Cholesterol-Induced Buckling in Physisorbed Polymer-Tethered Lipid Monolayers

The influence of cholesterol concentration on the formation of buckling structures is studied in a physisorbed polymer-tethered lipid monolayer system using epifluorescence microscopy (EPI) and atomic force microscopy (AFM). The monolayer system, built using the Langmuir-Blodgett (LB) technique, consists of 3 mol % poly(ethylene glycol) (PEG) lipopolymers and various concentrations of the phospholipid, 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and cholesterol (CHOL). In the absence of CHOL, AFM micrographs show only occasional buckling structures, which is caused by the presence of the lipopolymers in the monolayer. In contrast, a gradual increase of CHOL concentration in the range of 0–40 mol % leads to fascinating film stress relaxation phenomena in the form of enhanced membrane buckling. Buckling structures are moderately deficient in CHOL, but do not cause any notable phospholipid-lipopolymer phase separation. Our experiments demonstrate that membrane buckling in physisorbed polymer-tethered membranes can be controlled through CHOL-mediated adjustment of membrane elastic properties. They further show that CHOL may have a notable impact on molecular confinement in the presence of crowding agents, such as lipopolymers. Our results are significant, because they offer an intriguing prospective on the role of CHOL on the material properties in complex membrane architecture

Fluorescence Measurements of Aromatic Amino Acids in the Presence of Lipid Membranes

Amphiphilic peptides are capable of finding their way to, and occasionally through, cellular membranes using a mechanism that includes specific amino acid sequences. Physical measurements of amino acid-lipid interactions are of interest for a quantitative description of peptide affinities to biological membranes. In this study, we investigate small peptide-lipid interactions using the fluorescence of the aromatic amino acids tyrosine (Tyr), tryptophan (Trp) and phenylalanine (Phe). Reference spectra in deuterated isopropanol solutions are obtained to mimic hydrophobic environments and are used to quantify the interaction of Lys-Tyr-Lys, Trp-Gly, and Gly-Phe with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and palmitoyl-oleoyl phosphatidylserine (POPS) lipid membranes. These fluorescence data complement previously reported UV absorption data and have the advantage of eliminating background and scatter from solution. Together with NMR data, these results can be used to more fully characterize lipid-aromatic amino residue interactions

Near-field studies of anisotropic variations and temperature-induced structural changes in a supported single lipid bilayer

Temperature-controlled polarization modulation near-field scanning optical microscopy measurements of a single supported Lβ′ 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayer are presented. The effective retardance (S=2π(ne−no)tλ, where t is the thickness of the bilayer and λ is the wavelength of light used) and the direction of the projection of the acyl chains (θ) were measured simultaneously. We demonstrate how one is able to align the system over the sample and measure a relative retardance ΔS, a crucial step in performing temperature-controlled experiments. Maps of ΔS and θ, with a lateral resolution on the order of ∼100 nm are presented, highlighting variations deriving from changes in the average molecular orientation across a lipid membrane at room temperature. A discussion of how this information can be used to map the average three-dimensional orientation of the molecules is presented. From ΔS and the known thickness of the membrane t the birefringence (ne−no) of the bilayer was determined. Temperature-controlled measurements yielded a change of ΔS∼(3.8±0.3) mrad at the main transition temperature (Tm∼41∘C) of a single planar bilayer of DPPC, where the membrane transitioned between the gel Lβ′ to liquid disorder Lα state. The result agrees well with previous values of (ne−no) in the Lβ′ phase and translates to an assumed average acyl chain orientation relative to the membrane normal (⟨ϕ⟩∼32∘) when TTm. Evidence of super heating and cooling are presented. A discussion on how the observed behavior as Tm is approached, could relate to the existence of varying microconfigurations within the lipid bilyer are presented. This conversation includes ideas from a Landau-Ginzburg picture of first-order phase transitions in nematic-to-isotropic systems

Phase Coexistence in Single-Lipid Membranes Induced by Buffering Agents

Recent literature has shown that buffers affect the interaction between lipid bilayers through a mechanism that involves van der Waals forces, electrostatics, hydration forces and membrane bending rigidity. This letter shows an additional peculiar effect of buffers on the mixed chain 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers, namely phase coexistence similar to what was reported by Rappolt et al. for alkali chlorides. The data presented suggest that one phase appears to dehydrate below the value in pure water, while the other phase swells as the concentration of buffer is increased. However, since the two phases must be in osmotic equilibrium with one another, this behavior challenges theoretical models of lipid interactions

Structural Changes of Alpha 1-Antitrypsin under Osmotic Pressure and in the Presence of Lipid Membranes

poster abstractAlpha 1-Antitrypsin (A1AT) is a glycoprotein that has been shown to have protective roles of lung cells against emphysema, a disease characterized by lung tissue destruction. Most known glycoproteins have been shown to play a role in cellular interactions but the exact role of the glycan chains is still under investigation. Previous electrophysiological measurements show that A1AT has a strong affinity to lipid bilayers perturbing the function of ion channels present in the membrane. We have performed contrastmatching small-angle neutron scattering (SANS) experiments to study the conformational changes of the glycosylated form of A1AT for different concentrations of the osmolyte poly(ethelene glycol) (PEG) and in the presence of two different lipid membranes: POPC and POPS. We also monitor the structural changes of the lipid vesicles in the presence of A1AT by SANS. Guinier fits were used as a first approximation to obtain the radius of gyration (Rg) of A1AT. Bragg peaks were used to study structural changes of lipid vesicles. We observed that the Rg of A1AT changes as a function of PEG concentration in solution and when in the presence of lipid vesicles. The deformations monitored through changes in A1AT’s Rg in the presence of lipid vesicles are compared to the deformations of the glycoprotein observed under osmotic pressure and to the structural changes observed in the lipid vesicles

Programmable Colloidal Approach to Hierarchical Structures of Methylammonium Lead Bromide Perovskite Nanocrystals with Bright Photoluminescent Properties

Systematic tailoring of nanocrystal architecture could provide unprecedented control over their electronic, photophysical, and charge transport properties for a variety of applications. However, at present, manipulation of the shape of perovskite nanocrystals is done mostly by trial-and-error-based experimental approaches. Here, we report systematic colloidal synthetic strategies to prepare methylammonium lead bromide quantum platelets and quantum cubes. In order to control the nucleation and growth processes of these nanocrystals, we appropriately manipulate the solvent system, surface ligand chemistry, and reaction temperature causing syntheses into anisotropic shapes. We demonstrate that both the presence of chlorinated solvent and a long chain aliphatic amine in the reaction mixture are crucial for the formation of ultrathin quantum platelets (∼2.5 nm in thickness), which is driven by mesoscale-assisted growth of spherical seed nanocrystals (∼1.6 nm in diameter) through attachment of monomers onto selective crystal facets. A combined surface and structural characterization, along with small-angle X-ray scattering analysis, confirm that the long hydrocarbon of the aliphatic amine is responsible for the well ordered hierarchical stacking of the quantum platelets of 3.5 nm separation. In contrast, the formation of ∼12 nm edge-length quantum cubes is a kinetically driven process in which a high flux of monomers is achieved by supplying thermal energy. The photoluminescence quantum yield of our quantum platelets (∼52%) is nearly 2-fold higher than quantum cubes. Moreover, the quantum platelets display a lower nonradiative rate constant than that found with quantum cubes, which suggests less surface trap states. Together, our research has the potential both to improve the design of synthetic methods for programmable control of shape and assembly and to provide insight into optoelectronic properties of these materials for solid-state device fabrication, e.g., light-emitting diodes, solar cells, and lasing materials

Fluorescence Measurements Of Phototrophic Sulfur Bacteria For Applications In Water Column Profiling

poster abstractUnlike plants that produce oxygen during photosynthesis, phototrophic sulfur bacteria use sulfide and sunlight to produce carbohydrates and elemental sulfur. These bacteria require a unique aquatic environment to thrive: one that is anoxic (depleted of oxygen) and rich in hydrogen sulfide. Such conditions are found in a number of stratified lakes around the world including several in Northern Indiana. Studying the ecology and geochemical conditions that promote habitable conditions for phototrophic bacteria in lakes provides insight into the Early Earth (thought to be anoxic), ocean anoxic events of the Mesozoic (70-250 million years ago) and modern low oxygen conditions of coastal environments such as the Dead Zone of the Gulf of Mexico. However, locating and directly sampling these bacterial populations in vast bodies of water is not an easy task. In this project, we investigate fluorescent properties of purple sulfur bacteria in order to develop a dependable sensor that can be deployed in the water column. We report a number of measurements of purple sulfur bacterium fluorescence in the near infrared region when excited at discrete wavelengths in the UV range. We use these bench-top measurements to design a water-proof apparatus equipped with an absorption and luminescent detector for localization of bacteria in lake water. This device will be deployed in anoxic lakes of Northern Indiana to find the in situ water column position of phototrophic bacteria

The Effect of Dimethyl Sulfoxide on Macromolecular Structures

poster abstractHemorrhagic and thrombotic disorders are a consequence of the body’s inability to form effective blood clots. A major component of the hemostatic clot is a polymerized fibrin network. The network is formed by polymerization of fibrin which is generated by the enzymatic processing of the precursor fibrinogen in blood by the coagulation enzyme thrombin. In our study we investigated how dimethyl sulfoxide (DMSO) effects the formation of the fibrin mesh. These networks were grown over micron-sized pores in polymer membranes with varying concentrations of DMSO. The samples were characterized optically using confocal and differential interference contrast microscopies. Image analysis was performed to determine the structural changes in the fibrin organization

Solvent-like ligand-coated ultrasmall cadmium selenide nanocrystals: Strong electronic coupling in a self-organized assembly

Strong inter-nanocrystal electronic coupling is a prerequisite for delocalization of exciton wave functions and high conductivity. We report 170 meV electronic coupling energy of short chain poly(ethylene glycol) thiolate-coated ultrasmall (<2.5 nm in diameter) CdSe semiconductor nanocrystals (SNCs) in solution. Cryo-transmission electron microscopy analysis showed the formation of a pearl-necklace assembly of nanocrystals in solution with regular inter-nanocrystal spacing. The electronic coupling was studied as a function of CdSe nanocrystal size where the smallest nanocrystals exhibited the largest coupling energy. The electronic coupling in spin-cast thin-film (<200 nm in thickness) of poly(ethylene glycol) thiolate-coated CdSe SNCs was studied as a function of annealing temperature, where an unprecedentedly large, ∼400 meV coupling energy was observed for 1.6 nm diameter SNCs, which were coated with a thin layer of poly(ethylene glycol) thiolates. Small-angle X-ray scattering measurements showed that CdSe SNCs maintained an order array inside the films. The strong electronic coupling of SNCs in a self-organized film could facilitate the large-scale production of highly efficient electronic materials for advanced optoelectronic device application