Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R31: Membranes, Micelles, and Vesicles |
Hide Abstracts |
Sponsoring Units: DSOFT Chair: Joseph Zasadzinski, University of Minnesota Room: 503 |
Thursday, March 5, 2020 8:00AM - 8:12AM |
R31.00001: A super-swelled lamellar lipid system Jacob Rueben, Dylan Steer, Cecilia Leal Lipid bilayers are an integral structure found in myriad biological systems. Sometimes lipid bilayers stack to make a lamellar phase, in which bilayers are separated by water layers with thicknesses determined by intermolecular interactions such as undulation repulsion, attractive van der Waals forces, and in the case of charged lipids, double layer forces. Water layers in charged lipid systems have demonstrated swelling abilities up to 30 nm, but soft (low bending rigidity) bilayers usually lose periodicity approaching these values due to higher undulation force effects [1]. Here we report a simple lipid system of low-bending-rigidity lipid doped with small amounts of charged lipid that forms a previously undocumented super-swelled lamellar phase with layer spacings upwards of 80 nm at low dilution. These results differ from previous studies into similar systems, and defy currently held theories of maximum lamellar spacings for soft membranes. The formulated lipid systems were characterized with confocal fluorescence microscopy, cryogenic TEM, and ultra-small angle X-ray scattering. |
Thursday, March 5, 2020 8:12AM - 8:24AM |
R31.00002: Characterization of phospholipid monolayer viscoelasticity using microbubble acoustic radiation force Awaneesh Upadhyay, Outi Supponen, Francesco Guidi, Hendrik Vos, Pero Tortoli, Mark Borden Lipid monolayers (LM) stabilize the gas/water interface of the lung, ear and eye, and serve as model membrane for understanding the properties of bilayer leaflets. However, the nature of viscoelasticity of gel-phase LM for dilatational deformations remains poorly characterized. Microbubbles offer a novel experimental platform to study LM viscoelasticity, and their behavior is directly relevant to medical ultrasonics applications. The acoustic radiation force acting on a microbubble (MB) is maximal at the resonance frequency, which depends on its size and the LM shell viscoelastic properties. Here, we demonstrate a method using multi-gate spectral Doppler with an open source ultrasound scanner to measure the acoustic radiation force displacements of individual resonant MBs as we scan a frequency range between 3 and 7 MHz. The LM shell viscoelastic properties are determined by fitting the measured displacements to a modified Rayleigh-Plesset equation with Sarkar shell terms. Our results show the effects of lipid composition and temperature. These results provide mechanistic insights into LM dilatational viscoelasticity and illustrate rational design principles to engineer MBs for ultrasound imaging and therapy. |
Thursday, March 5, 2020 8:24AM - 8:36AM |
R31.00003: Tuning the microstructure of phospholipid monolayers using substrate curvature Joseph Barakat, Todd M Squires Two-dimensional monolayers and bilayers of phospholipids exhibit a heterogeneous microstructure, which is essential for fluidity, mechanical stability, and spatial localization of proteins, cholesterol, and signaling molecules. There is a growing body of evidence that lipid morphology is influenced by the curvature of the underlying template. Although this geometric dependence remains poorly understood, it becomes extremely relevant to the study of monolayers (e.g., in lung alveoli and microemulsions) and bilayers (in cells, organelles, and vesicles) with radii of curvature on the order of 10-100 microns. In this work, I will discuss how continuum-level theory can predict the effect of curvature on the phase behavior of lipid monolayers, using planar and spherical geometries as model templates. First, I will demonstrate that curvature decreases the expected radius of condensed domains due to the attenuation of dipolar repulsions. Second, I will show that curvature stabilizes circular domains (“droplets”) from elliptic distortions into elongated structures (“stripes”). The transition between these states is determined by the substrate curvature and condensed-phase area fraction, which parametrize the phase space of lipid morphologies on curved templates. |
Thursday, March 5, 2020 8:36AM - 8:48AM |
R31.00004: Molecular dynamics study on encapsulation of ibuprofen and indomethacin in Triton X-100 micelles Hrachya Ishkhanyan, Dave Barlow, Christian D. Lorenz Surfactants are widely used in biological applications including the extraction of proteins from cell membranes, the enhancement of the permeability of cell membranes1 and the encapsulation of poorly soluble drug molecules2. In this study, drug encapsulation capabilities of a non-ionic surfactant Triton X-100 were studied using molecular dynamics (MD) simulations. An all-atomic simulation has been performed in an aqueous solution of pre-assembled surfactants and drug molecules using the CHARMM force-field. The micelle becomes saturated with ibuprofen molecules after approximately around 50% of them are encapsulated within 50ns. Whereas the micelle does not get saturated with indomethacin until approximately 70% of the available molecules are encapsulated. Furthermore, the simulation shows that in the presence of drug molecules the semi-spherical micelle becomes increasingly rod-like in shape. More analysis shows that the volume as well as the surface area of the micelle expand significantly in the presence of indomethacin, and are fairly the same in the presence of ibuprofen. |
Thursday, March 5, 2020 8:48AM - 9:00AM |
R31.00005: Faceted Shapes of Cooled Emulsion Droplets of CTAB and Hexadecane Abdullah Hashim, Subas Dhakal Recent experiments [Denkov et al., Nature 528 (2015) 392] have shown that the emulsion droplets resulting from the spontaneous self-assembly of cetyltrimethylammonium bromide (CTAB) surfactants with alkanes of similar carbon chain length such as Hexadecane in water “self- shape” into regular polyhedral, platelets, and rods on cooling. To understand the mechanism of the faceting of those emulsion droplets, we performed the coarse-grained (CG) molecular dynamics simulation of aqueous solutions of CTAB and Hexadecane using the Martini force field in GROMACS. In 1 (CTAB): 1(Hexadecane) solutions, we find that the average droplet size scales as~C0.4 with the concentration of Hexadecane (C). A variety of polyhedral shapes of the emulsion droplets observed at low temperatures in simulations will be compared with those of the experiments. The mechanism for faceting of the droplets will be discussed. |
Thursday, March 5, 2020 9:00AM - 9:12AM |
R31.00006: Pore Formation: How Cubic Lipid-siRNA Constructs Escape the Endosome Lining Zheng, Cecilia Leal Small interfering RNA (siRNA) silences gene expression and has shown great potential in medical applications. However, intracellular delivery of siRNA remains a great challenge. Lipid nanoparticles have been one of the most successful siRNA carriers to date, but their delivery efficiency is limited due to low endosomal release. We have shown that cubic structured lipid nanoparticles (cubosomes) loaded with siRNA - cuboplexes, show greater siRNA knockdown compared to traditional lamellar structured liposomes. In this work, we aim to elucidate the reason behind this difference in delivery efficiency. We will show confocal microscopy (CF), flow cytometry and live cell imaging data demonstrating higher cellular uptake of cubosomes compared to liposomes. CF, fluorescent based fusion assays, cryogenic transmission electron microscopy and dynamic light scattering measurements indicate that compared to liposomes, cubosomes are more prone to fuse and form aggregates with endosomes, which implies a stronger pore forming capability that leads to siRNA escape. These results support our suggestion that cubosomes can lower the free energy required to promote endosome pore formation and establish a topologically active delivery mechanism. |
Thursday, March 5, 2020 9:12AM - 9:24AM |
R31.00007: Osmotic Shrinking of Vesicles Embedded in Hydrogels Natasha Rigby, Margarita Staykova Lipid vesicles can be dispersed in various gel matrices as an encapsulation technology and as a model system for cells supported by an extracellular matrix. |
Thursday, March 5, 2020 9:24AM - 9:36AM |
R31.00008: Mechanically mediated interactions between solid domains in composite vesicles Hao Wu, Maria Santore, Gregory Grason We explore the structure and interactions of fluid/solid composite vesicles using continuum models compared with experimental studies of phase-separated two-component vesicles. In contrast to well-known fluid-fluid phase-separated domain structure, we show that shear rigidity of solid phospholipid domains coexisting with a fluid phase produces qualitatively new collective behaviors in composite vesicles. Beyond simply an enhanced bending stiffness, shear rigidity of the solid domains tends to expel Gaussian curvature into the fluid membrane phase, an effect which generically competes with the global spherical topology of the vesicle. We show that fluid membrane elasticity leads to bending-mediated interactions between solid domains, controlled by their size and the vesicle area-to-volume ratio. For sufficiently tensed vesicles we find that fluid phase bending induces a depletion-like attraction between solid domains of sufficiently large size, driven by tendency to consolidate elastic high-bending “hinges” that flank the solid domains at high pressure. This result suggests a qualitatively new picture of large solid domains as “2D colloids” dispersed in a fluid background, whose effective interactions are tunable through global vesicle properties. |
Thursday, March 5, 2020 9:36AM - 9:48AM |
R31.00009: Biomimetic Lung Surfactant Nanodrops for Acoustic Droplet Vaporization Alec N Thomas, Mark Borden Acoustic droplet vaporization (ADV) involves the liquid-to-gas phase conversion of a superheated emulsion droplet by ultrasound to form an echogenic bubble. This technology may be useful for medical ultrasound, as nanodroplets small enough to leak through endothelial fenestrations may be converted to echogenic microbubbles for extravascular ultrasound imaging of inflamed and angiogenic vasculature. Additionally, droplets may be transformed to acoustically pulsating microbubbles to enhance ultrasound-guided drug delivery. However, surfactant coverage on the droplet often fails to stabilize the expanding interface during ADV, resulting in transitory microbubbles with limited utility. Here we show that interfacial melting and spreading by lung surfactant during surface dilation can be harnessed to increase the echogenicity and stability of post-ADV microbubbles. Lung surfactant, whose composition in the mammalian lung has been honed over millions of years of evolution, has thus proven to be a superior coating material for ADV droplets, and its biomimicry will be an important step toward clinical translation of ADV in ultrasound imaging and therapy. |
Thursday, March 5, 2020 9:48AM - 10:00AM |
R31.00010: Microsecond Coarse-Grained MD Simulations of Beta-Amyloid Fibrils Binding to Phase Separated Lipid Raft Domains Reveal Diverse Membrane-bounded Conformations and Binding Kinetics of Protein Depending on the Fibril size and Lipid Domain Structure Kwan Cheng, Sara Cheng, Yiyi Cao, M Rouzbehani, Ronald J Davenport-Dendy, A Urby, S Spurlock An early event associated with several protein aggregation diseases is the binding of amyloidogenic fibrils to cell membranes. At present, the binding conformation and kinetics of these fibrils on structurally heterogeneous and dynamic cholesterol-enriched raft domains remain elusive. We have constructed 4 coarse-grained (CG) beta-amyloid fibrils, ranging from dimer to pentamer, and 5 phase-separated CG lipid rafts, containing phospholipid and cholesterol or tail- or headgroup modified cholesterol. Using CG-MD simulations, the binding conformation, kinetics and interaction energetics of each fibril from solution to the rafts were studied. Within 20 μs, all fibrils bound to the raft surface and formed stable fibril/raft complexes at the liquid-ordered and -disordered phase boundary. The dimer fibrils bound to the raft exclusively via its hydrophobic C-terminal (C-state). Other than the C-state, the larger fibrils bound to the rafts via the hydrophilic N-terminal (N-state) and termini (T-state) with weaker binding energies. Interestingly, a transmembrane inserted state (I-state) of a trimer fibril was found for the raft containing tailgroup modified cholesterol. Our study indicates diverse conformations, kinetics and energetics of fibril/raft interactions in cells. |
Thursday, March 5, 2020 10:00AM - 10:12AM |
R31.00011: An implicit lipid model to simulate reaction-diffusion of proteins
binding to membrane surface YIBEN FU Localization of proteins to a membrane surface is an essential step in a broad range of biological processes such as signaling, virion formation, and clathrin-mediated endocytosis. The strength and specificity of proteins binding to a membrane depend on the lipid composition. Single-particle reaction-diffusion method is a powerful tool for capturing lipid-specific binding to membrane by treating lipids explicitly as individual diffusable binding sites. However, modeling lipid particles is computationally expensive. Here we present an algorithm for reversible binding of proteins to continuum surfaces with implicit lipids, providing dramatic speed-ups to many body simulations. The kinetics show excellent agreement between our method and the full explicit lipid model. Crucially, we demonstrate our method's application to membranes of arbitrary curvature and topology, modeled via a subdivision limit surface. We also utilize this method to describe experimental data of membrane binding and the feedback from the curvature generation. Our method will enable efficient cell-scale simulations involving proteins localizing to realistic membrane models, which is a critical step for predictive modeling and quantification of in vitro and in vivo dynamics. |
Thursday, March 5, 2020 10:12AM - 10:24AM |
R31.00012: Relationship among Phase Behavior, Micellar Structure and Thin Film Drainage in Aqueous Surfactant Solutions Shang Gao, Chrystian Ochoa, Vivek Sharma, Samanvaya Srivastava Sodium naphthenates (NaNs) are petrochemical anionic surfactants that stabilize petroleum emulsions and foams and pose significant environmental challenges when released into water bodies. Relatively little is known about the phase behavior and self-assembly (including micelle formation) as well as thin film drainage kinetics of aqueous NaN self-assemblies, impeding the development of strategies for NaN sequestration and petroleum foams and emulsions destabilization. |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R31.00013: Molecular Assembly of Surfactant Mixtures in Oil-Swollen Micelles: Implications for High Salinity Colloidal Stability Hsieh Chen, Ayrat Gizzatov, Amr I Abdel-Fattah Alkylbenzene sulfonates are one of the most important synthetic surfactant families, considering their wide applicability, cost-effectiveness, and overall consumption levels. Nevertheless, their low salt tolerance (especially divalent ion contents) prevented wider applications such as enhanced oil recovery in high salinity reservoirs. Here, using experiments and atomistic molecular dynamics simulations, we demonstrated that the high salinity colloidal stability of alkylbenzene sulfonates can be dramatically increased by mixing with zwitterionic co-surfactants in oil-swollen micelles. By mixing with co-surfactants we had two important observations: (1) The polydispersity of surfactant mixture oil-swollen micelles were largely decreased due to the less rigid oil/water interfaces with mixed surfactants, which formed fewer but larger uniform micelles. (2) Strong dehydration of sulfonates due to the shielding from protruding more extended zwitterionic co-surfactants at the oil/water interfaces. Both observed molecular assembly characteristics triggered by the co-surfactants effectively reduced the total exposure of sulfonates to the water phase which may form divalent ion bridging and large aggregates, thus increasing the high salinity colloidal stability. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700