Molecular Dynamics Simulations of Mixed Acidic/Zwitterionic Phospholipid Bilayers

Anionic lipids are key components in the cell membranes. Many cell-regulatory and signaling mechanisms depend upon a complicated interplay between them and membrane-bound proteins. Phospholipid bilayers are commonly used as model systems in experimental or theoretical studies to gain insight into the structure and dynamics of biological membranes. We report here 200-ns-long MD simulations of pure (DMPC and DMPG) and mixed equimolar (DMPC/DMPG, DMPC/DMPS, and DMPC/DMPA) bilayers that each contain 256 lipids. The intra- and intermolecular interaction patterns in pure and mixed bilayers are analyzed and compared. The effect of monovalent ions (Na⁺) on the formation of salt-bridges is investigated. In particular, the number of Na⁺-mediated clusters in the presence of DMPS is higher than with DMPG and DMPA. We observe a preferential clustering of DMPS (and to some extent DMPA) lipids together rather than with DMPC molecules, which can explain the phase separation observed experimentally for DMPC/DMPS and DMPC/DMPA bilayers.     

Weak interaction of spectrin with phosphatidylcholine-phosphatidylserine multilayers: a 2H and 31P NMR study

Spectrin from human erythrocytes binds to bilayer dispersions of both DMPC and DMPS:DMPC (1:1, w/w). However, no effect of bound spectrin on the conformation of the lipid head groups, as measured from the deuterium quadrupolar splittings of DMPC or DMPS specifically deuterated in the polar head groups, was detected in 1:1 mixtures of the two lipids containing either deuterated DMPC or DMPS. Neither the phase transition of the DMPS:DMPC mixtures, nor the spin-lattice relaxation time (T1) of the deuterated DMPS head group, was affected by spectrin. These results argue against any strong interaction of spectrin with phosphatidylserine and rule out the possibility that spectrin is responsible for the maintenance of PS in the inner monolayer of the erythrocyte membrane during the whole life-span of this cell.     

Reconstitution of transferrin receptor in mixed lipid vesicles. An example of the role of elastic and electrostatic forces for protein/lipid assembly

We studied the interaction of transferrin receptors (of cell line Molt-4) with mixed model membranes as a function of lipid chain length (phospholipids with C14:0 and C18:1 hydrocarbon chains) and of the surface charge of the membrane using mixtures of C14:0 lecithin (DMPC) with C14:0 phosphatidylglycerol (DMPG) and C14:0 phosphatidylserine (DMPS). Spontaneous self-assembly of receptors and lipids was achieved by freeze-thaw cycles of a codispersion of mixed vesicles and receptors in buffer and subsequent separation of receptor-loaded and receptor-free vesicles by density gradient centrifugation. Information on specific lipid/protein interaction mechanisms was obtained by evaluation of protein-induced shifts of phase boundaries of lipid mixtures by calorimetry and by FTIR spectroscopy of partially deuterated lipid mixtures. The important role (1) of minimizing the elastic forces caused by the mismatch of the lengths of hydrophobic cores of the protein (lp) and the bilayer (lL) and (2) of the electrostatic coupling of protein head groups with the charged membrane/water interface for the lipid/protein self-assembly is established. The electrostatic interaction energy per receptor is about 10(3) kBT (by coupling to about 1000 charged lipids) which is sufficient to overcompensate the elastic energy associated with a mismatch of lp - lL approximately 1.0 nm. The maximum receptor concentration incorporated was measured as a function of membrane surface charge and lipid chain length. The maximum receptor molar fraction varied from xpmax = 5 x 10(-5) for DMPC to xpmax = 4 x 10(-4) for 1:1 DMPC/DMPG; moreover xpmax is higher for DMPS than for DMPG as charged component. For the long-chain lipids, xpmax is higher for a 9:1 DEPE/DEPC mixture [(4.2-9) x 10(-4)] than for pure DEPC (ca. 3.5 x 10(-4)). By decomposition of reconstituted receptors with proteases, we demonstrated the homogeneous orientation of the receptor with its extracellular head group pointing to the convex side of the vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Orientation and lipid-peptide interactions of gramicidin A in lipid membranes: polarized attenuated total reflection infrared spectroscopy and spin-label electron spin resonance

Gramicidin A was incorporated at a peptide/lipid ratio of 1:10 mol/mol in aligned bilayers of dimyristoyl phosphatidylcholine (DMPC), phosphatidylserine (DMPS), phosphatidylglycerol (DMPG), and phosphatidylethanolamine (DMPE), from trifluoroethanol. Orientations of the peptide and lipid chains were determined by polarized attenuated total reflection infrared spectroscopy. Lipid-peptide interactions with gramicidin A in DMPC bilayers were studied with different spin-labeled lipid species by using electron spin resonance spectroscopy. In DMPC membranes, the orientation of the lipid chains is comparable to that in the absence of peptide, in both gel and fluid phases. In gel-phase DMPC, the effective tilt of the peptide exceeds that of the lipid chains, but in the fluid phase both are similar. For gramicidin A in DMPS, DMPG, and DMPE, the degree of orientation of the peptide and lipid chains is less than in DMPC. In the fluid phase of DMPS, DMPG, and DMPE, gramicidin A is also less well oriented than are the lipid chains. In DMPE especially, gramicidin A is largely disordered. In DMPC membranes, three to four lipids per monomer experience direct motional restriction on interaction with gramicidin A. This is approximately half the number of lipids expected to contact the intramembranous perimeter of the gramicidin A monomer. A selectivity for certain negatively charged lipids is found in the interaction with gramicidin A in DMPC. These results are discussed in terms of the integration of gramicidin A channels in lipid bilayers, and of the interactions of lipids with integral membrane proteins.     

Conformational changes of phospholipid headgroups induced by a cationic integral membrane peptide as seen by deuterium magnetic resonance

Deuterium nuclear magnetic resonance (2H NMR) was used to study the interaction of a cationic amphiphilic peptide with pure DMPC membranes and with mixed bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS). The choline and serine headgroups were selectively deuteriated at the alpha and beta positions. The amphiphilic peptide, with 20 leucine residues in the hydrophobic core and two cationic hydrophilic lysine residues at each end, spanned the lipid bilayer. Although 2H NMR experiments using DMPC with perdeuteriated fatty acyl chains showed that the average order parameter of the hydrophobic region was not significantly modified by the incorporation of the amphiphilic peptide, for either DMPC or DMPC/DMPS (5:1) bilayers, large perturbations of the quadrupolar splittings of the choline and serine headgroups were observed. The results obtained with the DMPC headgroup suggest that the incorporation of the cationic peptide in both DMPC and DMPC/DMPS (5:1) bilayers leads to a structural perturbation directly related to the net charge on the membrane surface. The magnitude of the observed effect seems to be similar to those observed previously with other cationic molecules [Seelig, J., MacDonald, P.M., & Scherer, P.G. (1987) Biochemistry 26, 7535-7541]. Two of the three quadrupolar splittings of the PS headgroup exhibited large variations in the presence of the amphiphilic peptide, while the third one remained unchanged. Our data have led us to propose a model describing the influence of membrane surface charges on headgroup conformation. In this model, the surface charge is represented as a uniform charge distribution. The electric field due to the charges produces a torque which rotates the polar headgroups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of myotoxin a with artificial membranes: Raman spectroscopic investigation

Myotoxin a from the venom of Crotalus viridis viridis (prairie rattlesnake) is a small protein which is responsible for myonecrosis. It is a basic protein with 42 amino acid residues of known sequence. Three disulfide bonds give it a highly compact structure. Microscopic examination of the toxin's effects reveals that the most pronounced and earliest visible damage occurs intracellularly, in the sarcoplasmic reticulum membrane system of skeletal muscle. A better understanding of its mechanism of action is therefore of particular interest. The interaction of myotoxin a with artificial membranes (multibilamellar phospholipid dispersions) was investigated by using dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS). Two regions of the Raman spectrum were examined for information: the C-H stretching region between 2800 and 3000 cm-1 and the C-C stretching region between 1000 and 1300 cm-1. The effects of myotoxin a on the thermotropic phase behavior of the artificial membranes were determined. This was done by monitoring three structurally sensitive Raman intensity ratios, I2932/2880, I2880/2850, and I1088/1126. It was found that myotoxin alpha destabilized the ordered structure of the gel phase of phospholipid bilayers. This effect was seen with both DMPC and DMPS. The pretransition of DMPC was perturbed by myotoxin a, while the main gel to liquid-crystal phase transition temperature was decreased. The effect of myotoxin a on the phase behavior of DMPS was found to be pH dependent with the least effect observed at low pH values. These results suggest the involvement of negatively charged phosphate groups of phospholipids in the interaction of myotoxin a with artificial membranes.     

Comparative study of the interaction of fullerenol nanoparticles with eukaryotic and bacterial model membranes using solid-state NMR and FTIR spectroscopy

Native fullerene is notoriously insoluble in water and forms aggregates toxic to cell membranes, thus limiting its use in nanomedicine. In contrast, water-soluble fullerenol is compatible with biological systems and shows low in vivo toxicity on human cell lines. The interaction mechanism between these hydrophilic nanoparticles and biological membranes is however not well understood. Therefore, in this work, the effect of fullerenol on model eukaryotic and bacterial membranes was investigated using (31)P- and (2)H solid-state NMR as well as FTIR spectroscopy. DPPC/cholesterol and DPPC/DPPG bilayers were used to mimic eukaryotic and bacterial cell membranes, respectively. Our results show low affinity of fullerenol for DPPC/cholesterol bilayers but a clear interaction with model bacterial membranes. A preferential affinity of fullerenol for the anionic phospholipids DPPG in DPPC/DPPG membranes is also observed. Our data suggest that fullerenol remains at the water/bilayer interface of eukaryote-like membranes. They also indicate that the presence of a polar group such as DPPG's hydroxyl moiety at the bilayer surface plays a key role in the interaction of fullerenol with membranes. Hydrogen bonding of fullerenol nanoparticles with DPPGs' OH groups is most likely responsible for inducing lipid segregation in the lipid bilayer. Moreover, the location of the nanoparticles in the polar region of DPPG-rich regions appears to disturb the acyl chain packing and increase the membrane fluidity. The preferential interaction of fullerenol with lipids mostly found in bacterial membranes is of great interest for the design of new antibiotics.     

2H and 31P NMR study of pentalysine interaction with headgroup deuterated phosphatidylcholine and phosphatidylserine

The interaction of cationic pentalysine with phospholipid membranes was studied by using phosphorus and deuterium Nuclear Magnetic Resonance (NMR) of headgroup deuterated dimyristoyl phosphatidylcholine (DMPC) and dimyristoyl phosphatidylserine (DMPS). In the absence of pentalysine, some of the deuterium and phosphorus spectra of DMPC/DMPS 5:1 (m:m) membranes gave lineshapes similar to those of partially-oriented bilayers with the planes of the bilayers being parallel to the magnetic field. The deuterium NMR data show that the quadrupolar splittings of the deuterated methylenes of the DMPC headgroup are not affected by adsorption of pentalysine on the PC/PS membranes. By contrast, the pentalysine produces significant changes in the quadrupolar splittings of the negatively charged DMPS headgroup. The results are discussed in relation to previous ²H NMR investigations of phospholipid headgroup perturbations arising from bilayer interaction with cationic molecules.Abbreviations NMR nuclear magnetic resonance - DMPC 1,2-dimyristoyl-sn-glycero-3-phosphocholine - DMPS 1,2-dimyristoyl-sn-glycero-3-phosphoserine - POPC 1-palmitoyl, 2-oleyl-sn-glycero-3-phosphocholine - POPG 1-palmitoyl-2-oleyl-sn-glycero-3-phosphoglycerol - PC phosphatidylcholine - PS phosphatidyl serine - PG phosphatidylglycerol - HEPES N-(2-hydroxy-ethyl)piperazine-N-2-ethanesulfonic acid - TRIS tris-(hydroxymethyl)aminoethane - EDTA ethylenediamine-tetra-acetic acid     

Effect of Lipid Head Groups on Double-Layered Two-Dimensional Crystals Formed by Aquaporin-0

Aquaporin-0 (AQP0) is a lens-specific water channel that also forms membrane junctions. Reconstitution of AQP0 with dimyristoyl phosphatidylcholine (DMPC) and E. coli polar lipids (EPL) yielded well-ordered, double-layered two-dimensional (2D) crystals that allowed electron crystallographic structure determination of the AQP0-mediated membrane junction. The interacting tetramers in the two crystalline layers are exactly in register, resulting in crystals with p422 symmetry. The high-resolution density maps also allowed modeling of the annular lipids surrounding the tetramers. Comparison of the DMPC and EPL bilayers suggested that the lipid head groups do not play an important role in the interaction of annular lipids with AQP0. We now reconstituted AQP0 with the anionic lipid dimyristoyl phosphatidylglycerol (DMPG), which yielded a mixture of 2D crystals with different symmetries. The different crystal symmetries result from shifts between the two crystalline layers, suggesting that the negatively charged PG head group destabilizes the interaction between the extracellular AQP0 surfaces. Reconstitution of AQP0 with dimyristoyl phosphatidylserine (DMPS), another anionic lipid, yielded crystals that had the usual p422 symmetry, but the crystals showed a pH-dependent tendency to stack through their cytoplasmic surfaces. Finally, AQP0 failed to reconstitute into membranes that were composed of more than 40% dimyristoyl phosphatidic acid (DMPA). Hence, although DMPG, DMPS, and DMPA are all negatively charged lipids, they have very different effects on AQP0 2D crystals, illustrating the importance of the specific lipid head group chemistry beyond its mere charge.     

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.     

A reliable and rapid procedure to estimate drug partitioning in biomembranes

A direct method using derivative spectrophotometry was developed for determining membrane-water molar partition coefficients (Kp) of the anticancer drugs tamoxifen (TAM) and 4-hydroxytamoxifen (OHTAM). This method explores a shift in the absorption spectra of the drugs when removed from the aqueous phase to a hydrophobic environment. Partition of TAM and OHTAM depends on membrane composition and on drug concentration, temperature and presence of cholesterol. Unlike OHTAM, partition of TAM in DMPC bilayers, liposomes of sarcoplasmic reticulum (SR) lipids and native membranes of SR and mitochondria decreases linearly with drug concentration. Additionally, the partition of these drugs is higher in SR native membranes than in liposomes of SR lipids. The partition also depends on membrane type, being higher in mitochondria than in SR membranes. Maximal partitionings in DMPC are observed at temperatures in the range of the main phase transition. Cholesterol strongly affects the incorporation of drugs and maximal inhibition was observed in DMPC bilayers.     

Quinidine is a strong perturber of acidic phospholipid bilayer order and fluidity

The effect of an antiarrhythmic drug, quinidine, on the organization of model phospholipid membranes was studied by the spin-labeling technique. Quinidine strongly perturbs the molecular organization of lipid bilayers prepared from acidic phospholipids (phosphatidylserine, phosphatidic acid) and has only a slight effect on neutral phosphatidylcholine membranes. The interaction of the drug with acidic phospholipids manifests itself in a pronounced increase in the order parameter of the region close to the polar surface of the bilayer and in some decrease in its inner hydrocarbon core fluidity. It is suggested that the perturbation in the organization of membrane lipids may contribute to the mechanisms by which quinidine exerts its pharmacological effects.     

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.     

The membrane lipid environment modulates drug interactions with the P-glycoprotein multidrug transporter.

The P-glycoprotein multidrug transporter functions as an ATP-driven efflux pump for a large number of structurally unrelated hydrophobic compounds. Substrates are believed to gain access to the transporter after partitioning into the membrane, rather than from the extracellular aqueous phase. The binding of drug substrates to P-glycoprotein may thus be modulated by the properties of the lipid bilayer. The interactions with P-glycoprotein of two drugs (vinblastine and daunorubicin) and a chemosensitizer (verapamil) were characterized by quenching of purified fluorescently labeled protein in the presence of various phospholipids. Biphasic quench curves were observed for vinblastine and verapamil, suggesting that more than one molecule of these compounds may bind to the transporter simultaneously. All three drugs bound to P-glycoprotein with substantially higher affinity in egg phosphatidylcholine (PC), compared to brain phosphatidylserine (PS) and egg phosphatidylethanolamine (PE). The nature of the lipid acyl chains also modulated binding, with affinity decreasing in the order egg PC > dimyristoyl-PC (DMPC) > dipalmitoyl-PC (DPPC). Following reconstitution of the transporter into DMPC, all three compounds bound to P-glycoprotein with 2-4-fold higher affinity in gel phase lipid relative to liquid-crystalline phase lipid. The P-glycoprotein ATPase stimulation/inhibition profiles for the drugs were also altered in different lipids, in a manner consistent with the observed changes in binding affinity. The ability of the drugs to partition into bilayers of phosphatidylcholines was determined. All of the drugs partitioned much better into egg PC relative to DMPC and DPPC. The binding affinity increased (i.e., the value of Kd decreased) as the drug-lipid partition coefficient increased, supporting the proposal that the effective concentration of the drug substrate in the membrane is important for interaction with the transporter. These results provide support for the vacuum cleaner model of P-glycoprotein action.     

Differential interactions of a biological photosensitizer with liposome membranes having varying surface charges

The present work demonstrates the interaction of promising cancer cell photosensitizer, harmane (HM), with liposome membranes of varying surface charges, dimyristoyl-l-α-phosphatidylcholine (DMPC) and dimyristoyl-l-α-phosphatidylglycerol (DMPG). Electrostatic interaction of the cationic probe (HM) with the surface charges of the lipids is responsible for differential modulation of the spectral properties of the drug in different lipid environments. Estimation of partition coefficient (K(p) (±10%) = 5.58 × 10(4) in DMPC and 3.28 × 10(5) in DMPG) of HM between aqueous buffer and lipid phases reflect strong binding interaction of the drug with both the lipids. Evidence for greater degree of partitioning of HM into DMPG membrane compared to DMPC membrane has been deduced and further substantiated from experimental studies such as steady-state fluorescence anisotropy, micropolarity determination. The molecular modeling investigation by docking simulation coupled with fluorescence quenching experiment has been exploited to substantiate the location of drug at the lipid head-group region. Modulation of the dynamical properties of the drug within the lipid environments has also been addressed. Rotational relaxation dynamics studies unravel the impartation of a significant degree of motional restriction on the probe molecule within the lipids and reinforce the differential interactions of HM with the two lipid systems along the lines of other findings. Fluorescence kinetics studies reveal a faster association (in terms of apparent rate constants describing the process of interaction) of the drug with DMPG membrane compared to DMPC. This result is argued in connection with the electrostatic interaction between the drug and the liposome surface charges.     

AFM study of the interaction of cytochrome P450 2C9 with phospholipid bilayers

Cytochromes P450 (CYP) are key enzymes involved in the metabolism of drugs and other lipophilic xenobiotics and endogenous compounds. In this study, atomic force microscopy was applied to characterise the association of CYP2C9 to dimyristoylphosphatidylcholine (DMPC) supported phospholipid bilayers. CYP2C9 was found to exclusively localise in the gel domains of partially melted DMPC bilayers. Despite lacking the N-terminus transmembrane spanning domain, the CYP2C9 protein appeared to partially embed into the membrane bilayer, as evidenced by an increase in melting temperature of surrounding phospholipids. Reversible binding of CYP2C9 via an engineered His tag to a phospholipid bilayer was facilitated using nickel-chelating lipids, presenting potential applications for biosensor technologies.     

Interaction of furosemide with lipid membranes

Summary The interaction of furosemide with different phospholipids was investigated. Its influence on the lipid structure was inferred from its effect on the phase transition properties of lipids and on the conductance of planar bilayer membranes. The thermotropic properties of dipalmitoyl phosphatidylcholine, phosphatidylethanolamine (natural), dipalmitoyl phosphatidylethanolamine, brain sphingomyelin, brain cerebrosides and phosphatidylserine in the presence and absence of furosemide were investigated by differential scanning calorimetry,. The modifying effect of furosemide seems to be strongest on phosphatidylethanolamine (natural) and sphingomyelin bilayers. The propensity of furosemide to decrease the electrical resistance of planar lipid membranes was also studied and it is shown that the drug facilitates the transport of ions. Partition coefficients of furosemide between lipid bilayers and water were measured.Abbreviations DSC differential scanning calorimetry - PLM planar lipid membranes - DPPC dipalmitoyl phosphatidylcholine - DMPC dimyristoyl phosphatidylcholine - PE phosphatidyl ethanol     

Interaction of beta-lactoglobulin with phospholipid bilayers: a molecular level elucidation as revealed by infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy has been used to study, at a molecular level, the interactions between beta-lactoglobulin (BLG), the most abundant globular protein in milk, and some lipids (sphingomyelin, SM; dimyristoylphosphatidylcholine, DMPC; dipalmytoylphosphatidylcholine, DPPC; dimyristoylphosphatidylserine-sodium salt, DMPS; dipalmitoylphosphatidylserine-sodium salt, DPPS) constituting the milk fat globule membrane (MFGM). The interactions were monitored with respect to alteration in the secondary structure of BLG, as registered by the amide I' band, and phospholipid conformation, as revealed by the acyl chain and carbonyl bands. The results show that neither the conformation nor the thermotropism of neutral bilayers containing DMPC or DPPC is affected by BLG. Reciprocally, the secondary structure and thermal behaviour of pure BLG remain the same in the presence of PC. These results suggest that no interaction occurs between PC and BLG, in agreement with previous studies. However, it is found that BLG interacts with neutral bilayers constituted by milk SM lipids, increasing gauche conformers and thus conformational disorder of the lipid acyl chains. This perturbing effect has been attributed to a partial penetration of BLG into the hydrophobic core of the bilayer, which allows hydrophobic interactions between BLG and SM. Moreover, the fact that SM possesses the same headgroup of PC implies that the head group does not prevent the occurrence of BLG-lipid interactions and other lipid regions can control the binding of BLG to lipids. Furthermore, BLG was found to interact electrostatically with charged bilayers containing PS, leading to a rigidification of the lipid hydrocarbon chains and a dehydration of the interfacial region. This last effect suggests that the protein limits the accessibility of water molecules to the interfacial region of the phospholipids by its presence at the membrane surface.     

The membrane-activity of Ibuprofen, Diclofenac, and Naproxen: A physico-chemical study with lecithin phospholipids

Nonsteroidal anti-inflammatory drugs (NSAIDs) represent non-specific inhibitors of the cycloxygenase pathway of inflammation, and therefore an understanding of the interaction process of the drugs with membrane phospholipids is of high relevance. We have studied the interaction of the NSAIDs with phospholipid membranes made from dimyristoylphosphatidylcholine (DMPC) by applying Fourier-transform infrared spectroscopy (FTIR), Förster resonance energy transfer spectroscopy (FRET), differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC). FTIR data obtained via attenuated total reflectance (ATR) show that the interaction between DMPC and NSAIDs is limited to a strong interaction of the drugs with the phosphate region of the lipid head group. The FTIR transmission data furthermore are indicative of a strong effect of the drugs on the hydrocarbon chains inducing a reduction of the chain-chain interactions, i.e., a fluidization effect. Parallel to this, from the DSC data beside the decrease of T_m a reduction of the peak height of the melting endotherm connected with its broadening is observed, but leaving the overall phase transition enthalpy constant. Additionally, phase separation is observed, inducing the formation of a NSAID-rich and a NSAID-poor phase. This is especially pronounced for Diclofenac. Despite the strong influence of the drugs on the acyl chain moiety, FRET data do not reveal any evidence for drug incorporation into the lipid matrix, and ITC measurements performed do not exhibit any heat production due to drug binding. This implies that the interaction process is governed by only entropic reactions at the lipid/water interface.     

Selectivity in rhodopsin-phospholipid interactions

This series of experiments systematically evaluated the effect of phospholipid headgroup structure on the interaction between rhodopsin and phospholipids. Two types of experiments were reported. First, ESR experiments involving spin-labeled phosphatidylserine, phosphatidic acid, and phosphatidylcholine demonstrated that, in the fluid-isotropic phase of dimyristoylphosphatidylcholine (DMPC)-rhodopsin membranes, the relative order of rhodopsin-induced immobilization was phosphatidic acid greater than phosphatidylcholine greater than phosphatidylserine. Second, the effect of rhodopsin incorporation on the dimyristoylphosphatidylserine (DMPS) gel to liquid-crystalline phase transition was analyzed with ESR techniques. A partial, binary phase diagram for the DMPS-rhodopsin system at pH 7.0 was constructed by studying the partitioning of Tempo between polar and hydrophobic domains as a function of temperature and system composition. A main result of this analysis was the finding that rhodopsin broadens and reduces the amplitude of the DMPS phase transition to a much smaller extent than it does the DMPC phase transition. When interpreted in terms of theoretical treatments of integral protein-lipid interactions, this indicates that rhodopsin has a lower affinity for DMPS than DMPC.