X-ray Reflectivity Study Research Articles

How Tim proteins differentially exploit membrane features to attain robust target sensitivity

Immune surveillance cells such as T cells and phagocytes utilize integral plasma membrane receptors to recognize surface signatures on triggered and activated cells such as those in apoptosis. One such family of plasma membrane sensors, the transmembrane immunoglobulin and mucin domain (Tim) proteins, specifically recognize phosphatidylserine (PS) but elicit distinct immunological responses. The molecular basis for the recognition of lipid signals on target cell surfaces is not well understood. Previous results suggest that basic side chains present at the membrane interface on the Tim proteins might facilitate association with additional anionic lipids including but not necessarily limited to PS. We, therefore, performed a comparative quantitative analysis of the binding of the murine Tim1, Tim3, and Tim4, to synthetic anionic phospholipid membranes under physiologically relevant conditions. X-ray reflectivity and vesicle binding studies were used to compare the water-soluble domain of Tim3 with results previously obtained for Tim1 and Tim4. Although a calcium link was essential for all three proteins, the three homologs differed in how they balance the hydrophobic and electrostatic interactions driving membrane association. The proteins also varied in their sensing of phospholipid chain unsaturation and showed different degrees of cooperativity in their dependence on bilayer PS concentration. Surprisingly, trace amounts of anionic phosphatidic acid greatly strengthened the bilayer association of Tim3 and Tim4, but not Tim1. A novel mathematical model provided values for the binding parameters and illuminated the complex interplay among ligands. In conclusion, our results provide a quantitative description of the contrasting selectivity used by three Tim proteins in the recognition of phospholipids presented on target cell surfaces. This paradigm is generally applicable to the analysis of the binding of peripheral proteins to target membranes through the heterotropic cooperative interactions of multiple ligands.

Investigation of structural, magnetic and electronic properties of FeTa films for varying Ta concentration at different annealing temperatures

In the present work, temperature dependent magnetic, structural and electronic properties of FeTa films with increasing Ta concentration are investigated. X-ray reflectivity (XRR) studies reveal densification of the film with increasing Ta concentration. Thermal annealing also results in an increased electron density relative to as-deposited film, concurrent with increase in grain size as observed from X-ray diffraction (XRD) and results of Soft-X-ray absorption spectroscopy (SXAS). However, grain growth is inhibited with increasing Ta concentration by presence of Ta in grain boundary (GB) region. This also results in increase in magnetic coercivity (greater than150 %) as observed from magneto-optical Kerr effect (MOKE). The structural properties were further correlated with magnetic properties as observed from Polarized neutron reflectivity (PNR) and Ferromagnetic resonance (FMR). Effective magnetic moment of the samples, as observed from PNR decreases with increasing Ta concentration and increases with increasing annealing temperature, which is in agreement with FMR results. Scattering length density profiles, observed from simultaneous fitting of XRR and PNR show presence of an oxide layer at surface, as also confirmed from SXAS measurements.

Discerning perturbed assembly of lipids in a model membrane in presence of violacein

Violacein is a naturally found pigment that is used by some gram negative bacteria to defend themselves from various gram positive bacteria. As a result, this molecule has caught attention for its potential biomedical applications and has already shown promising outcomes as an antiviral, an antibacterial, and an anti-tumor agent. Understanding the interaction of this molecule with a cellular membrane is an essential step to extend its use in the pharmaceutical paradigm. Here, the interaction of violacein with a lipid monolayer formed at the air–water interface is found to depend on electrostatic nature of lipids. In presence of violacein, the two dimensional (2D) pressure–area isotherms of lipids have exhibited changes in their phase transition pressure and in-plane elasticity. To gain insights into the out-of-plane structural organization of lipids in a membrane, X-ray reflectivity (XRR) study on a solid supported lipid monolayer on a hydrophilic substrate has been performed. It has revealed that the increase in membrane thickness is more pronounced in the zwitterionic and positively charged lipids compared to the negatively charged one. Further, the lipid molecules are observed to decrease their tilt angle made with the normal of lipid membrane along with an alteration in their in-plane ordering. This has been quantified by grazing incidence X-ray diffraction (GIXD) experiments on the multilayer membrane formed in an environment with controlled humidity. The structural reorganization of lipid molecules in presence of violacein can be utilized to provide a detailed mechanism of the interaction of this molecule with cellular membrane.

Atomic layer deposition of chromium oxide—An interplay between deposition and etching

Atomic layer deposition (ALD) of chromium oxide (Cr2O3) thin films is investigated in a custom built hot wall viscous flow reactor configuration at 300 °C. Chromium(III) 2,4-pentanedionate [Cr(acac)3] and ozone (O3) are employed as the metal and the oxygen sources, respectively. In situ quartz crystal microbalance (QCM) and ex situ x-ray reflectivity studies are utilized as the two complementary techniques to monitor the growth mechanism and self-limiting deposition chemistry during Cr2O3 ALD. In situ QCM studies reveal a negligible nucleation period on the previously grown Al-OH* terminated surface before revealing the perfectly linear growth mechanism at 300 °C. The saturated growth rate is found to be ca. 0.28 Å/cycle. In addition, excessive O3 exposure also reveals an alternative, controlled, and spontaneous etching pathway of the growing film as a result of the partial surface oxidation of Cr2O3. The as-deposited thin films are found to exhibit a polycrystalline rhombohedral structure without any preferential orientation. X-ray photoelectron spectroscopy studies reveal uniform distribution of Cr and O throughout the stack of ca. 40 nm film with minimum C impurities. High resolution scans of Cr 2p core level also confirm the presence of Cr in the +3 oxidation state with the corresponding multiplet spectrum.

Structure, morphology and reversible hysteresis nature of human serum albumin (HSA) monolayer on water surface

Compression-decompression surface pressure (π)-specific molecular area (A) isotherm cycle of human serum albumin (HSA) monolayer is performed on water surface at four different subphase pH conditions, i.e., below and above the isoelectric point (pI ≈ 4.7) of HSA molecule. For all pH conditions, the decompression curve nearly follows the compression curve, however, at pH ≈ 5.0, hysteresis is observed at higher surface pressure. Out-of-plane structures and in-plane morphologies obtained from the X-ray reflectivity and AFM studies show that only the film thickness variation takes place with the change in surface pressure, which is also evidenced from the BAM images. With increase in surface pressure, the oblate-shaped HSA molecules start tilting making an angle with the water surface and as the monolayer is decompressed the molecules regain their initial untilted monomolecular configuration. Depending upon the subphase pH and local surface charge of the specific protein molecule, electrostatic repulsive interaction dominates over the van der Waals attraction and as a result decompression curve follows the compression curve as the molecules repel each other, however, closer to the isoelectric point as strength of the interactions reverses, a hysteresis is obtained at higher surface pressure and accordingly monolayer behaviour modifies on the water surface.

X-ray Reflectivity Study of Ion-implanted Layer on Si Substrate

X-ray Reflectivity Study of Ion-implanted Layer on Si Substrate

Potential Dependent Ion Arrangement Near the Electrode/Electrolyte Interface

The electric double layer (EDL) is the ion arrangement evolving at electrolyte interfaces. Despite its importance in many applications such as super-capacitors, lithium ion batteries (LIB), water purification, and stabilization of nanoparticles in solution, the structural motifs of the EDL are still under debate.In LIB, the molecular scale ion arrangement near the interface or EDL controls solvation and hence mass/charge transfer across the interface. Additionally, the interfacial speciation governs the reductive and oxidative processes at the interface. Thus, it has a great impact on the stability of the electrolyte. Hence, the near-interface structure plays a vital role in the solid electrolyte interphase (SEI) formation which is still poorly understood. The SEI protects the electrolyte from further decomposition and its stability, low electrical conductance, and good Li-ion transport properties are crucial for the cycling ability and long-term stability of the cell. Therefore, an atomic level elucidation of the electrolyte/electrode interface and its rearrangement dynamics due to potential changes is of utmost importance for understanding the behaviors of LIBs, and will ultimately lead to the minimization of ion transport resistance and improved long-term stability.We present an in-situ X-ray reflectivity (XR) study on the single crystalline boron-doped diamond – electrolyte interface. The metal-like electric conductivity and wide electrochemical window of the single crystalline boron-doped diamond allow us to apply a potential across the interface. We obtain structural information by modeling the potential dependent EDL to reproduce our experimental data. First proof-of-concept results on 1m CsCl(aq) show a clear trend of the XR profile on the applied potential of -1.2V – 1.2V vs AgCl. The length scale of π/qz = 4Å of the observed minima at qz = 0.75 Å−1 corresponds well to the expected length-scale of the inner-Helmholtz layer of the EDL. Our analysis suggests that the population of the inner-Helmholtz layer decreases with increasing potential.Our efforts allow to challenge the current theoretical models and simulations and will ultimately help to understand the mechanisms involved in the SEI formation and lead to the minimization of ion transport resistance as well as to an improvement of the LIB cycling ability. Figure 1

Transition from a uni- to a bimodal interfacial charge distribution in hbox {LaAlO}_3/hbox {SrTiO}_3 upon cooling

We present a combined resonant soft X-ray reflectivity and electric transport study of hbox {LaAlO}_3/hbox {SrTiO}_3 field effect devices. The depth profiles with atomic layer resolution that are obtained from the resonant reflectivity reveal a pronounced temperature dependence of the two-dimensional electron liquid at the hbox {LaAlO}_3/hbox {SrTiO}_3 interface. At room temperature the corresponding electrons are located close to the interface, extending down to 4 unit cells into the hbox {SrTiO}_3 substrate. Upon cooling, however, these interface electrons assume a bimodal depth distribution: They spread out deeper into the hbox {SrTiO}_3 and split into two distinct parts, namely one close to the interface with a thickness of about 4 unit cells and another centered around 9 unit cells from the interface. The results are consistent with theoretical predictions based on oxygen vacancies at the surface of the hbox {LaAlO}_3 film and support the notion of a complex interplay between structural and electronic degrees of freedom.

X-ray reflectivity and X-ray photoelectron spectroscopy studies on reactively sputtered $$\hbox {Nb}_{2}\hbox {O}_{5}$$-based thin-film devices

The reactively sputtered $$\hbox {Nb}_{2}\hbox {O}_{5}$$ -based metal–oxide–semiconductor devices deposited under different $$\hbox {O}_{2}/\hbox {Ar}$$ ratios have been studied using laboratory-based X-ray reflectivity (XRR) and synchrotron-based X-ray photoelectron spectroscopy techniques. The $$I-V$$ characteristics reveal a sharp transition in the magnitude of current ( $$\sim$$ up to five orders) signifying a switching phenomenon between high-resistance and low-resistance states. The electron density profile, derived from XRR data, shows a dependence of the switching properties on the film porosity. Further, the non-lattice oxygen in the film also plays a role in determining the on/off ratio of the devices. Therefore, besides the presence of non-lattice oxygen, film porosity also influences the resistive switching behavior of the $$\hbox {Nb}_{2}\hbox {O}_{5}$$ -based devices.

X-ray and Neutron Reflectivity Studies of Styrene-Maleic Acid Copolymer Interactions with Galactolipid-Containing Monolayers.

Styrene-maleic acid (SMA) copolymers have recently gained attention for their ability to facilitate the detergent-free solubilization of membrane protein complexes and their native boundary lipids into polymer-encapsulated, nanosized lipid particles, referred to as SMALPs. However, the interfacial interactions between SMA and lipids, which dictate the mechanism, efficiency, and selectivity of lipid and membrane protein extraction, are barely understood. Our recent finding has shown that SMA 1440, a chemical derivative of the SMA family with a functionalized butoxyethanol group, was most active in galactolipid-rich membranes, as opposed to phospholipid membranes. In the present work, we have performed X-ray reflectometry (XRR) and neutron reflectometry (NR) on the lipid monolayers at the liquid-air interface followed by the SMA copolymer adsorption. XRR and Langmuir Π-A isotherms captured the fluidifying effect of galactolipids, which allowed SMA copolymers to infiltrate easily into the lipid membranes. NR results revealed the detailed structural arrangement of SMA 1440 copolymers within the membranes and highlighted the partition of butoxyethanol group into the lipid tail region. This work allows us to propose a possible mechanism for the membrane solubilization by SMA.

X-ray and Neutron Reflectivity Studies of Styrene-Maleic Acid Polymer Interactions with Galactolipid-Containing Monolayers

X-ray and Neutron Reflectivity Studies of Styrene-Maleic Acid Polymer Interactions with Galactolipid-Containing Monolayers

PH-dependent structure, pattern and hysteresis behaviour of lipid (DMPA)-protein (BSA) monolayer complex

Understanding lipid-protein interactions in model membranes is a challenging task. Limited information exist to-date regarding the relative influence of hydrophobic and electrostatic forces on the organization of proteins inside model membranes, while these forces determine the structure of lipid-protein complexes. We measured the surface pressure (π) - molecular area (A) isotherm cycles of protein (BSA) – lipid (DMPA) mixed monolayers below and above the isoelectric point of BSA (≈ 4.8). At pH ≈ 4.0, below the isoelectric point, BSA is positively charged, and exposes few hydrophobic groups at its surface, compression-decompression isotherms show a nearly reversible hysteresis. At pH ≈ 7.0, above the isoelectric point, BSA is negatively charged and more hydrophobic. At this pH, compression-decompression isotherms show an irreversible hysteresis. This behaviour indicates that the deformation of BSA molecules under pressure is reversible below the isoelectric point, while it becomes irreversible above it. X-ray reflectivity studies for protein-lipid mixed monolayers show that BSA molecules move from the zone close to water and near the lipid polar heads toward the zone occupied by their hydrocarbon tails s when surface pressure increases. Mostly the surface pressure in combinations with hydrophobic and electrostatic interactions is responsible for such structural modifications.

Pulsed laser deposited Co2FeSi Heusler alloy thin films: Effect of different thermal growth processes

Heusler alloy of Co2FeSi (CFS) is a promising candidate for spintronics applications due to its high magnetic moment and high spin polarization. In this report two series of CFS thin films of approximate thickness of 1000 Å were prepared using Pulsed Laser Deposition (PLD) technique by two separate routes, viz., (i) by depositing at elevated substrate temperature and (ii) by depositing at room temperature followed by post-deposition annealing under vacuum. The effects of these two thermal growth processes on the structural and magnetic properties of the films have been studied in detail here. X-ray diffraction study suggests that similar to the bulk target cubic Heusler phase is maintained in the thin films prepared by both the processes, however, X-ray reflectivity study shows that the films deposited at elevated substrate temperatures have higher density and surface roughness than the other set. Co/Fe atomic ratio in the films was found to remain near stoichiometry up to high temperature in both the series of samples though atomic percentage of Si is found to be higher in the samples. Synchrotron based Extended X-ray absorption fine structure (EXAFS) measurements indicate higher 3d-3p (Co-Si) hybridization for the series of samples prepared at elevated temperature while stronger 3d-3d (Co-Fe) hybridization for the samples prepared with post deposition annealing. EXAFS study also indicated presence of Co/Fe type antisite disorder which increases with increase in the substrate temperature. Finally from detail magnetic measurements it was observed that the films prepared by post-deposition annealing process have lesser crystallinity, magnetic ordering and magneto crystalline anisotropy in comparison to the films grown with elevated substrate heating.

Multilayer Density Analysis of Cellulose Thin Films.

An approach for the multilayer density analysis of polysaccharide thin films at the example of cellulose is presented. In detail, a model was developed for the evaluation of the density in different layers across the thickness direction of the film. The cellulose thin film was split into a so called “roughness layer” present at the surface and a “bulk layer” attached to the substrate surface. For this approach, a combination of multi-parameter surface plasmon resonance spectroscopy (SPR) and atomic force microscopy (AFM) was employed to detect changes in the properties, such as cellulose content and density, thickness and refractive index, of the surface near layer and the bulk layer. The surface region of the films featured a much lower density than the bulk. Further, these results correlate to X-ray reflectivity studies, indicating a similar layered structure with reduced density at the surface near regions. The proposed method provides an approach to analyse density variations in thin films which can be used to study material properties and swelling behavior in different layers of the films. Limitations and challenges of the multilayer model evaluation method of cellulose thin films were discussed. This particularly involves the selection of the starting values for iteration of the layer thickness of the top layer, which was overcome by incorporation of AFM data in this study.

Structure and morphology of atomic layer deposition grown ZnO thin film/nanostructure on polymeric template

Abstract Zinc oxide (ZnO) was grown on polymer template, namely Poly (methyl methacrylate) (PMMA), by atomic layer deposition (ALD) technique. The detailed structural characterizations of the composite ZnO/PMMA film, through X-ray scattering techniques, indicate that on the PMMA template crystalline ZnO thin film was formed with its c-axis oriented along the surface normal. Combination of X-ray reflectivity (XRR) and atomic force microscopy (AFM) studies demonstrate the formation of high quality ZnO thin film (about 25 nm thick) on PMMA template, with more than 92% coverage and low surface/interface roughness. AFM results clearly show that the ZnO film consists of almost uniform nanoparticles with ∼24-30 nm in-plane size distribution, which is very much consistent with the observed size of the ZnO crystallites obtained from X-ray diffraction (XRD) study.

Study of in-situ structural and chemical changes of ultrathin polymer films

Simulation of X-ray reflectivity (XRR) curves based on standard theoretical formalism has been performed by changing the structural parameters such as thickness, density, and roughness. Analysis of simulated curves shows that the density of thin films can be obtained within the specified range of thickness assuming chemical and physical changes occur in the system. Also along with this simulation, we have performed experimental XRR studies using polymer thin films of different thicknesses prepared by spin-coating method. In-situ XRR data were collected while chemical and structural changes occur in thin films. Results show that the instant density of the films can be determined during chemical and structural changes in the films as observed by comparison with simulated curves.

Photoisomerization Dynamics in a Densely Packed Optically Transformable Azobenzene Monolayer.

Molecular monolayers that can be reconfigured through the use of external stimuli promise to enable the creation of interfaces with precisely selected dynamically adjustable physical and electronic properties with potential impact ranging from electronics to energy storage. Azobenzene-containing molecular monolayers have multiple stable molecular conformations but face a challenging nanoscale problem associated with understanding the basic mechanisms of reconfiguration. Time-resolved X-ray reflectivity studies show that the reconfiguration of a densely packedrhenium-azobenzene monolayer occurs in a period of many seconds. The degree of reconfiguration from trans to cis forms depends on the integrated UV fluence and has kinetics that are consistent with a mechanism in which the transformation occurs through the nucleation and growth of nanoscale two-dimensional regions of the cis isomer.

Structural changes in cellular membranes induced by ionic liquids: From model to bacterial membranes

Ionic liquids (ILs) have generated considerable attention recently because of their cytotoxicity and application as antibiotics. However, the mechanism of how they damage cell membranes is not currently well understood. In this paper, the antibacterial activities of two imidazolium-based ILs, namely 1-butyl- 3-methylimidazolium tetrafluroborate ([BMIM][BF4]) and 1-ethyl- 3-methylimidazolium tetrafluroborate ([EMIM][BF4]) have been investigated. The activity of [BMIM][BF4] on gram negative bacteria E. coli is observed to be stronger compared with the short chained [EMIM][BF4]. To explain this observation, the effects of these ILs on the self-assembled structures of model cellular membranes have been investigated. The in-plane elasticity of a monolayer formed at air-water interface by 1,2-dipalmitoyl- sn-glycero- 3-phosphocholine (DPPC) lipids was reduced in the presence of the ILs. The x-ray reflectivity studies on polymer supported lipid bilayer have shown the bilayer to shrink and correspondingly exhibit an increase in electron density. The presence of a certain mol% of negatively charged lipid, 1,2-dipalmitoyl-rac-glycero-3-phospho-L-serine (DPPS), in DPPC mono- and bi-layers enhances the effect considerably.

Sample cell for studying liquid interfaces with an insitu electric field using X-ray reflectivity and application to clay particles at oil-oil interfaces.

Commissioning results of a liquid sample cell for X-ray reflectivity studies with an insitu applied electrical field are presented. The cell consists of a Plexiglas container with lateral Kapton windows for air-liquid and liquid-liquid interface studies, and was constructed with grooves to accept plate electrodes on the walls parallel to the direction of the beam. Both copper and ITO plate electrodes have been used, the latter being useful for simultaneous optical studies. Commissioning tests were made at the I07 beamline of the Diamond Light Source.

Adsorption Behavior of Lysozyme at Titanium Oxide-Water Interfaces.

We present an in situ X-ray reflectivity study of the adsorption behavior of the protein lysozyme on titanium oxide layers under variation of different thermodynamic parameters, such as temperature, hydrostatic pressure, and pH value. Moreover, by varying the layer thickness of the titanium oxide layer on a silicon wafer, changes in the adsorption behavior of lysozyme were studied. In total, we determined less adsorption on titanium oxide compared with silicon dioxide, while increasing the titanium oxide layer thickness causes stronger adsorption. Furthermore, the variation of temperature from 20 to 80 °C yields an increase in the amount of adsorbed lysozyme at the interface. Additional measurements with variation of the pH value of the system in a region between pH 2 and 12 show that the surface charge of both protein and titanium oxide has a crucial role in the adsorption process. Further pressure-dependent experiments between 50 and 5000 bar show a reduction of the amount of adsorbed lysozyme with increasing pressure.