Structural models of lipid surface monolayers from X-ray and neutron reflectivity measurements

Structural investigations of phospholipid monolayers on aqueous subphases on the submolecular level using X-ray and neutron reflectivity measurements are reviewed. While such investigations have been limited in the past by a relatively restricted accessible momentum transfer range, recent developments in synchrotron technology--almost doubling this range--have considerably improved the capabilities of the technique. Until recently, data interpretation has entirely relied on 'box models' which describe the structures as molecularly homogeneous slabs--one hydrophobic and one hydrophilic. It is shown that box models of phospholipid monolayers are rather inadequate to model data at the high momentum transfer available nowadays in X-ray measurements. As an alternative, a hybrid data inversion strategy is proposed that treats the hydrophobic alkane phase as a homogeneous slab and describes the position of submolecular fragments of the lipid headgroups by means of distribution functions along the interface. Within this approach, composition-space refinement--enabling the coupling of data sets from various X-ray and neutron contrasts--in connection with volumetric constraints enables structural characterization of lipid monolayers in unprecedented detail. Extending a recent characterization of dimyristoylphosphatidic acid (DMPA) monolayers on pure water [Schalke et al., Biochim. Biophys. Acta 1464 (2000) 113-126] it is shown that stoichiometric binding of the divalent cations--DMPA-:Cat2+= 2:1--occurs only at exceedingly low areas per molecule, A lipid. At low surface pressure pi, both cations and anions are incorporated into the headgroup in significant amounts, approximately 0.68 Ba2+ and approximately 0.35 Cl- per PA molecule at pi = 2 mN m(-1). They are continuously squeezed out upon compression, until upon approaching Alipid = 41 A2, the stoichiometric ratio between bound cations and acidic headgroups is observed. The average inclination angle alpha of the headgroups as well as their water content is constant along the whole isotherm. The intrinsic contribution to the distribution width--i.e. the spread that is due to a distribution of the fragments within the headgroup without the action of capillary waves--increases with compression up to pi approximately 30 mN m(-1) and drops sharply thereafter in a regime of the isotherm where Alipid approaches its limiting value. The same general picture is observed for DMPA on subphases with 10 mM Ca2+, although the lower electron density of that cation limits the precision of the results.     

Artificial neural networks in chemometrics: History, examples and perspectives

Artificial neural networks (ANNs) are non-linear computational tools suitable to a great host of practical application due to their flexibility and adaptability. However, their application to the resolution of chemometric problems is relatively recent (early 1990s).In this communication, different artificial neural networks architectures are presented and their application to different kinds of chemometric problems (mainly classification and regression) is discussed by means of examples taken from the authors' experience, stressing the pros and cons of ANNs with respect to traditional chemometric techniques.     

PEG-covered lipid surfaces: bilayers and monolayers

In this review paper we survey the ways in which various micropipet techniques have been used to study the mechanochemical and interactive features of lipid bilayer vesicles and monolayer-coated gas bubbles. Special emphasis will be made on characterizing the barrier properties of grafted PEG layers and how a hierarchical approach that uses a short barrier and extended ligand allows us to start to mimic nature's own solution to the problem of ubiquitous repulsion and specific attraction. The information gained from such studies not only characterizes the membrane and other lipid surfaces and their intersurface interactions from a fundamental materials science perspective, but also provides essential materials property data that are required for the successful design and deployment of lipid-based carriers and other capsules in applications involving this so-called ‘stealthy’ surface.     

化学计量学方法在光谱解析中的应用进展

本文对近20年来化学计量学在光谱分析测试中的应用进行了评述。指出了化学计量学在光谱分析中的重要性和应用前景。引用文献共129篇。     

Formation of dimers in lipid monolayers

In this work, we analyse theoretically the hypothesis that zwitterionic lipids form dimers in adsorption monolayers on water/ hydrocarbon phase boundary. A dimer can be modelled as a couple of lipid molecules whose headgroup lateral dipole moments have antiparallel orientation. Properties including surface pressure, chemical potentials and activity coefficients are deduced from a general expression for the free energy of the monolayer. The theoretical model is in a good agreement with experimental data for surface pressure and surface potential of lipid monolayers. The results favour the hypothesis about formation of dimers in equilibrium with monomers, with the amount of the species depending on the area per molecule and temperature. The reaction of dimerisation turns out to be exothermic with a heat of about 2.5kT per dimer. The results may be applied to the molecular models of membrane structures and mechanisms.     

DC photoelectric signals from bacteriorhodopsin adsorbed on lipid monolayers and thiol/lipid bilayers supported by mercury

Purple membrane (PM) fragments were adsorbed on a dioleoylphosphatidylcholine (DOPC) monolayer and on a mixed alkanethiol/DOPC bilayer supported by mercury to investigate the kinetics of light-driven proton transport by bacteriorhodopsin (bR). The light-on and light-off capacitive currents on an alkanethiol/DOPC bilayer at pH 6.4 were interpreted on the basis of a simple equivalent circuit. The pH dependence of the biphasic decay kinetics of the light-on currents was analyzed to estimate the pK(a) values for the transitions releasing protons to, and taking up protons from, the solution. The linear dependence of the stationary light-on current of bR on a DOPC monolayer self-assembled on mercury upon the applied potential was interpreted on the basis of an equivalent circuit.     

Stability of lipid films formed on gamma-aminopropyl monolayers

The stability of supported lipid membranes (SLMs) deposited on planar substrates derivatized with (gamma-aminopropyl)silane (GAPS) was examined. Ellipsometry, fluorescence microscopy, and atomic force microscopy were used to characterize SLMs exposed to repeated drying and rehydration. Vesicle fusion on GAPS-coated substrates produced SLMs with a thickness significantly greater than that of a single lipid bilayer. Exposure to even one cycle of drying/rehydration significantly decreased the thickness of a SLM on GAPS, and repeated drying/rehydration resulted in near quantitative lipid desorption. Thus SLMs on GAPS do not appear to be significantly more stable than the single bilayer SLM that is formed on bare glass or SiO2 under equivalent conditions.     

Analytical problem-solving, reference materials, and multivariate quality control: A chemometrics approach

Summary Breakthroughs in sensor technology have augmented the chemist's measurement repertoire by introducing new kinds of detectors with improved selectivity and the capacity to perform simultaneous multi-species measurements. Thus, the electronic revolution has qualitatively and quantitatively changed the data matrices to which the analyst/problem-solver has access. The new chemical subdiscipline of chemometrics is developing powerful mathematical and statistical data analysis tools to exploit the electronic windfall and enhance data interpretation. Principal component analysis and graphical procedures have been used to examine the multivariate suitability of current reference materials in matching the concentration ranges and matrices for various food analyses. Principal component analysis has been useful in developing and exploring quality control information for the routine analysis laboratory.     

Immobilization of acetylcholinesterase in lipid membranes deposited on self-assembled monolayers

Human red blood cell acetylcholinesterase was incorporated into planar lipid membranes deposited on alkanethiol self-assembled monolayers (SAMs) on gold substrates. Activity of the protein in the membrane was detected with a standard photometric assay and was determined to be similar to the protein in detergent solution or incorporated in lipid vesicles. Monolayer and bilayer lipid membranes were generated by fusing liposomes to hydrophobic and hydrophilic SAMs, respectively. Liposomes were formed by the injection method using the lipid dimyristoylphosphatidylcholine (DMPC). The formation of alkanethiol SAMs and lipid monolayers on SAMs was confirmed by sessile drop goniometry, ellipsometry, and electrochemical impedance spectroscopy. In this work, we report acetylcholinesterase immobilization in lipid membranes deposited on SAMs formed on the gold surface and compare its activity to enzyme in solution.     

Temperature dependence of poloxamer insertion into and squeeze-out from lipid monolayers

F68, a triblock copolymer of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), is found to effectively seal damaged cell membranes. To better understand the molecular interaction between F68 and cells, we have modeled the outer leaflet of a cell membrane with a dipalmitoylphosphatidylcholine (DPPC) monolayer spread at the air-water interface and introduced poloxamer into the subphase. Subsequent interactions of the polymer with the monolayer either upon expansion or compression were monitored using concurrent Langmuir isotherm and fluorescence microscopy measurements. To alter the activity of the poloxamer, a range of subphase temperatures from 5 to 37 degrees C was used. Lower temperatures increase the solubility of the poloxamer in the subphase and therefore lessen the amount of material at the interface, resulting in a lower equilibrium spreading pressure. Additionally, changes in temperature affect the phase behavior of DPPC. Below the triple point, the monolayer is condensed at pertinent polymer insertion pressures; for temperatures immediately above the triple point, the monolayer is a heterogeneous mix of liquid expanded and condensed phase; for the highest temperature measured, the DPPC monolayer remains completely fluid. At all temperatures, F68 inserts into DPPC monolayers at its equilibrium spreading pressure. Upon compression of the monolayer, polymers are squeezed-out at surface pressures notably higher than those for insertion, with higher temperatures leading to a higher squeeze-out pressure. An increase in temperature decreases the solvent quality of water for the poloxamer, lowering solubility of the polymer in the subphase and thus increasing its propensity to be maintained within the monolayer to higher pressures.     

Supported lipid bilayers lifted from the substrate by layer-by-layer polyion cushions on self-assembled monolayers

The formation of lipid bilayers, lifted from the solid substrate by layer-by-layer polyion cushions, on self-assembled monolayers (SAMs) on gold was investigated by surface plasmon resonance (SPR) and fluorescence recovery after photobleaching (FRAP). The polyions poly(diallyldimethylammonium chloride) (PDDA) and polystyrene sulfonate (PSS) sodium salt were used for the layer-by-layer polyion macromolecular assembly. The cushion was formed by electrostatic interaction of PDDA/PSS/PDDA layers with a negatively charged surface of an SAM of 11-mercaptoundecanoic acid (MUA) on gold. The lipid bilayer membranes were deposited by vesicle fusion with different compositions of SOPS (an anionic lipid, 1-stearoyl-2-oleoyl-phosphatidylserine) and POPC (a zwitterionic lipid, 1-palmitoyl-2-oleoylphosphatidylcholine). In the case of pure SOPS and for lipid mixtures with a POPC composition up to 25%, single bilayers were deposited. FRAP experiments showed that single bilayers supported on PDDA/PSS/PDDA/MUA were mobile at room temperature, with lateral coefficients of approximately (1.2–2.1)×10⁻⁹ cm²/s. The kinetics of the addition of the ion-channel-forming peptide protegrin-1 to the supported bilayers was detected by SPR. A two-step interaction was observed, similar to the association behavior of protegrin-1 with bilayers supported on PDDA/MUA. The results are similar to that of supported lipid bilayers without a layer-by-layer cushion. The model membrane system in this work is a potential biosensor for mimicking the natural activities of biomolecules and is a possible tool to investigate the fundamental properties of biomembranes.     

Supported bilayer lipid membrane arrays on photopatterned self-assembled monolayers

This work demonstrates the use of photocleavable cholesterol derivatives to create supported bilayer lipid membrane arrays on silica. The photocleavable cholesteryl tether is attached to the surface by using the reaction of an amine-functionalized self-assembled monolayer (SAM) and the N-hydroxysuccinimide-based reagent 9. The resultant SAM contains an ortho-nitrobenzyl residue that can be cleaved by photolysis by using soft (365 nm) UV light regenerating the original amine surface, and which can be patterned using a mask. The photoreaction yield was approximately 75 % which was significantly higher than previously found for related ortho-nitrobenzyl photochemistry on gold substrates. The SAMs were characterized by means of contact angle measurements, ellipsometry and X-ray photoelectron spectroscopy. Patterned surfaces were characterized with SEM and AFM. After immersing the patterned surface into a solution containing small unilamellar vesicles of egg phosphatidylcholine (PC), supported lipid membranes were formed comprised of lipid bilayer over the amine functionalized "hydrophilic" regions and lipid monolayer over the cholesteryl "hydrophobic" regions. This was confirmed by fluorescence microscopy and AFM. FRAP studies yielded a lateral diffusion coefficient for the probe molecule of 0.14+/-0.05 microm(2) s(-1) in the bilayer regions and approximately 0.01 microm(2) s(-1) in the monolayer regions. This order of magnitude difference in diffusion coefficients effectively serves to isolate the bilayer regions from one another, thus creating a bilayer array.     

Acyl chain ordering and crystallization in lipid monolayers

The condensed phases of phospholipid monolayers on an air/water interface are described by means of a microscopic interaction model which incorporates intra-chain flexibility as well as crystal orientation variables. The phase transitions and microstructures are studied as functions of lateral pressure. The model predicts, in accordance with recent synchrotron X-ray experiments, that the chain-ordering transition and the crystallization of the monolayer need not take place at the same lateral pressure.     

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.     

Influence of alpha-hydroxylation of glycolipids on domain formation in lipid monolayers

By means of fluorescence and scanning force microscopy (SFM), we investigated the phase behavior of lipid monolayers composed of a mixture of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, sphingomyelin, and cholesterol (5/2/3) with either alpha-hydroxylated or nonhydroxylated galactocerebroside. Fluorescence images of lipid monolayers at the air-water interface demonstrate that, independent of the lipid mixture, phase separation occurs at low surface pressure up to 4-6 mN m(-1), while an almost homogeneous phase is observed at larger surface pressures. However, by means of SFM of lipid monolayers transferred by the Langmuir-Blodgett technique at around 30 mN m(-1), nanometer-sized domains became discernible in those lipid mixtures that contained galactocerebroside, while, in that without a glycolipid, no such domain formation was visible. Moreover, the alpha-hydroxy group of the galactocerebroside alters the size and the total area of the domains significantly.     

Probing DNA hybridization in thiolipid monolayers by means of impedance spectroscopy

A composite monolayer consisting of a thiolipid and a nucleic acid probe (1), acts as a microenvironment for probing hybridization with the complementary strand (2) by means of impedance spectroscopy. The impedance measurements indicate a significant decrease in the resistance at the electrode surface upon the formation of the (1)/(2) duplex. The impedance measurements were performed in the absence of any amplifying label or added redox label. In order to characterize the electrodes surface and to follow the immobilization processes kinetics, surface plasmon resonance measurements were performed.     

Interfacial behavior of lipid nanocapsules spread on model membrane monolayers

The lipid nanocapsules (LNCs) spread at the air–water interface (A/W) undergo destabilization and disaggregation leading to formation of a triglyceride (TG) surface film. The kinetics of reorganization and formation of TG surface film were followed by measuring either the change of surface pressure at constant area or the surface area at constant surface pressure. From the obtained experimental data were determined the effectiveness of TG spreading and the rate of LNC disaggregation at A/W interface covered with preformed model membrane monolayers of DPPC, Curosurf®, and mucus. Partial LNC stabilization due to their interaction with the model membrane monolayers was observed and characterized by atomic force microscopy (AFM). The obtained results demonstrated that the LNCs spread on mucus surface layer, which models the epithelial surface were more stable than if they were spread either on DPPC or Curosurf® surface layers, which emulate the alveolar surface.     

DC- and RF-GD-OES measurements of adsorbed organic monolayers on copper

Our direct current (DC)- and radiofrequency glow discharge optical emission spectroscopy (RF-GD-OES) measurements of adsorbed organic monolayers were inspired by the work of Shimizu et al., who presented the first example of depth profile analysis of an adsorbed monolayer by RF-GD-OES in 2004. The great potential of RF-GD-OES for analyses of layers with thicknesses in the subnanometer range was surprising. Shimizu et al. discussed not only the qualitative detection of atoms of the organic monolayer (C, H, N, S), but also the determination of the different orientation of the molecules relative to the surface due to a significant peak sequence. This latter assumption was questioned in the analytical community. We intend to demonstrate the potential of the GD-OES technique for surface analysis in terms of reliability and reproducibility by using an advanced vacuum instrumentation and presputtering with silicon. It will be shown that comparable measurements can be reproduced not only with RF-GD-OES but, above all, also with DC-GD-OES. The experimental steps to adsorb thiourea molecules on a copper substrate are described in detail. Further experiments with other organic molecules, e.g. benzotriazole (BTA) or benzothiazole (BTH), disprove the predicted correlation between the orientation of the molecules relative to the surface and the occurrence of peak separation. Ultimately, a quantification of compounds of the organic monolayer in the case of adsorbed thiourea is achieved.