Humidity-Responsive Polymer Cushion-Supported Biomimetic Membrane: A Model System for X-ray Studies.

An effort aimed at replacing the conventional water column by a relative humidity (RH) environment for structural investigation of a soft polymer cushion-supported model phospholipid membrane has been reported. An RH-responsive well-hydrated polymer cushion layer capable of approximately 2-fold swellability under RH 96% has been employed for phospholipid model membrane fabrication. To validate the proposed method, supported lipid bilayers (SLBs) of phosphocholine and phosphoethanolamine were deposited and structurally characterized at molecular level by the X-ray scattering method. In addition, the molecular interaction of the porphyrin-based hemin molecule, having a drug-like structure, with the supported membrane has been studied for further validation. The swelling behavior of the polymer cushion has been studied at a range of RH values prior to the bilayer deposition. The RH environment, in comparison to the conventional water column, enhanced the dynamic range approximately by 100-fold and the structural resolution by 2-fold. Thus, the bilayer structural features can be assessed without being overwhelmed by the background signals from the traditional water column. This facilitates in extracting reliable layer parameters and exogenous molecule-induced minute changes from the model fit. The proposed method will have far-reaching implications in biosensor engineering, protein-lipid, and drug-lipid interaction studies, X-ray microscopy, imaging, and photon correlation spectroscopy studies from SLBs where acquiring sufficient scattered intensity is still a challenge. This study also predicts that lab-based rotating-anode X-ray instruments can potentially be an alternative to the hard-access synchrotron experiments on biomimetic membranes, keeping the dynamic range and structural resolution uncompromised.

Origin of Intense Luminescence from Supramolecular 2D Molecular Crystals.

Luminescence enhancement in 2D molecular crystals (2D crystals) is promising for a variety of optical applications, yet the availability is limited because of unclear mechanism and inefficient design strategy of luminescence control. Herein, the room temperature phosphorescence from micron long molecular thin free-standing 2D crystals of a mono-cyclometalated Ir(III) complex designed at the water surface is reported. A large luminescence enhancement is observed from the 2D crystals at 300 K, which is comparable with the rigidified solution at 77 K suggesting room temperature phosphorescence origin of the luminescence. In situ synchrotron grazing incidence X-ray diffraction measurements determine the constituent centered rectangular unit cells with precise molecular conformation that promotes the formation of 2D crystals. The molecular crystal design leads to a reduced singlet-triplet energy gap (ΔEST ) and mixing of singlet-triplet states by spin-orbit coupling (SOC) for efficient intersystem crossing, which explains the phosphorescence origin at room temperature and luminescence enhancement. The supramolecular assembly process provides an elegant design strategy to realize room temperature phosphorescence from 2D crystals by rigid intermolecular interactions.

Status of Membrane Asymmetry in Erythrocytes: Role of Spectrin.

Spectrin-based proteinaceous membrane skeletal network has been found to be implicated in membrane disorders like hereditary spherocytosis (HS). HS greatly affects eryptosis via loss of membrane asymmetry which is seen to be the case in haemoglobin disorders like thalassemia and sickle cell disease as well. The biological implications of the status of membrane asymmetry are strongly correlated to spectrin interactions with aminophospholipids, e.g. PE and PS. Fluorescence and X-ray reflectivity (XRR) measurements of spectrin interactions with small unilamellar vesicles (SUVs) and cushioned bilayers of phospholipids, respectively, were studied. Both the XRR and fluorescence measurements led to the characterization of spectrin orientation on the surface of lipid bilayer of phosphatidylcholine (PC) and PC/aminophospholipid mixed membrane systems showing formation of a uniform layer of spectrin on top of the mixed phospholipid bilayer. Fluorescence studies show that spectrin interacts with PC and phosphatidylethanolamine (PE)/phosphatidylserine (PS) membranes with binding dissociation constants (Kd) in the nanomolar range indicating the role of spectrin in the maintenance of the overall membrane asymmetry of erythrocytes.

Ganglioside GM1 Drives Hemin and Protoporphyrin Adsorption in Phospholipid Membranes: A Structural Study.

Monosialoganglioside (GM1), a ubiquitous component of lipid rafts, and hemin, an integral part of heme proteins such as hemoglobin, are essential to the cell membranes of brain neurons and erythrocyte red blood cells for regulating cellular communication and oxygen transport. Protoporphyrin IX (PPIX) and its derivative hemin, on the contrary, show significant cytotoxic effects when in excess causing hematological diseases, such as thalassemia, anemia, malaria, and neurodegeneration. However, the in-depth molecular etiology of their interactions with the cell membrane has so far been poorly understood. Herein, the structure of the polymer cushion-supported lipid bilayer (SLB) of the binary mixture of phospholipid and GM1 in the presence of PPIX and its derivative hemin has been investigated to predict the molecular interactions in model phospholipid membranes. A high-resolution synchrotron-based X-ray scattering technique has been employed to explore the out-of-plane structure of the assembly at different compositions and concentrations. The structural changes have been complemented with the isobaric changes in the mean molecular area obtained from the Langmuir monolayer isotherm to predict the additive-induced membrane condensation and fluidization. PPIX-induced fluidization of phospholipid SLB without GM1 was witnessed, which was reversed to condensation with 2-fold higher structural changes in the presence of GM1. A hemin concentration-dependent linear condensing effect was observed in the pristine SLB. The effect was significantly reduced, and the linearity was observed to be lost in the mixed SLB containing GM1. Our study shows that GM1 alters the interaction of hemin and PPIX with the membrane, which could be explained with the aid of hydrophobic and electrostatic interactions. Our study indicates favorable and unfavorable interactions of GM1 with PPIX and hemin, respectively, in the membrane. The observed structural changes in both SLB and the underlying polymer cushion layer lead to the proposal of a molecule-specific interaction model that can benefit the pharmaceutical industries specialized for drug designing. Our study potentially enriches our fundamental biophysical understanding of neurodegenerative diseases and drug-membrane interactions.

Structural Flexibility of Proteins Dramatically Alters Membrane Stability─A Novel Aspect of Lipid-Protein Interaction.

Protein isoforms are structural variants with changes in the overall flexibility predominantly at the tertiary level. For membrane associated proteins, such structural flexibility or rigidity affects membrane stability by playing modulatory roles in lipid-protein interaction. Herein, we investigate the protein chain flexibility mediated changes in the mechanistic behavior of phospholipid model membranes in the presence of two well-known isoforms, erythroid (ER) and nonerythroid (NER) spectrin. We show dramatic alterations of membrane elasticity and stability induced by spectrin in the Langmuir monolayers of phosphatidylocholine (PC) and phosphatidylethanolamine (PE) by a combination of isobaric relaxation, surface pressure-area isotherm, X-ray scattering, and microscopy measurements. The NER spectrin drives all monolayers to possess an approximately equal stability, and that required 25-fold increase and 5-fold decrease of stability in PC and PE monolayers, respectively. The untilting transition of the PC membrane in the presence of NER spectrin observed in X-ray measurements can explain better membrane packing and stability.

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.

Effects of GM1 on brain spectrin-aminophospholipid interactions.

Spectrin, a major component of the membrane skeletal meshwork of metazoan cells, is implicated to associate with membrane domains and is known to act as a scaffold for stabilization and activation of different signalling modules. We have studied the effect of GM1 (monosialotetrahexosyl ganglioside), a well-known model ganglioside and a signalling moiety, on the interaction of non-erythroid brain spectrin with both saturated and unsaturated aminophospholipids by spectroscopic methods. We observe that GM1 modulates brain spectrin-aminophospholipid interaction to the greatest degree whereas its effect on erythroid spectrin is not as pronounced. Fluorescence quenching studies show that brain spectrin interacts with DMPC/DMPE-based vesicles with a 10-fold increased affinity in presence of very low amounts of 2% and 5% GM1, and the extent of quenching decreases progressively in presence of increasing amounts of GM1. Interaction of brain spectrin with unsaturated membrane systems of DOPC/DOPE weakens in presence GM1. Increase in the mean lifetime of the Trp residues of brain spectrin in presence of GM1 indicates change in the microenvironment of spectrin, without affecting the secondary structure of the protein significantly. Studies on pressure - area isotherm of Langmuir-Blodgett monolayer and Brewster's angle microscopy show that GM1 has an expanding effect on the aminophospholipid monolayers, and ordered regions in DMPC/DMPE mixed monolayers are formed and are stabilized at higher pressure. GM1-induced fluidization of the phospholipid membranes and probable physical contact between bulky sugar head group of GM1 and spectrin, may explain the modulatory role of GM1 on aminophospholipid interactions with nonerythroid brain spectrin.

Evidence of 2D anti-ferromagnetic ordering in rare-earth Langmuir-Blodgett films.

In recent years the ordering of spins in two-dimensions has received considerable attention due to both the fundamental physics interest and for the possible technological applications. Langmuir-Blodgett (LB) films with magnetic ions are ideal systems to study two-dimensional (2D) magnetic ordering as the distances of the magnetic-ions along the out-of-plane and in-plane directions differ by almost an order of magnitude and the effect of the substrate can be neglected. In particular, vortex formation in ferro and antiferro 2D magnetic structures are of current interest and LB films are ideal to study this evolving physics. We show here that 2D magnetic ordering along the in-plane direction of multilayered LB films changes from ferromagnetic to anti-ferromagnetic as the rare-earth magnetic ion is changed from Gadolinium (Gd) to Holmium (Ho). The in-plane magnetization results have shown that Gd based LB films exhibit a temperature dependent saturation moment due to the existence of a vortex structure. The results of the magnetization study presented here show that the Ho based LB films exhibit an in-plane anti-ferromagnetic ordering and the saturation moment is found to be almost independent of temperature indicating the absence of spin vortex structures. From a 1/χ - T plot the asymtotic Curie point θ a and the Neel temperature θ N of the Ho-St LB film were found to be 66 K and 42 K respectively.

Cholesterol-Induced Structural Changes in Saturated Phospholipid Model Membranes Revealed through X-ray Scattering Technique.

Lateral and out-of-plane organization of cholesterol and its effect on regulating the physicochemical properties of zwitterionic phospholipid model membranes have been investigated by a pressure-area isotherm study from the Langmuir monolayer, atomic force microscopy (AFM), and X-ray reflectivity (XRR) measurements from supported binary monolayer films. The systematic isotherm studies on the Langmuir monolayer of phospholipids and the subsequent extraction of excess Gibbs free energy (ΔGexc) revealed the mechanism of cholesterol interaction and the molecular cooperativeness for different arrangements in the phospholipid model membranes. We have found a critical cholesterol molar concentration (χc) up to which the lipid-cholesterol miscibility gradually increases and then further increase in the concentration leads to an inhomogeneous structure formation similar to raft structures. The thickening in the lipid acyl chain and the subsequent lowering of the lipid head group thickness up to χc are also evident from the XRR study. Beyond χc, large-sized domains are observed in the AFM images from the deposited monolayer. χc has also been observed to depend on the phase of the monolayer, in particular, ∼25 molar % in the gel phase and ∼40 molar % in the fluid phase, wherein a regular distribution has been found with the highest separation between the cholesterol molecules. The extracted isothermal compressibility coefficient (CS) and ΔGexc from the monolayer isotherms indicate that the molecular arrangement at χc are the most stable configurations of the monolayer. Our study provides direct evidence into cholesterol-induced evolution in phase behavior and the consequent model on the structure at different phases in the phospholipid Langmuir monolayers.

In-Situ GISAXS Study of Supramolecular Nanofibers having Ultrafast Humidity Sensitivity.

Self assembled nanofibers derived from donor-acceptor (D-A) pair of dodecyl methyl viologen (DMV) and potassium salt of coronene tetracarboxylate (CS) is an excellent material for the development of organic electronic devices particularly for ultrafast response to relative humidity (RH). Here we have presented the results of in-situ grazing incidence small angle x-ray scattering (GISAXS) measurements to understand aridity dependent self reorganization of the nanofibers. The instantaneous changes in the organization of the nanofibers was monitored with different equilibrium RH conditions. Additionally formation of nanofibers during drying was studied by GISAXS technique - the results show two distinct stages of structural arrangements, first the formation of a lamellar mesophase and then, the evolution of a distorted hexagonal lattice. The RH dependent GISAXS results revealed a high degree of swelling in the lattice of the micelles and reduction in the distortion of the hexagonal structure with increase in RH. In high RH condition, the nanofibers show elliptical distortion but could not break into lamellar phase as observed during formation through drying. This observed structural deformation gives insight into nanoscopic structural changes of the micelles with change in RH around it and in turn explains ultrafast sensitivity in its conductivity for RH variation.

Counterion effects on nano-confined metal-drug-DNA complexes.

We have explored morphology of DNA molecules bound with Cu complexes of piroxicam (a non-steroidal anti-inflammatory drug) molecules under one-dimensional confinement of thin films and have studied the effect of counterions present in a buffer. X-ray reflectivity at and away from the Cu K absorption edge and atomic force microscopy studies reveal that confinement segregates the drug molecules preferentially in a top layer of the DNA film, and counterions enhance this segregation.

Entanglement effects in capillary waves on liquid polymer films.

Overdamped surface capillary wave relaxations on molten polymer films were measured using x-ray photon correlation spectroscopy. We found a transition from a single through a stretched to another single exponential regime as the temperature is decreased from well above to near the bulk glass transition temperature. A universal scaling of the dynamics was discovered over a wide range of film thicknesses, temperatures, and molecular weights (except in the multiple relaxation regime). These observations are justified by hydrodynamic theory and the time-temperature superposition principle by considering an effective viscosity instead of the bulk zero shear viscosity.