Sum-frequency spectroscopy analysis of two-component langmuir monolayers and the associated interfacial water structure

Probing the Influence of Hydrophobicity of Modified Gold Nanoparticles in Modulating the Lipid Surface Behavior Using Vibrational Sum Frequency Generation Spectroscopy

A deep understanding of how the surface modifications of nanoparticles impact their interactions with cell membranes is vital for advancing safe and effective biomedical applications. Among the pivotal factors governing these interactions, the hydrophobicity of nanoparticles plays a crucial role, predominantly driven by the hydrophobic interactions with the cell membrane. Herein, we study the influence of the hydrophobic alkyl chain length of thiol-capped gold nanoparticles (GNPs) on lipid surfaces with the help of vibrational sum frequency generation spectroscopy. We have utilized the zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid monolayer as a representative model of cell membranes on the water surface. Our findings revealed that GNPs capped with the thiol ligand having a shorter alkyl chain such as heptanethiol (HT, C7) show minimal changes in the C-H stretching vibrations while interacting with the lipid monolayer. These observations could be attributed to the perturbation of the lipid chain due to hydrophobic-hydrophobic interactions between the alkyl chain of thiol-capped GNPs and the hydrophobic group of the lipid membrane or simply by the adsorption of GNPs at the interface without disrupting the monolayer structure. However, with increasing the chain length of thiol-capped GNPs from decanethiol (DDT, C10) to octadecanethiol (ODT, C18), the extent of spectral change in the C-H stretching vibration is increased. The controlled experiment performed with the deuterated lipids conforms that the changes observed in the C-H stretching vibration after adding HT (C7) GNPs are only because of their presence in the surface without altering the monolayer structure. However, in the case of DT (C10) and DDT (C12) GNPs, the strong hydrophobic interactions between the monolayer and the alkyl chain of the thiol-capped GNPs result in the increased orientational order of the monolayer. Moreover, in the case of ODT (C18) GNPs, the very long alkyl chain induces pronounced perturbations in the monolayer structure with net disordering of the monolayer. These observations are further supported by the spectral changes observed in the O-H vibration of the interfacial water molecules. Our findings reveal the crucial role of the hydrophobic nature of GNPs in influencing the interface. Understanding these effects is crucial for drug delivery applications and improving the stability and effectiveness of lipid-based systems.

Molecular Explanation for the Abnormal Flux of Material into a Hot Spot in Ester Monolayers

Langmuir monolayers of certain surfactants show a negative derivative of the surface pressure with respect to temperature. In these monolayers, a local temperature gradient leads to local yielding of the solid phase to a kinetically flowing liquid, so that the material flows toward the hotter regions that act as sinks. The accumulation of material leads to the formation of nonequilibrium multilamellar bubbles of different sizes. Here we investigate the molecular factors leading to such a peculiar behavior. First, we identify the required structural molecular moieties, and second we vary the composition of the subphase in order to analyze its influence. We conclude that esters appear to be unique in two key aspects: they form monolayers whose compression isotherms shift to lower areas as the temperature increases, and thus collapse into a hot spot; and they bind weakly to the aqueous subphase, i.e., water does not attach to the monolayer at the molecular level, but only supports it. Molecular simulations for a selected system confirm and help explain the observed behavior: surfactant molecules form a weak hydrogen bonding network, which is disrupted upon heating, and also the molecular tilting changes with temperature, leading to changes in the film density.

Structure and lateral interaction in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC) and stearyl alcohol

π-A isotherms, atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy are employed to investigate the molecular structure and lateral interactions in mixed monolayers of dioctadecyldimethylammonium chloride (DOAC) and stearyl alcohol (SA) at air/water and air/solid interfaces. To avoid possible interference between the two molecules in the SFG spectroscopic measurements, perprotonated DOAC and perdeuterated SA (dSA) were used. The thermodynamic analyses for the π-A isotherms show that DOAC is miscible with dSA. SFG observations reveal that DOAC molecules become conformationally ordered as dSA molecules are introduced into the monolayer. AFM observations demonstrate coexistence of DOAC-rich and dSA-rich domains in the mixed monolayer with ratios different from their initial composition in the subphase. The present study suggests that DOAC molecules in the mixed monolayer are condensed by mixing with dSA in which the repulsive interactions between positively charged head groups of the DOAC molecules are largely reduced along with an increase of van der Waals interactions with dSA.

Protein-phospholipid interactions in nonclassical protein secretion: problem and methods of study

Extracellular proteins devoid of signal peptides use nonclassical secretion mechanisms for their export. These mechanisms are independent of the endoplasmic reticulum and Golgi. Some nonclassically released proteins, particularly fibroblast growth factors (FGF) 1 and 2, are exported as a result of their direct translocation through the cell membrane. This process requires specific interactions of released proteins with membrane phospholipids. In this review written by a cell biologist, a structural biologist and two membrane engineers, we discuss the following subjects: (i) Phenomenon of nonclassical protein release and its biological significance; (ii) Composition of the FGF1 multiprotein release complex (MRC); (iii) The relationship between FGF1 export and acidic phospholipid externalization; (iv) Interactions of FGF1 MRC components with acidic phospholipids; (v) Methods to study the transmembrane translocation of proteins; (vi) Membrane models to study nonclassical protein release.

Environmental chemistry at vapor/water interfaces: insights from vibrational sum frequency generation spectroscopy

The chemistry that occurs at surfaces has been an intense area of study for many years owing to its complexity and importance in describing a wide range of physical phenomena. The vapor/water interface is particularly interesting from an environmental chemistry perspective as this surface plays host to a wide range of chemistries that influence atmospheric and geochemical interactions. The application of vibrational sum frequency generation (VSFG), an inherently surface-specific, even-order nonlinear optical spectroscopy, enables the direct interrogation of various vapor/aqueous interfaces to elucidate the behavior and reaction of chemical species within the surface regime. In this review we discuss the application of VSFG to the study of a variety of atmospherically important systems at the vapor/aqueous interface. Chemical systems presented include inorganic ionic solutions prevalent in aqueous marine aerosols, small molecular solutes, and long-chain fatty acids relevant to fat-coated aerosols. The ability of VSFG to probe both the organization and reactions that may occur for these systems is highlighted. A future perspective toward the application of VSFG to the study of environmental interfaces is also provided.

Single Lipid Bilayers Constructed on Polymer Cushion Studied by Sum Frequency Generation Vibrational Spectroscopy

Planar solid supported single lipid bilayers on mica, glass, or other inorganic surfaces have been widely used as models for cell membranes. To more closely mimic the cell membrane environment, soft hydrophilic polymer cushions were introduced between the hard inorganic substrate and the lipid bilayer to completely avoid the possible substrate-lipid interactions. In this article, sum frequency generation (SFG) vibrational spectroscopy was used to examine and compare single lipid bilayers assembled on the CaF(2) prism surface and on poly (L-lactic acid) (PLLA) cushion. By using asymmetric lipid bilayers composed of a hydrogenated 1,2-dipalmitoyl-sn-glycerol-3-phosphoglycerol (DPPG) leaflet and a deuterated 1,2-dipalmitoyl-(d62)-sn-glycerol-3-phosphoglycerol (d-DPPG) leaflet, it was shown that the DPPG lipid bilayers deposited on the CaF(2) and PLLA surfaces have similar structures. SFG has also been applied to investigate molecular interactions between an antimicrobial peptide Cecropin P(1) (CP1) and the lipid bilayers on the above two different surfaces. Similar results were again obtained. This research demonstrated that the hydrophilic PLLA cushion can serve as an excellent substrate to support single lipid bilayers. We believe that it can be an important cell membrane model for future studies on transmembrane proteins, for which the possible inorganic substrate-bilayer interactions may affect the protein structure or function.

Investigations of the interactions between synthetic antimicrobial polymers and substrate-supported lipid bilayers using sum frequency generation vibrational spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy was used to analyze interactions between solid-supported lipid bilayers acting as models for cellular membranes and several membrane-active random copolymers with different lipophilic side chains, named 0R (no group), 33Me (methyl group), 11Bz (benzyl group), and 33Bu (butyl group), according to both the identity and percentage of the side chains within the polymer. Biological tests of the minimum inhibitory concentration (MIC) and hemolytic concentration were performed. The inherent surface sensitivity of SFG allowed for independent monitoring of isotopically labeled lipid bilayer leaflets as a function of concentration to study polymer-bilayer interaction mechanisms. Concentrations at which each bilayer leaflet was disrupted were quantitatively determined for each copolymer. Spectroscopic evidence of interaction with the bilayer below the critical concentrations was observed for the 11Bz polymer. The lipophilic butyl side chain of the 33Bu polymer was found to be oriented parallel to the surface normal. This research shows that SFG is a useful analytical technique which provides unique details regarding the interaction mechanisms of these membrane-active copolymers and lipid bilayers.

In situ molecular level studies on membrane related peptides and proteins in real time using sum frequency generation vibrational spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study the molecular structures of surfaces and interfaces in different chemical environments. This review summarizes recent SFG studies on hybrid bilayer membranes and substrate-supported lipid monolayers and bilayers, the interaction between peptides/proteins and lipid monolayers/bilayers, and bilayer perturbation induced by peptides/proteins. To demonstrate the ability of SFG to determine the orientations of various secondary structures, studies on the interactions between different peptides/proteins (melittin, G proteins, alamethicin, and tachyplesin I) and lipid bilayers are discussed. Molecular level details revealed by SFG in these studies show that SFG can provide a unique understanding on the interactions between a lipid monolayer/bilayer and peptides/proteins in real time, in situ and without any exogenous labeling.

Molecular view of the interaction between iota-carrageenan and a phospholipid film and its role in enzyme immobilization

Proteins incorporated into phospholipid Langmuir-Blodgett (LB) films are a good model system for biomembranes and enzyme immobilization studies. The specific fluidity of biomembranes, an important requisite for enzymatic activity, is naturally controlled by varying phospholipid compositions. In a model system, instead, LB film fluidity may be varied by covering the top layer with different substances able to interact simultaneously with the phospholipid and the protein to be immobilized. In this study, we immobilized a carbohydrate rich Neurospora crassa alkaline phosphatase (NCAP) in monolayers of the sodium salt of dihexadecylphosphoric acid (DHP), a synthetic phospholipid that provides very condensed Langmuir films. The binding of NCAP to DHP Langmuir-Blodgett (LB) films was mediated by the anionic polysaccharide iota-carrageenan (iota-car). Combining results from surface isotherms and the quartz crystal microbalance technique, we concluded that the polysaccharide was essential to promote the interaction between DHP and NCAP and also to increase the fluidity of the film. An estimate of DHP:iota-car ratio within the film also revealed that the polysaccharide binds to DHP LB film in an extended conformation. Furthermore, the investigation of the polysaccharide conformation at molecular level, using sum-frequency vibrational spectroscopy (SFG), indicated a preferential conformation of the carrageenan molecules with the sulfate groups oriented toward the phospholipid monolayer, and both the hydroxyl and ether groups interacting preferentially with the protein. These results demonstrate how interfacial electric fields can reorient and induce conformational changes in macromolecules, which may significantly affect intermolecular interactions at interfaces. This detailed knowledge of the interaction mechanism between the enzyme and the LB film is relevant to design strategies for enzyme immobilization when orientation and fluidity properties of the film provided by the matrix are important to improve enzymatic activity.

Water structure and its influence on the flotation of carbonate and bicarbonate salts

Interfacial water structure is a most important parameter that influences the collector adsorption by salt minerals such as borax, potash and trona. According to previous studies, salts can be classified as water structure makers and water structure breakers. Water structure making and breaking properties of salt minerals in their saturated brine solutions are essential to explain their flotation behavior. In this work, water structure making-breaking studies in solutions of carbonate and bicarbonate salts (Na(2)CO(3), K(2)CO(3), NaHCO(3) and NH(4)HCO(3)) in 4 wt% D(2)O in H(2)O mixtures have been performed by FTIR analysis of the OD stretching band. This method reveals a microscopic picture of the water structure making/breaking character of the salts in terms of the hydrogen bonding between the water molecules in solution. The results from the vibrational spectroscopic studies demonstrate that carbonate salts (Na(2)CO(3) and K(2)CO(3)) act as strong structure makers, whereas bicarbonate salts (NaHCO(3) and NH(4)HCO(3)) act as weak structure makers. In addition, the changes in the OD band parameters of carbonate and bicarbonate salt solutions are in agreement with the viscosity characteristics of their solutions.

Vast Magnetic Monolayer Film with Surfactant-Stabilized Fe3O4 Nanoparticles Using Langmuir−Blodgett Technique

Although several methods (e.g., self-assembly, spin coating, etc.) have been explored for making a monolayer film of nanoparticles, the monolayer on a substrate is typically smaller than 1 micromx1 microm in certain regions. The approach is not ideally suitable for generating a highly ordered and close-packed homogeneous vast monolayer of nanoparticles, which is potentially important for applications. In this report, the preparation of the vast monolayer films of Fe3O4 nanoparticles with a wide range such as that over 3.25 micromx3.95 microm is reported. Their TEM images showed a two-dimensional assembly of Fe3O4 nanoparticles, demonstrating the uniformity of these nanoparticles. The formation of a Langmuir monolayer of the oleic acid-coated Fe3O4 nanoparticles mixed with stearic acid molecules at the air/water interface and its stability were studied with a pressure-area isotherm curve. TEM and BAM studies demonstrated that increasing surface pressure resulted in a transition from well-separated domains of nanoparticles complex to well-compressed, monoparticulate layers.

Probing chitosan and phospholipid interactions using Langmuir and Langmuir-Blodgett films as cell membrane models

The interaction between chitosan and Langmuir and Langmuir-Blodgett (LB) films of dimyristoyl phosphatidic acid (DMPA) is investigated, with the films serving as simplified cell membrane models. At the air-water interface, chitosan modulates the structural properties of DMPA monolayers, causing expansion and decreasing the monolayer elasticity. As the surface pressure increased, some chitosan molecules remained at the interface, but others were expelled. Chitosan could be transferred onto solid supports alongside DMPA using the LB technique, as confirmed by infrared spectroscopy and quartz crystal microbalance measurements. The analysis of sum-frequency vibration spectroscopy data for the LB films combined with surface potential measurements for the monolayers pointed to chitosan inducing the ordering of the DMPA alkyl chains. Furthermore, the morphology of DMPA LB films, studied with atomic force microscopy, was affected significantly by the incorporation of chitosan, with the mixed chitosan-DMPA films displaying considerably higher thickness and roughness, in addition to chitosan aggregates. Because chitosan affected DMPA films even at pressures characteristic of cell membranes, we believe this study may help elucidate the role of chitosan in biological systems.

Real-time investigation of lung surfactant respreading with surface vibrational spectroscopy

The respreading of a lung surfactant monolayer at the air-water interface is investigated with broad bandwidth sum frequency generation (BBSFG) spectroscopy. The lung surfactant mixture contains chain perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62), palmitoyloleoylphosphatidylglycerol (POPG), palmitic acid (PA), and KL4 (a 21-residue polypeptide analogue to the surfactant protein SP-B). DPPC-d62 serves as a probe molecule for the spectroscopic investigation. The BBSFG spectra of DPPC-d62 in the lung surfactant mixture are obtained in the C-D stretching region in real-time during film compression and expansion in a Langmuir trough. The BBSFG intensity of the CD3 stretch peak from DPPC-d62 terminal methyl groups is used as a measure of the interfacial density of DPPC-d62 after careful consideration of orientation effects. For the first time, the interfacial loss of DPPC in a complex lung surfactant mixture is quantified. Spectroscopic results reveal that there is an 18% DPPC-d62 interfacial loss during film respreading. However, the surface pressure-area isotherm measurements demonstrate that there is a rather large trough area reduction (37%) during film expansion. The relatively small interfacial loss of DPPC-d62 and the rather large trough area reduction indicate that the respreading of DPPC and non-DPPC components in the lung surfactant is not uniform and a surface refinement process exists during film compression and expansion. This refinement process results in a DPPC-enriched monolayer with a significant depletion of non-DPPC components after film respreading. Implication for replacement surfactant design from this work is discussed.