolamine, a type 2 cannabinoid receptor agonist

The molecular structure, packing properties, and intermolecular interactions of two structural polymorphs of N-palmitoylethanolamine (NPEA) have been determined by single-crystal X-ray diffraction. Polymorphs alpha and beta crystallized in monoclinic space group P2(1)/c and orthorhombic space group Pbca, respectively. In both polymorphs, NPEA molecules are organized in a tail-to-tail manner, resembling a bilayer membrane. Although the molecular packing in polymorph alpha is similar to that in N-myristoylethanolamine and N-stearoylethanolamine, polymorph beta is a new form. The acyl chains in both polymorphs are tilted by approximately 35 degrees with respect to the bilayer normal, with their hydrocarbon moieties packed in an orthorhombic subcell. In both structures, the hydroxy group of NPEA forms two hydrogen bonds with the hydroxy groups of molecules in the opposite leaflet, resulting in extended, zig-zag type H-bonded networks along the b-axis in polymorph alpha and along the a-axis in polymorph beta. Additionally, the amide N-H and carbonyl groups of adjacent molecules are involved in N-H...O hydrogen bonds that connect adjacent molecules along the b-axis and a-axis, respectively, in alpha and beta. Whereas in polymorph alpha the L-shaped NPEA molecules in opposite layers are arranged to yield a Z-like organization, in polymorph beta one of the two NPEA molecules is rotated 180 degrees , leading to a W-like arrangement. Lattice energy calculations indicate that polymorph alpha is more stable than polymorph beta by approximately 2.65 kcal/mol.     

Electrochemical and PM-IRRAS studies of the effect of cholesterol on the structure of a DMPC bilayer supported at an Au (111) electrode surface, part 1: properties of the acyl chains

Charge density measurements and polarization modulation infrared reflection absorption spectroscopy were employed to investigate the spreading of small unilamellar vesicles of a dimyristoylphosphatidylcholine (DMPC)/cholesterol (7:3 molar ratio) mixture onto an Au (111) electrode surface. The electrochemical experiments demonstrated that vesicles fuse and spread onto the Au (111) electrode surface, forming a bilayer, at rational potentials -0.4 V < (E - Epzc) < 0.4 V or field strength <6 x 10(7) V m(-1). Polarization modulation infrared reflection absorption spectroscopy experiments provided information concerning the conformation and orientation of the acyl chains of DMPC molecules. Deuterated DMPC was used to subtract the contribution of C-H stretching bands of cholesterol and of the polar head region of DMPC from spectra in the C-H stretching region. The absorption spectra of the C-H stretch bands in the acyl chains were determined in this way. The properties of the DMPC/cholesterol bilayer have been compared with the properties of a pure DMPC bilayer. The presence of 30% cholesterol gives a thicker and more fluid bilayer characterized by a lower capacity and lower tilt angle of the acyl chains.     

Interaction of aspirin (acetylsalicylic acid) with lipid membranes

We studied the interaction of Aspirin (acetylsalicylic acid) with lipid membranes using x-ray diffraction for bilayers containing up to 50 mol% of aspirin. From 2D x-ray intensity maps that cover large areas of reciprocal space we determined the position of the ASA molecules in the phospholipid bilayers and the molecular arrangement of the molecules in the plane of the membranes. We present direct experimental evidence that ASA molecules participate in saturated lipid bilayers of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) and preferably reside in the head group region of the membrane. Up to 50 mol% ASA molecules can be dissolved in this type of bilayer before the lateral membrane organization is disturbed and the membranes are found to form an ordered, 2D crystal-like structure. Furthermore, ASA and cholesterol were found to co-exist in saturated lipid bilayers, with the ASA molecules residing in the head group region and the cholesterol molecules participating in the hydrophobic membrane core.