Penetration and interactions of the antimicrobial peptide, microcin J25, into uncharged phospholipid monolayers

Microcin J25 forms stable monolayers at the air-water interface showing a collapse at a surface pressure of 5 mN/m, 220 mV of surface potential, and 6 fV per squared centimeter of surface potential per unit of molecular surface density. The adsorption of microcin J25 from the subphase at clean interfaces leads to a rise of 10 mN/m in surface pressure and a surface potential of 220 mV. From these data microcin appears to be a poor surfactant per se. Nevertheless, the interaction with the lipid monolayer further increase the stability of the peptide at the interface depending on the mode in which the monolayer is formed. Spreading with egg PC leads to nonideal mixing up to 7 mN/m, with hyperpolarization and expansion of components at the interface, with a small excess free energy of mixing caused by favorable contributions to entropy due to molecular area expansion compensating for the unfavorable enthalpy changes arising from repulsive dipolar interactions. Above 7 mN/m microcin is squeezed out, leaving a film of pure phospholipid. Nevertheless, the presence of lipid at 10 and 20 mN/m stabilize further microcin at the interface and adsorption from the subphase proceeds up to 30 mN/m, equivalent to surface pressure in bilayers.     

Adsorption of amyloid beta (1-40) peptide to phosphatidylethanolamine monolayers.

The aggregation of soluble, nontoxic amyloid beta (Abeta) peptide to beta-sheet containing fibrils is assumed to be a major step in the development of Alzheimer's disease. Interactions of Abeta with neuronal membranes could play a key role in the pathogenesis of the disease. Herein, we study the adsorption of synthetic Abeta peptide to DPPE and DMPE monolayers (dipalmitoyl- and dimyristoylphosphatidylethanolamine). Both lipids exhibit a condensed monolayer state at 20 degrees C and form a similar lattice. However, at low packing densities (at large area per molecule), the length of the acyl chains determines the phase behavior, therefore DPPE is fully condensed whereas DMPE exhibits a liquid-expanded state with a phase transition at approximately 5-6 mNm(-1). Adsorption of Abeta to DPPE and DMPE monolayers at low surface pressure leads to an increase of the surface pressure to approximately 17 mNm(-1). The same was observed during adsorption of the peptide to a pure air-water interface. Grazing incidence X-ray diffraction (GIXD) experiments show no influence of Abeta on the lipid structure. The adsorption kinetics of Abeta to a DMPE monolayer followed by IRRAS (infrared reflection absorption spectroscopy) reveals the phase transition of DMPE molecules from liquid-expanded to condensed states at the same surface pressure as for DMPE on pure water. These facts indicate no specific interactions of the peptide with either lipid. In addition, no adsorption or penetration of the peptide into the lipid monolayers was observed at surface pressures above 30 mNm(-1). IRRAS allows the measurement of the conformation and orientation of the peptide adsorbed to the air-water interface and to a lipid monolayer. In both cases, with lipids at surface pressures below 20 mNm(-1) and at the air-water interface, adsorbed Abeta has a beta-sheet conformation and these beta-sheets are oriented parallel to the interface.     

Aggregation of a peptide antibiotic alamethicin at the air-water interface and its influence on the viscoelasticity of phospholipid monolayers

The aggregation properties of an antibiotic membrane-active peptide alamethicin at the air-water interface have been studied using interfacial rheology and fluorescence microscopy techniques. Fluorescence microscopy of alamethicin monolayers revealed a coexistence of liquid expanded (LE) and solid phases at the surface concentrations studied. Interfacial oscillatory shear measurements on alamethicin monolayers indicate that its viscoelastic properties are determined by the area fraction of the solid domains. The role of zwitterionic phospholipids dioleoylphosphatidyl choline (DOPC) and dioleoylphosphatidyl ethanolamine (DOPE) on the peptide aggregation behavior was also investigated. Fluorescence microscopy of alamethicin/phospholipid monolayers revealed an intermediate phase (I) in addition to the solid and LE phase. In mixed monolayers of phospholipid (L)/alamethicin (P), with increase in L/P, the monolayer transforms from a viscoelastic to a viscous fluid with the increase in area fraction of the intermediate phase. Further, a homogeneous mixing of alamethicin/lipid molecules is observed at L/P > 4. Our studies also confirm that the viscoelasticity of alamethicin/phospholipid monolayers is closely related to the alamethicin/phospholipid interactions at the air-water interface.     

Reorganization of lipid nanocapsules at air-water interface. I. Kinetics of surface film formation

The size, the electrical properties and the behaviour at air-water interface of lipid nanocapsules (LNC) with various compositions were investigated. Two populations of LNC are presented in the suspension after the preparation: with (LNC II) and without (LNC I) phospholipid molecules. After the spreading at air-water interface, a rapid disaggregation of LNC I, located in the vicinity of interface, occurs leading to formation of surface film. The phospholipid molecules stabilize the structure of nanocapsules and LNC II are more stable at the interface in comparison with LNC I. The formation of a surface film was followed after by measuring the evolution of the surface pressure, relative surface area change and surface potential. A kinetic approach describing the various processes during the surface film formation was proposed. The corresponding kinetic constants were estimated.     

Dynamic Behaviors and Morphology Change of Anionic Phospholipid DPPG Monolayer Caused by Bovine Serum Albumin at Air-Water Interface

The interaction between bovine serum albumin (BSA) and the anionic 1.2-dipalmitoyl-snglycero- 3-(phospho-rac-(1-glycerol)) (sodium salt) (DPPG) phospholipid at different subphase pH values was investigated at air-water interface through surface pressure measurements and atomic force microscopy (AFM) observation. By analyzing surface pressure-mean molecular area (π-A) isotherms, the limiting molecular area in the closed packing state-the concentration of BSA (A_lim-[BSA]) curves, the compressibility coefficient-surface pressure (C_S^-1-π) curves and the difference value of mean molecular area-the concentration of BSA (ΔA-[BSA]) curves, we obtained that the mean molecular area of DPPG monolayer became much larger when the concentration of BSA in the subphase increased at pH=3 and 5. But the isotherms had no significant change at different amount of BSA at pH=10. In addition, the amount of BSA molecules adsorbed onto the lipid monolayer reached a threshold value when [BSA]>5×10^-8 mol/L for all pHs. From the surface pressure-time (π-t) data, we obtained that desorption and adsorption processes occurred at pH=3, however, there was only desorption process occurring at pH=5 and 10. These results showed that the interaction mechanism between DPPG and BSA molecules was affected by the pH of subphase. BSA molecules were adsorbed onto the DPPG monolayers mainly through the hydrophobic interaction at pH=3 and 5, and the strength of hydrophobic interaction at pH=3 was stronger than the case of pH=5. At pH=10, a weaker hydrophobic interaction and a stronger electrostatic repulsion existed between DPPG and BSA molecules. AFM images revealed that the pH of subphase and [BSA] could affect the morphology features of the monolayers, which was consistent with these curves. The study provides an important experimental basis and theoretical support to understand the interaction between lipid and BSA at the air-water interface.     

Penetration behaviour of alkylbetainate chlorides into lipid monolayers

In this paper, the penetration behaviour of the alkylbetainate chloride surfactants (C(n)BC, n=10-16) into lipid monolayers of dipalmitoylphosphatidylserine (DPPS), dipalmitoylphosphatidic acid (DPPA), dipalmitoylphosphatidylethanolamine (DPPE), palmitoyoleoylphosphatidylcholine (POPC) and cholesterol (CHOL) is investigated using the Langmuir trough technique. The penetration of C(n)BC is followed by measurement of the surface pressure increase (Δπ) at a constant surface area after the injection of C(n)BC into the aqueous phase, underneath the lipid monolayer previously spread at the air-water interface at 25°C and at different initial surface pressures (π(i)). The influence of both the lipid head group and the surfactant hydrocarbon chain length on the effectiveness of C(n)BC penetration into these monolayers is discussed. The results have shown that C(n)BC adsorb at the air-water interface giving evidence of their surface-active properties. The adsorption kinetics of C16BC into different lipid monolayers are lipid head charge and lipid head volume-dependent. The magnitude of the surface pressure increase (Δπ) arises in the following order: DPPA>DPPS?CHOL≈DPPE>POPC. C(n)BC penetration into negatively-charged (DPPS and DPPA) monolayers does not seem to depend on surfactant alkyl-chain length compared to uncharged (CHOL) and zwitterionic (DPPE and POPC) monolayers for which Δπ increases with a larger alkyl-chain length. Electrostatic interactions are mainly involved in the affinity of C(n)BC with monolayers but the hydrophobic effect plays also a role.     

Modeling the influence of adsorbed DNA on the lateral pressure and tilt transition of a zwitterionic lipid monolayer

Certain lipid monolayers at the air-water interface undergo a second-order transition from a tilted to an untilted liquid-crystalline state of their lipid hydrocarbon chains at sufficiently large lateral pressure. Recent experimental observations demonstrate that in the presence of divalent cations DNA adsorbs onto a zwitterionic lipid monolayer and decreases the tilt transition pressure. Lowering of the tilt transition pressure indicates that the DNA condenses the lipid monolayer laterally. To rationalize this finding we analyze a theoretical model that combines a phenomenological Landau approach with an extension of the Poisson-Boltzmann model to zwitterionic lipids. Based on numerical calculations of the mean-field electrostatic free energy of a zwitterionic lipid monolayer-DNA complex in the presence of divalent cations, we analyze the thermodynamic equilibrium of DNA adsorption. We find that adsorbed DNA induces a 10% reduction of the electrostatic contribution to the lateral pressure exerted by the monolayer. This result implies a small but notable decrease in the tilt transition pressure. Additional mechanisms due to ion-ion correlations and headgroup reorientations are likely to further enhance this decrease.     

Interaction of small amounts of bovine serum albumin with phospholipid monolayers investigated by surface pressure and atomic force microscopy

The influence of small amounts of bovine serum albumin (BSA) (nM concentration) on the lateral organization of phospholipid monolayers at the air-water interface and transferred onto solid substrates as one-layer Langmuir-Blodgett (LB) films was investigated. The kinetics of adsorption of BSA onto the phospholipid monolayers was monitored with surface pressure isotherms in a Langmuir trough, for the zwitterionic dipalmitoylphosphatidyl ethanolamine (N,N-dimethyl-PE) and the anionic dimyristoylphosphatidic acid (DMPA). A monolayer of N,N-dimethyl-PE or DMPA incorporating BSA was transferred onto a solid substrate using the Langmuir-Blodgett technique. Atomic force microscopy (AFM) images of one-layer LB films displayed protein-phospholipid domains, whose morphology was characterized using dynamic scaling theories to calculate roughness exponents. For DMPA-BSA films the surface is characteristic of self-affine fractals, which may be described with the Kardar-Parisi-Zhang (KPZ) equation. On the other hand, for N,N-dimethyl-PE-BSA films, the results indicate a relatively flat surface within the globule. The height profile and the number and size of globules varied with the type of phospholipid. The overall results, from kinetics of adsorption on Langmuir monolayers and surface morphology in LB films, could be interpreted in terms of the higher affinity of BSA to the anionic DMPA than to the zwitterionic N,N-dimethyl-PE. Furthermore, the effects from such small amounts of BSA in the monolayer point to a cooperative response of DMPA and N,N-dimethyl-PE monolayers to the protein.     

Formation of complex Langmuir and Langmuir-Blodgett films of water soluble rosebengal

This communication reports the formation of complex Langmuir monolayer at the air-water interface by charge transfer types of interaction with the water soluble N-cetyl N,N,N-trimethyl ammonium bromide (CTAB) molecules doped with rosebengal (RB), with the stearic acid (SA) molecules of a preformed SA Langmuir monolayer. The reaction kinetics of the formation of RB-CTAB-SA complex monolayer was monitored by observing the increase in surface pressure with time while the barrier was kept fixed. Completion of interaction kinetics was confirmed by FTIR study. This complex Langmuir films at the air-water interface was transferred onto solid substrates at a desired surface pressure to form multilayered Langmuir-Blodgett films. Spectroscopic characterizations reveal some molecular level interactions as well as formation of microcrystalline aggregates depending upon the molar ratios of CTAB and RB within the complex LB films. Presence of two types of species in the complex LB films was confirmed by fluorescence spectroscopy.     

Hybridization of DNA at the surface of phospholipid monolayers. Effect of orientation of oligonucleotide chains

We studied the properties of lipid monolayers formed at the air-water interface composed of dioleoylphosphatidylcholine (DOPC) with incorporated short (19-mer) oligonucleotides. These oligonucleotides were modified by oleylamine at both (3' and 5') terminals or only at one (3') terminal. Interaction of single-stranded (19-mer) oligonucleotides without oleylamine with DOPC monolayers resulted only in slight increase of surface pressure and the area per phospholipid molecule, while more substantial and significant increase of these values were observed following incorporation of oligonucelotides modified by oleylamine. This influence is similar for both types of oligonucleotide modifications. However, considerable differences in changes of monolayer properties took place after hybridization with complementary oligonucleotides. The hybridization of oligonucleotides with the DNA modified by oleic acid at both 3' and 5' terminals at the surface of lipid monolayer resulted in further increase of surface pressure and in the increase of the area per phospholipid molecule, while decrease of both the surface pressure and the area per phospholipid molecules were observed for hybridization with DNA modified by oleic acid at 3' terminal. It is possible that in latter case, the hybridization caused the loss of hybridized molecules from monolayers. Interaction of noncomplementary chains with DOPC monolayers with incorporated oleyl acid-modified DNA also influenced the properties of monolayers, but the effect was weaker in comparison with that observed for complementary chains.     

Surface behavior and peptide-lipid interactions of the cyclic neuropeptide melanin concentrating hormone

The kinetics and the thermodynamics of melanin concentrating hormone (MCH) adsorption, penetration, and mixing with membrane components are reported. MCH behaved as a surface active peptide, forming stable monolayers at a lipid-free air-water interface, with an equilibrium spreading pressure, a collapse pressure, and a minimal molecular area of 11 mN/m, 13 mN/m, and 140 A (2), respectively. Additional peptide interfacial stabilization was achieved in the presence of lipids, as evidenced by the expansion observed at pi > pi sp in monolayers containing premixtures of MCH with zwitterionic or charged lipids. The MCH-monolayer association and dissociation rate constants were 9.52 x 10 (-4) microM (-1) min (-1) and 8.83 x 10 (-4) min (-1), respectively. The binding of MCH to the dpPC-water interface had a K d = 930 nM at 10 mN/m. MCH penetration in lipid monolayers occurred even up to pi cutoff = 29-32 mN/m. The interaction stability, binding orientation, and miscibility of MCH in monolayers depended on the lipid type, the MCH molar fraction in the mixture, and the molecular packing of the monolayer. This predicted its heterogeneous distribution between different self-separated membrane domains. Our results demonstrated the ability of MCH to incorporate itself into biomembranes and supports the possibility that MCH affects the activity of mechanosensitive membrane proteins through mechanisms unrelated with binding to specific receptors.     

The study of the interaction of a model alpha-helical peptide with lipid bilayers and monolayers

We studied the interaction of the alpha-helical peptide acetyl-Lys(2)-Leu(24)-Lys(2)-amide (L(24)) with tethered bilayer lipid membranes (tBLM) and lipid monolayers formed at an air-water interface. The interaction of L(24) with tBLM resulted in adsorption of the peptide to the surface of the bilayer, characterized by a binding constant K(c)=2.4+/-0.6 microM(-1). The peptide L(24) an induced decrease of the elasticity modulus of the tBLM in a direction perpendicular to the membrane surface, E(radial). The decrease of E(radial) with increasing peptide concentration can be connected with a disordering effect of the peptide to the tBLM structure. The pure peptide formed a stable monolayer at the air/water interface. The pressure-area isotherms were characterized by a transition of the peptide monolayer, which probably corresponds of the partial intercalation of the alpha-helixes at higher surface pressure. Interaction of the peptide molecules with lipid monolayers resulted in an increase of the mean molecular area of phospholipids both in the gel and liquid crystalline states. With increasing peptide concentration, the temperature of the phase transition of the monolayer shifted toward lower temperatures. The analysis showed that the peptide-lipid monolayer is not an ideally miscible system and that the peptide molecules form aggregates in the monolayer.     

Comparative study of lipolysis by PLA2 of DOPC substrates organized as monolayers, bilayer vesicles and nanocapsules

The water-soluble lipolytic enzymes act at the interface of insoluble lipid substrates, where the catalytical step is coupled with various interfacial phenomena as enzyme penetration, solubilization of reaction products, loss of mechanical stability of organized assemblies of phospholipids molecule, etc. One biologically relevant example is the enzymatic hydrolysis of DOPC by PLA(2), which results in cleavage of phospholipids molecules into water insoluble lipolytic products, namely oleic acid and lysophospholipid. In general, the enzymatic activity depends on the substrate organization and molecular environment of the catalytic reaction. The lipolysis by phospholipase A(2) of dioleoylphosphatidylcholine substrates organized as monolayer, bilayers vesicles and lipid nanocapsules was studied by measuring the decrease of the surface area at constant surface pressure or increase of the surface pressure at constant area at air-water interface. A kinetic model describing the coupling of the catalytic act with corresponding interfacial phenomena was developed. By using the kinetic model the values for the global hydrolytic kinetic constants were obtained. The obtained value for the monolayer is five orders of magnitude higher than this obtained with small unilamellar vesicles and six orders of magnitude higher then those obtained with lipid nanocapsules. The comparison shows that the enzymatic catalytic act occurring in the lipid environment of the monolayer is more efficacious than at the vesicle and nanocapsules interfaces.     

Effects of lipid chain length on molecular interactions between paclitaxel and phospholipid within model biomembranes

Molecular interactions between an anticancer drug, paclitaxel, and phosphatidylcholine (PC) of various chain lengths were investigated in the present work by the Langmuir film balance technique and differential scanning calorimetry (DSC). Both the lipid monolayer at the air-water interface and lipid bilayer vesicles (liposomes) were employed as model biological cell membranes. Measurement and analysis of the surface pressure versus molecular area curves of the mixed monolayers of phospholipids and paclitaxel under various molar ratio showed that phospholipids and paclitaxel formed a nonideal miscible system at the interface. Paclitaxel exerted an area-condensing effect on the lipid monolayer at small molecular surface areas and an area-expanding effect at large molecular areas, which could be explained by the intermolecular forces and geometric accommodation between the two components. Paclitaxel and phospholipids could form thermodynamically stable monolayer systems: the stability increased with the chain length in the order DMPC (C14:0)>DPPC (C16:0)>DSPC (C18:0). Investigation of paclitaxel penetration into the pure lipid monolayer showed that DMPC had a higher ability to incorporate paclitaxel and the critical surface pressure for paclitaxel penetration also increased with the chain length in the order DMPC>DPPC>DSPC. A similar trend was testified by DSC studies on vesicles of the mixed paclitaxel/phospholipids bilayer. Paclitaxel showed the greatest interaction with DMPC while little interaction could be measured in the paclitaxel/DSPC liposomes. Paclitaxel caused broadening of the main phase transition without significant change at the peak melting temperature of the phospholipid bilayers, which demonstrated that paclitaxel was localized in the outer hydrophobic cooperative zone of the bilayer. The interaction between paclitaxel and phospholipid was nonspecific and the dominant factor in this interaction was the van der Waals force or hydrophobic force. As the result of the lower net van der Waals interaction between hydrocarbon chains for the shorter acyl chains, paclitaxel interacted more readily with phospholipids of shorter chain length, which also increased the bilayer intermolecular spacing.     

Effects of cholesterol component on molecular interactions between paclitaxel and phospholipid within the lipid monolayer at the air-water interface

Cholesterol is a main component of the cell membrane and could have significant effects on drug-cell membrane interactions and thus the therapeutic efficacy of the drug. It also plays an important role in liposomal formulation of drugs for controlled and targeted delivery. In this research, Langmuir film technique, atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) are employed for a systematic investigation on the effects of cholesterol component on the molecular interactions between a prototype antineoplastic drug (paclitaxel) and 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) within the cell membrane by using the lipid monolayer at the air-water interface as a model of the lipid bilayer membrane and the biological cell membrane. Analysis of the measured surface pressure (pi) versus molecular area (a) isotherms of the mixed DPPC/paclitaxel/cholesterol monolayers at various molar ratios shows that DPPC, paclitaxel and cholesterol can form a non-ideal miscible system at the air-water interface. Cholesterol enhances the intermolecular forces between paclitaxel and DPPC, produces an area-condensing effect and thus makes the mixed monolayer more stable. Investigation of paclitaxel penetration into the mixed DPPC/cholesterol monolayer shows that the existence of cholesterol in the DPPC monolayer can considerably restrict the drug penetration into the monolayer, which may have clinical significance for diseases of high cholesterol. FTIR and AFM investigation on the mixed monolayer deposited on solid surface confirmed the obtained results.     

Mitochondrial creatine kinase adsorption to biomimetic membranes: a Langmuir monolayer study

Interaction of mitochondrial creatine kinase (mtCK) with either synthetic or natural zwitterionic or acidic phospholipids was monitored by surface pressure measurements. Injection of mtCK beneath a monolayer at very low surface pressure results in a large increase in the apparent area per lipid molecule reflecting the intrinsic surface activity of the protein. This effect is particularly pronounced with anionic phospholipid-containing films. Upon compression to high lateral pressure, the protein is squeezed out of the lipid monolayer. On the contrary, mtCK injected beneath a monolayer compressed at 30 mN/m, does not insert into the monolayer but is concentrated below the surface by anionic phospholipids as evidenced by the immediate and strong increase in the apparent molecular area occurring upon decompression. Below 8 mN/m the protein adsorbs to the interface and remains intercalated until the lateral pressure increases again. The critical pressure of insertion is higher for anionic lipid-containing monolayers than for films containing only zwitterionic phospholipids. In the former case it is markedly diminished by NaCl. The adsorption of mtCK depends on the percentage of negative charges carried by the monolayer and is reduced by increasing NaCl concentrations. However, the residual interaction existing in the absence of a global negative charge on the membrane may indicate that this interaction also involves a hydrophobic component.     

Study of adsorption and penetration of E2(279-298) peptide into Langmuir phospholipid monolayers

The peptide corresponding to the sequence (279-298) of the Hepatitis G virus (HGV/GBV-C) E2 protein was synthesized, and surface activity measurements, pi-A compression isotherms, and penetration of E2(279-298) into phospholipid monolayers spread at the air-water interface were carried out on water and phosphate buffer subphases. The results obtained indicated that the pure E2(279-298) Langmuir monolayer exhibited a looser packing on saline-buffered than on pure water subphase and suggest that the increase in subphase ionic strength stabilizes the peptide monolayer. To better understand the topography of the monolayer, Brewster angle microscopy (BAM) images of pure peptide monolayers were obtained. Penetration of the peptide into the pure lipid monolayers of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) and into mixtures of dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DMPC/DMPG) at various initial surface pressures was investigated to determine the ability of these lipid monolayers to host the peptide. The higher penetration of peptide into phospholipids is attained when the monolayers are in the liquid expanded state, and the greater interaction is observed with DMPC. Furthermore, the penetration of the peptide dissolved in the subphase into these various lipid monolayers was investigated to understand the interactions between the peptide and the lipid at the air-water interface. The results obtained showed that the lipid acyl chain length is an important parameter to be taken into consideration in the study of peptide-lipid interactions.     

Effect of gramicidin on phospholipid-modified monolayers and on ion transfer at a liquid-liquid interface.

The interaction of hybrid lipid/gramicidin A (gA) monolayers with dextran sulfate (DS) and the effect of this interaction on ion transfer at a liquid-liquid interface is reported. The interfacial and physicochemical properties are studied with Langmuir-Blodgett (LB) and electrochemical techniques. The results obtained from compression isotherms demonstrate that the interactions between the different species in the hybrid monolayer vary according to the chemical nature of the lipid (hydrocarbon region and charge of the head group). Interfacial capacitance measured with AC voltammetry indicates that the DS chains form a rather flat and compact layer when adsorbed to either zwitterionic or negatively charged phospholipid monolayers, and that calcium, even at low concentrations, interacts with the monolayers. These results are successfully described by a model based on the solution of the Poisson-Boltzmann equation in the interfacial region. Ion transfer and interactions with the lipid/gA/DS-modified monolayers were also studied with electrochemical techniques. Admittance data show that although the studied ions are not using gA channels for the transfer through the lipid membranes, the incorporation of gA in the lipid domain and the adsorption of DS at the interface have a significant effect on ion transfer across the monolayers. This effect can be explained as a consequence of the modified surface charge and of the compactness of the lipid domain due to its interaction with gA and to calcium and DS adsorption at the interface. The ion-transfer rate, therefore, depends on the composition of the monolayer and the chemical nature of the ion.     

RNA and DNA association to zwitterionic and charged monolayers at the air-liquid interface

The objective of this work is to establish under which conditions short RNA molecules (similar to miRNA) associate with zwitterionic phospholipids and how this differs from the association with cationic surfactants. We study how the base pairing (i.e., single stranded versus double stranded nucleic acids) and the length of the nucleic acid and the charge of the lipid/surfactant monolayer affect the association behavior. For this purpose, we study the adsorption of nucleic acids to monolayers composed of dipalmitoyl phosphatidylcholine (DPPC) or dioctadecyl-dimethyl-ammoniumbromide (DODAB) using the surface film balance, neutron reflectometry, and fluorescence microscopy. The monolayer studies with the surface film balance suggested that short single-stranded ssRNA associates with liquid expanded zwitterionic phospholipid monolayers, whereas less or no association is detected for double-stranded dsRNA and dsDNA. In order to quantify the interaction and to determine the location of the nucleic acid in the lipid/surfactant monolayer we performed neutron reflectometry measurements. It was shown that ssRNA adsorbs to and penetrates the liquid expanded monolayers, whereas there is no penetration of nucleic acids into the liquid condensed monolayer. No adsorption was detected for dsDNA to zwitterionic monolayers. On the basis of these results, we propose that the association is driven by the hydrophobic interactions between the exposed hydrophobic bases of the ssRNA and the hydrocarbon chains of the phospholipids. The addition of ssRNA also influences domain formation in the DPPC monolayer, leading to fractal-like interconnected domains. The experimental results are discussed in terms of the implication for biological processes and new leads for applications in medicine and biotechnology.     

髓鞘碱性蛋白对细胞膜脂质分子DPPC、DPPE单层膜影响机理研究

本文通过Langmuir单层膜的表面压力-平均分子面积(π-A)曲线的测定与分析,分别对髓鞘碱性蛋白(MBP)与细胞膜中不同头部基团脂质分子二棕榈酰基磷脂胆碱(DPPC)和二棕榈酰基磷脂酰乙醇胺(DPPE)在空气/液体界面上的相互作用过程进行了系统研究.实验结果表明:(1)当界面上脂质含量一定时,亚相中随着MBP浓度的增大,DPPC、DPPE单层膜的等温线向平均分子面积较大的方向移动;(2)在单层膜表面压力为10 mN/m时,一个MBP分子分别结合140±3个DPPC分子和100±3个DPPE分子,随着表面压力增大,当MBP分子分别与两种磷脂分子相互作用时,MBP插入到磷脂单层界面的个数逐渐减少;(3)随着蛋白质浓度的增加,脂分子形成的单层膜变得较为疏松,且MBP分子易于插入到分子头部较小的DPPE单层膜中;(4)蛋白质的存在使DPPC单层膜的表面压力逐渐减小,且蛋白质浓度越大表面压力降低越多,DPPC被MBP带入到亚相中越多;(5)对于DPPE单层膜,蛋白质通过与DPPE相互作用插入到界面膜中,引起表面压力增大,且蛋白质浓度越高,压力变化量越大.