Gold nanoparticle self-assembly in two-component lipid Langmuir monolayers

Self-assembly processes are considered to be fundamental factors in supramolecular chemistry. Langmuir monolayers of surfactants or lipids have been shown to constitute effective 2D "templates" for self-assembled nanoparticles and colloids. Here we show that alkyl-coated gold nanoparticles (Au NPs) adopt distinct configurations when incorporated within Langmuir monolayers comprising two lipid components at different mole ratios. Thermodynamic and microscopy analyses reveal that the organization of the Au NP aggregates is governed by both lipid components. In particular, we show that the configurations of the NP assemblies were significantly affected by the extent of molecular interactions between the two lipid components within the monolayer and the monolayer phases formed by each individual lipid. This study demonstrates that multicomponent Langmuir monolayers significantly modulate the self-assembly properties of embedded Au NPs and that parameters such as the monolayer composition, surface pressure, and temperature significantly affect the 2D nanoparticle organization.     

Incorporation of Blood-Clotting Proteins into Phospholipid Langmuir Monolayers: A Fluorescence Microscopy Study

Phospholipid monolayers adsorbed at an air-water interface are model cell membranes and have been used in this work to study interactions with blood-clotting proteins. Factor I (non-membrane binding) was used as a control protein, and its association with L-alpha-dipalmitoylphosphatidylcholine Langmuir monolayers was compared to factor VII, a membrane-binding protein. Fluorescence micrographs indicated that factor I penetration of the lipid monolayers in the phase transition region occurred extensively, causing condensation of the lipid film. The association of factor I with phospholipid monolayers was deemed nonspecific. Factor VII was shown to associate with the periphery of lipid domains in the absence of calcium ions, causing flattening of domain edges. In the presence of calcium, factor VII induced expansion of the lipid monolayer. This effect is a specific interaction attributed to exposure of hydrophobic residues upon calcium binding, followed by protein association with lipid hydrocarbon chains. Copyright 2001 Academic Press.     

Incorporation of blood clotting factor X into phospholipid model membranes: fluorescence microscopy imaging and subphase effects surrounding the lipid phase transition region

Factor X is a blood clotting protein that associates at membrane surfaces to become activated during the coagulation cascade. A molecular level understanding of the protein-membrane phospholipid interactions has not been reached, although it is thought that the protein binds to phospholipids in the presence of calcium through a bridge with the Gla (gamma-carboxyglutamic acid) domain on the protein. In this work, phospholipid Langmuir monolayers have been utilized as model membranes to study factor X association with phospholipid membrane components. Surface pressure measurements indicate that subphase addition of sodium, magnesium, and calcium ions enhances protein penetration of the lipid monolayer, with the largest association found with calcium ions in the subphase. Fluorescence microscopy images collected after protein penetration of lipid monolayers indicate monolayer condensation in the presence of sodium and magnesium ions. Aggregation of lipid domains is induced when calcium is in the subphase, indicating binding-induced flocculation of surface lipid aggregates. Calcium binding to factor X likely causes a conformational change which allows protein-membrane interaction via hydrophobic association with lipid molecules.     

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.     

L-半胱氨酸自组装膜诱导草酸钙结晶研究

在CaCl2.H2O和Na2C2O4配制的过饱和溶液中,利用L-半胱氨酸(L-Cys)在金片上形成的自组装膜为模板,研究了草酸钙(CaOxa)在自组装膜上的结晶行为,并探讨了溶液pH对CaOxa晶体组成、晶型及其形貌的影响。采用X射线衍射(XRD)和扫描电子显微镜(SEM)等技术对CaOxa晶体的结构和形貌进行了表征。实验结果表明:当溶液pH=3.0时,溶液中可以形成一水草酸钙(CaC2O4.H2O,COM)和二水草酸钙(CaC2O4.2H2O,COD)晶体,而在同样pH条件下,在L-Cys自组装膜上只形成COD晶体,表明自组装单层对CaOxa晶体的成核和生长有重要影响。通过改变溶液的pH,在自组装单层上可以得到不同晶型和不同形状的CaOxa晶体。当pH=3.0时得到四方块状的COD晶体,而pH=5.0和pH=7.0时分别得到六边形和拉长六边形的COM晶体。     

Calcium oxalate monohydrate precipitation at membrane lipid rafts

The precipitation of calcium oxalate monohydrate (COM) was monitored at a Langmuir monolayer containing lipid raft domains. The raft-forming monolayer consists of a 2:1:1 mixture of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/sphingomyelin/dihydrocholesterol, where the raft liquid ordered phase is enriched in sphingomyelin and the sterol. COM crystals, monitored by Brewster angle microscopy, appear at the phase boundary between the raft domains and the expanded phase.     

Effect of Na+ and Ca2+ Ions on a Lipid Langmuir Monolayer: An Atomistic Description by Molecular Dynamics Simulations

Studying the effect of alkali and alkaline‐earth metal cations on Langmuir monolayers is relevant from biophysical and nanotechnological points of view. In this work, the effect of Na⁺ and Ca²⁺ on a model of an anionic Langmuir lipid monolayer of dimyristoylphosphatidate (DMPA^?) is studied by molecular dynamics simulations. The influence of the type of cation on lipid structure, lipid–lipid interactions, and lipid ordering is analyzed in terms of electrostatic interactions. It is found that for a lipid monolayer in its solid phase, the effect of the cations on the properties of the lipid monolayer can be neglected. The influence of the cations is enhanced for the lipid monolayer in its gas phase, where sodium ions show a high degree of dehydration compared with calcium ions. This loss of hydration shell is partly compensated by the formation of lipid–ion–lipid bridges. This difference is ascribed to the higher charge‐to‐radius ratio q/r for Ca²⁺, which makes ion dehydration less favorable compared to Na⁺. Owing to the different dehydration behavior of sodium and calcium ions, diminished lipid–lipid coordination, lipid–ion coordination, and lipid ordering are observed for Ca²⁺ compared to Na⁺. Furthermore, for both gas and solid phases of the lipid Langmuir monolayers, lipid conformation and ion dehydration across the lipid/water interface are studied.     

Design,synthesis and characterization of artificial phospholipids as model membrane receptors for specific binding of rabbit C-reactive protein

C-reactive protein (CRP) is an acute phase reactive protein, which has been shown to specifically bind to phosphorylcholine (PC) and phosphorylethanoamine (PE) moieties in the presence of calcium. In order to investigate the effect of steric hindrance on the specific binding of CRP to membranes, we designed and synthesized six phospholipids, each containing a long-arm spacer of 3, 6 or 8 atoms between the head group and hydrophobic tail. By mixing synthesized lipids and natural lipids the ligand-containing monolayers were prepared, which have PC or PE groups protruding out of the membrane surface. To characterize of the synthesized phospholipids, the thickness of the lipid monolayers was measured by surface plasmon resonance (SPR) technique, the phase behavior of the lipid monolayer at air/water interface was studied by pressure–area analysis, and the specific binding of rabbit C-reactive protein to the synthesized lipid containing membranes was studied by imaging ellipsometry.     

Brewster angle microscopy of calcium oxalate monohydrate precipitation at phospholipid monolayer phase boundaries

The precipitation of calcium oxalate monohydrate (COM) at phospholipid monolayers confined to the air/water interface is observed in situ with the aid of Brewster angle microscopy. COM crystals appear as bright objects that are easily identified and quantified to assess the effects of different conditions on crystallization. Crystal precipitation was monitored at monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in liquid condensed (LC) and liquid expanded (LE) phases. Within the LC phase, higher pressures reduce the incidence of crystallization at the interface, implying that within this phase precipitation is enhanced by higher compressibility or fluidity of the monolayer. Precipitation at biphasic LC/LE and LE/gas (G) monolayers was also studied. COM appears preferentially at phase boundaries of the DPPC LC/LE and LE/G monolayers. However, when an LC/LE phase boundary is created by two different phospholipids that are phase segregated, such as DPPC and 1,2-dimyristoyl-sn-glycero-3-phosphocholine, crystal formation occurs away from the interface within the DPPC LC phase. It is suggested that COM growth at phase boundaries is preferred only when there is molecular exchange between the phases.     

Preparation and characterization of asymmetric planar supported bilayers composed of poly(bis-sorbylphosphatidylcholine) on n-octadecyltrichlorosilane SAMs

Planar supported lipid bilayers (PSLBs) have been widely studied as biomembrane models and biosensor scaffolds. For technological applications, a major limitation of PSLBs composed of fluid lipids is that the bilayer structure is readily disrupted when exposed to chemical, mechanical, and thermal stresses. A number of asymmetric supported bilayer structures, such as the hybrid bilayer membrane (HBM) and the tethered bilayer lipid membrane (tBLM), have been created as an alternative to symmetric PSLBs. In both HBMs and tBLMs, the inner monolayer is covalently attached to the substrate while the outer monolayer is typically composed of a fluid lipid. Here we address if cross-linking polymerization of the lipids in the outer monolayer of an asymmetric supported bilayer can achieve the high degree of stability observed previously for symmetric PSLBs in which both monolayers are cross-linked [E.E. Ross, L.J. Rozanski, T. Spratt, S.C. Liu, D.F. O'Brien, S.S. Saavedra, Langmuir 19 (2003) 1752]. To explore this issue, HBMs composed of an outer monolayer of a cross-linkable lipid, bis-sorbylphosphatidylcholine (bis-SorbPC), and an inner SAM were prepared and characterized. Several experimental conditions were varied: vesicle fusion time, polymerization method, and polymerization time and temperature. Under most conditions, bis-SorbPC cross-linking stabilized the HBM such that its bilayer structure was largely preserved after drying; however these films invariably contained sub-micron scale defects that exposed the hydrophobic core of the HBM. The defects appear to be caused by desorption of low molecular weight oligomers when the film is removed from water, rinsed, and dried. In contrast, poly(bis-SorbPC) PSLBs prepared under similar conditions by Ross et al. were nearly defect free. This comparison shows that formation of a cross-linked network in the outer leaflet of an asymmetric supported bilayer is insufficient to prevent lipid desorption; inter-leaflet covalent linking appears to be necessary to create supported poly(lipid) assemblies that are impervious to repeated drying and rehydration. The difference in stability is attributed to inter-leaflet cross-linking between monolayers which can form in symmetric bis-SorbPC PSLBs.     

Interaction of rabbit C-reactive protein with phospholipid monolayers at air/water interface

C-reactive protein (CRP) is a major acute phase reactant in most mammalian species. The structure of CRP has been previously established by crystallography, and the significance of its interaction with lipid membranes is accepted in the literature. However, the nature of the interaction between CRP and phospholipids is not yet well understood. In this paper we use monolayer technique to study the characteristics of the interaction of rabbit C-reactive protein (rCRP) with the phospholipid membranes. The results show that rCRP is surface active and can spontaneously insert into the lipid monolayers. The critical pressure for rCRP inserting into the phospholipid monolayers is about 34.5 mN/m, which is not sensitive to the types of the lipid headgroups and the presence of calcium ions in the subphase. The findings of this paper may provide a clue to the further understanding of the mechanism of the interactions between rCRP and the biological membranes.     

Influence of the environment on photoinduced electron transfer: comparison between organized monolayers at the air-water interface and monolayer assemblies on glass

Photoinduced electron transfer (PET) has been investigated in organized monolayers at the air-water interface and in monolayer assemblies on glass in an effort to evaluate the influence of solvent reorganization and molecular dynamics on PET. The donor monolayer contained an amphiphilic thiacyanine dye, and the electron acceptors were methyl viologen and dioctadecyl viologen, respectively. The distance dependence is described here by a hard disk model, where an acceptor molecule within a disk with a radius rDA around the excited donor molecule quenches the donor fluorescence due to electron transfer. Acceptor molecules outside the disk are considered ineffective. The critical radius rDA is larger in monolayer assemblies on glass (rDA = 1.97 nm) than at the air-water interface (rDA = 1.15 nm) as evaluated from steady-state fluorescence quenching. This large difference indicates that the time between thermal collisions generating and destroying the energetic match required for electron tunneling from the excited donor molecule to the acceptor is quite different in the two systems that are compared.     

Fundamentals and application of ordered molecular assemblies to affinity biosensing

Organization of biomolecules in two/three dimensional assemblies has recently aroused much interest in nanobiotechnology. In this context, the development of techniques for controlling spatial arrangement and orientation of the desired molecules to generate highly-ordered nanostructures in the form of a mono/multi layer is considered highly significant. The studies of monolayer films to date have focused on three distinct methods of preparation: (i) the Langmuir-Blodgett (LB) technique, involving the transfer of a monolayer assembled at the gas-liquid interface; (ii) self-assembly at the liquid-solid interface, based on spontaneous adsorption of desired molecules from a solution directly onto a solid surface; and (iii) Layer-by-layer (LBL) self-assembly at a liquid-solid interface, based on inter-layer electrostatic attractions for fabrication of multilayers. A variety of monolayers have been utilized to fabricate biomolecular electronic devices including biosensors. The composition of a monolayer based matrix has been found to influence the activity(ies) of biomolecule(s). We present comprehensive and critical analysis of ordered molecular assemblies formed by LB and self-assembly with potential applications to affinity biosensing. This critical review on fundamentals and application of ordered molecular assemblies to affinity biosensing is likely to benefit researchers working in this as well as related fields of research (401 references).     

Calcium-induced phospholipid ordering depends on surface pressure

The effect of sodium and calcium ions on zwitterionic and anionic phospholipids monolayers is investigated using vibrational sum-frequency generation in conjunction with surface pressure measurements and fluorescence microscopy. Sodium ions only subtly affect the monolayer structure, while the effect of calcium is large and depends strongly on the surface pressure. At low surface pressures (approximately 5 mN/m), the presence on Ca2+ results in the unexpected appearance of ordered domains. For pressures between approximately 5 and approximately 25 mN/m, Ca2+ ions induce disorder in the monolayer. For pressures exceeding 25 mN/m, calcium cations expand the monolayer, while simultaneously ordering the lipid chains. Interestingly, effects are similar for both zwitterionic lipids and negatively charged lipids. In both vibrational sum-frequency generation and surface tension measurements, the molecular signature of the association of Ca2+ with the lipids is evident from Ca2+-induced changes in the signals corresponding to area changes of 4 A2/lipid-precisely the surface area of a Ca2+ ion, with evidence for a change in lipid Ca2+ complexation at high pressures.     

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.     

Ordering by collapse: formation of bilayer and trilayer crystals by folding Langmuir monolayers

Neutron and synchrotron X-ray studies of arachidic-acid monolayers compressed to the collapse region, beyond their densely packed molecular area, reveal that the resulting structures exhibit a surprising degree of reproducibility and of order. The structure of the collapsed monolayers differs for films that are spread on pure water or on CaCl2 solutions. On pure water, the collapsed monolayer forms a stable crystalline trilayer structure, with acyl-chain in-plane packing practically identical to the three-dimensional (3D) crystal structure of fatty acids. For monolayers spread on Ca2+ solutions, the collapsed film consists of a bi- and trilayer mixture with a ratio that changes by the collapse protocol. Our analysis suggests that the bilayer structure is inverted, i.e., with the hydrophobic tails in contact with the water surface and the calcium ions bridging the polar heads. The inverted bilayer structure possesses a well-ordered crystalline slab of calcium oxalate monohydrate intercalated between two acyl chains. We provide theoretical arguments rationalizing that the observed structures have lower free energies compared with other possible structures and contend that the collapsed structures may, under certain circumstances, form spontaneously.     

Study of the interaction of beta-cyclodextrin with phospholipid monolayers by surface pressure measurements and fluorescence microscopy

The interaction of beta-cyclodextrin (beta-CD) with different lipids has been studied, using Langmuir monolayers kept at constant surface pressure or constant spreading surface. Results show that beta-CD, injected beneath the monolayer, is able to desorb unsaturated palmitoyloleoylphosphatidylcholine (POPC) and sphingomyelin (SM) under specific experimental conditions. In this last case, SM monolayers, labeled with the fluorescent NBD-PC probe, were also observed by fluorescence microscopy, before and after beta-CD injection. Images show that SM monolayers are more homogeneous after beta-CD injection, because of the lipid desorption. At last, it seems that lipid desorption occurs only in a restricted surface pressure range, depending on the lipid.     

Impact of aluminum on the oxidation of lipids and enzymatic lipolysis in monomolecular films at the air/water interface

There is evidence that serious pathologies are associated with aluminum (Al). In the present work, the influence of Al on enzymatic lipolysis was studied with the aim to get more insight into the possible link between the Al-induced membrane modification and the cytotoxicity of the trivalent cation (AlIII). Lipid monolayers were used as model membranes. The monomolecular film technique allowed monitoring the Al-dependent modifications of the lipid monolayer properties and enzyme kinetics. Two enzymes, namely, Candida rugosa lipase and a calcium (CaII)-dependent phospholipase A2 from porcine pancreas, were used to catalyze the lipolysis of triglyceride and phosphoglyceride monolayers, respectively. The results obtained show that Al modifies both the monolayer structure and enzymatic reaction rates. While the enzymes used in this study can be considered as probes detecting lipid membrane properties, it cannot be excluded that in physiological conditions modulation of the enzyme action by the Al-bound membranes is among the reasons for Al toxicity.     

Evaluation of the interactions between lipids and γ-globulin protein at the air–liquid interface

The interactions between lipids (cholesterol, distearoylphosphatidylcholine, distearoylphosphatidylethanolamine and sphingomyelin) and the γ-globulin protein have been analyzed using the monolayer technique at the air–liquid interface. The analysis has been carried out using both state equations and an adequate thermodynamic formulation for the surface pressure (π)–molecular area (a) isotherms. Different parameters as the virial coefficients, have been estimated. For the uncharged lipid monolayers, the interactions between the molecules are of an attractive nature, at medium and long distance, and of a steric repulsive nature at short distance. At low surface pressures the lipid molecules form small domains. The net force between γ-Globulin molecules in the monolayers has been found to be attractive. Finally, it can be concluded that when the lipid monolayers are uncharged, there is practically no interaction between the protein and lipid molecules at the mentioned interface.     

Surface complexation of DNA with insoluble monolayers. Influence of divalent counterions

DNA interacts with insoluble monolayers made of cationic amphiphiles as well as with monolayers of zwitterionic lipids in the presence of divalent ions. Binding to dioctadecyldimethylammonium bromide (DODAB) or distearoyl-sn-glycero-3-phosphocholine (DSPC) monolayers in the presence of calcium is accompanied by monolayer expansion. For the positively charged DODAB monolayer, this causes a decrease of surface potential, while an increase is observed for the DSPC monolayers. Binding to dipalmitoyl-sn-glycero-3-phosphocholine preserves most of the liquid expanded-liquid condensed coexistence region. The liquid condensed domains adopt an elongated morphology in the presence of DNA, especially in the presence of calcium. The interaction of DNA with phospholipid monolayers is ion specific: the presence of calcium leads to a stronger interaction than magnesium and barium. These results were confirmed by bulk complexation studies.