Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y51: Fluids, Proteins, MicrobesFocus
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Sponsoring Units: DBIO Chair: Chih-Kuan Tung, North Carolina A&T State Univ Room: LACC 511C |
Friday, March 9, 2018 11:15AM - 11:51AM |
Y51.00001: Particle-Tracking Studies of Protein Layer Formation at Fluid Interfaces Invited Speaker: Robert Leheny The tendency for proteins to adsorb at air-water or oil-water interfaces and to create stiff interfacial layers impacts current and developing technologies in the biomedical and pharmaceutical industries. Knowledge about the evolution of the rheology of protein layers is crucial for understanding the mechanisms driving layer formation, which can further provide a useful perspective on issues of protein denaturation, protein-protein interactions, and the gel transition. However, characterizing the rheology of such layers is difficult due to their confined geometry, their fragility, and the possibility of spatial heterogeneity. Particle tracking, which employs colloidal probes to interrogate the mechanical properties of soft materials, is a promising approach to investigate interfacial protein layers. This talk will describe particle-tracking studies of the time-dependent behavior of layers of the proteins beta-lactoglobulin and lysozyme adsorbing at air-water and oil-water interfaces. The experiments combine complementary passive and active techniques. In the passive measurements, the Brownian motion of ensembles of spherical colloids at the interface sheds light on incipient layers with modest interfacial viscosities and reveals transient mesoscale heterogeneity. In the active measurements, ferromagnetic nanowires at the interface rotate in response to magnetic torques, extending the range of measurements to stiffer layers and enabling characterization of nonlinear mechanical response. The talk will discuss approaches to connect the measured particle mobility with rheological parameters of the layers and the challenges of such interfacial microrheology. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y51.00002: Sizing Particulate Debris Ejected from Protein-Stabilized Microbubbles Xujun Zhang, Jinxin Fu, Paul Russo Protein-stabilized air microbubbles sometimes exhibit solid-like interfaces. The high interfacial modulus allows long term stability of the microbubbles. Under certain circumstances, such as temperature and pressure changes, the microbubbles reconfigure their shape or size, expelling debris in the form of particles. The small size size and presence of remaining microbubbles complicate size determination of these particles by dynamic light scattering or even particle tracking. Instead, we studied the local diffusive dynamics of expelled particulate debris using differential dynamic microscopy. The results show the debris freely follow Brownian diffusion. DDM proves to be an easy and powerful technique for such study. This suggests potential for applications such as controlled drug release from microbubbles. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y51.00003: Fibrinogen Adsorption Onto Phospholipid Monolayers: Aging and Stiffening Ian Williams, Todd Squires All mammals utilize lung surfactant (LS) to reduce surface tension at the alveolar interface and facilitate respiration. Serum proteins like fibrinogen often leak into the lung following injury, inactivating LS and developing into acute respiratory distress syndrome (ARDS). Motivated by the mechanical role fibrinogen may play in the progression of ARDS, we study mixed systems of fibrinogen and dipalmitoylphosphatidylcholine (DPPC), the main consituent of LS. As fibrinogen adsorbs to air/water interfaces, the interfacial rheology increases dramatically, well before the surface pressure changes in any measurable way. We find that DPPC is ineffective at displacing preadsorbed fibrinogen monolayers, and the resulting mixed monolayer has a strongly elastic shear repsonse. By contrast, a pre-existing DPPC monolayer is effective at preventing fibrinogen adsorption in its liquid condensed (LC) state, at relatively high surface pressures. Fibrinogen adsorbs to DPPC interfaces in the LC/LE coexistence region. Furthermore, the monolayer domain structures are qualitatively different when fibrinogen is added to DPPC, giving insight into mechanics of ARDS progression. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y51.00004: Spatial arrangement and orientation of cholesterol affect cholesterol diffusivity in lipid membranes Younghoon Oh, Bong June Sung The lateral diffusion and flip-flop of cholesterol are involved in multiple functions of lipid membranes such as the formation of cholesterol-enriched nanodomains. The subdiffusion of cholesterol, where its lateral mean square displacement scales with time t as 〈Δr2(t)〉 ∼ tα with 0 < α < 1, spans up to several hundreds of nanoseconds. More interestingly, a recent study [1] reported that there were two regimes of subdiffusion with different exponents: the first regime with an exponent α1 at a short time scale (~ 1 ns) and the second one with α2 (0 < α1 < α2 < 1) at a longer time scale (~ 102 ns). The mechanism for such complicated cholesterol diffusion remains elusive. In this study, we employ coarse-grained molecular dynamics simulations to investigate the cholesterol transport in lipid membranes. We find that the cholesterol diffuses much faster when the cholesterol lies at the membrane center with horizontal orientation. On the other hand, the cholesterol diffusion is relatively slow when it stays at the leaflets. We also find that the center cholesterol undergoes subdiffusion at a short time scale (~ 1 ns), while leaflet cholesterol undergoes the subdiffusion at a longer timescale (~ 102 ns), which is consistent with previous studies. |
Friday, March 9, 2018 12:27PM - 1:03PM |
Y51.00005: Structural Changes in Proteins at Air-Water Interfaces Invited Speaker: Marek Cieplak We study the behavior of five proteins (protein G, egg-white lysozyme, hydrophobin, tryptophan cage and LTP1) at the air–water and oil–water interfaces by all-atom molecular dynamics. The proteins are found to change orientation and get distorted when pinned to the interface. This behavior is consistent with the empirical way of introducing the interfaces in a coarse-grained model through a force that depends on the hydropathy indices of the residues. Proteins couple to the oil–water interface stronger than to the air–water one. They diffuse slower at the oil–water interface but do not depin from it, whereas depinning events are observed at the other interface. The reduction of the disulfide bonds slows the diffusion down. We use the empirical coarse-grained model to study properties of protein layers at the air-water interface. In particular, we demonstrate existence of glassy effects as evidenced by slowing down of diffusion with increasing concentration of proteins. We also show that layers of two barley proteins, LTP1 and its ligand adduct LTP1b, flatten out at the interface and can make a continuous and dense film that should be responsible for formation and stability of foam in beer. The degree of the flattening depends on the protein - the layers of LTP1b should be denser than those of LTP1 – as well as on the presence of glycation and the degree of reduction in the number of disulfide bonds. We also show that the interfacial forces can untie proteins with shallow knots, but they can also make knots in proteins that are natively unknotted. The physics of proteins at the air-water interface can be captured by a simple lattice model which allows for larger statistics of the pinning-depinning processes and an analysis of a Marangoni-like effect induced by a temperature gradient. Collaboration with D. B. Allen, M. Chwastyk, R. L. Leheny, D. H. Reich and Y. Zhao. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y51.00006: Bilayer Mechanics of Protein-Induced Lipid Phase Separation Ahis Shrestha, Osman Kahraman, Christoph Haselwandter Recent experiments on the mechanosensitive channel of large conductance (MscL), as well as other membrane proteins, have suggested that membrane protein function can be mechanically modulated, in the highly heterogeneous membrane environments typically found in vivo, through the formation of lipid "platforms" of defined lipid composition around membrane proteins. We show here that the interplay of lipid bilayer mechanics and protein-induced lipid phase separation provides one possible physical origin of such lipid platforms. At the mean-field level, the lipid platforms surrounding MscL can be captured by coupling the energetics of protein-induced lipid bilayer thickness deformations to the Ginzburg-Landau model of phase separation. We find that protein-induced lipid phase separation can yield a rich dependence of bilayer-thickness-mediated interactions between membrane proteins on lipid bilayer composition. Our work provides a theoretical framework describing how the heterogeneous lipid compositions found in living cells affect the mechanics of bilayer-proteins interactions. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y51.00007: Controlled bioconvection of magnetotactic bacteria Nicolas Waisbord, Tyler Shendruk, Thomas Coons, Jorn Dunkel, Jeffrey Guasto Magnetotactic bacteria (MTB) synthesize magnetic nanoparticles (magnetosomes) in their cell membrane, and use earth magnetic field to efficiently navigate for resources in oceans and swamps. In this work, we demonstrate that active cell motility feeds back into the ambient Poiseuille flow to drive the formation of bioconvective cells when MTB are directed upstream. Dense suspensions of MTB are seeded with fluorescent tracers, and video microscopy simultaneously captures both the active bacterial and passive tracer motion, when MTB are driven upstream by a Helmholtz coil through a microfluidic channel. Experiments are complemented by numerical simulations to decipher the relative roles of the rotlet exerted by the external magnetic field on the MTB (passive) and the recirculating convective cells resulting from MTB self-propulsion (active) in the instability. Such collective behaviors in driven active matter systems may elucidate novel survival mechanisms in MTB ecosystems and have the potential to impact bioremediation and drug delivery applications that employ MTB. |
Friday, March 9, 2018 1:27PM - 1:39PM |
Y51.00008: Versatile phototactic behaviors of the chiral microswimmer Euglena gracilis Alan Cheng Hou Tsang, Amy Lam, Ingmar Riedel-Kruse The various taxis strategies of microorganisms regarding external stimuli highlight interesting solutions to control-feedback tasks bounded by biophysical constraints. Here we study the versatile phototactic behaviors of the microswimmer Euglena gracilis. These cells scan the surrounding light by rolling around their long axes while swimming along helical paths, thereby pointing their single directional photoreceptor periodically into various directions. How this detected time-varying light signal is coupled to active reorientation of the cell to achieve various phototaxis tasks effectively is not fully understood. We develop a biophysical model that describes the change of the cell’s pitch axis orientation and strength in dependence of the light intensity, which leads to a unified account of various phototactic behavioral states of these cells. We experimentally test this model using different spatiotemporal light stimuli and show how a simple monotonous dependence of the pitch axis orientation on light intensity can naturally evoke the appropriate taxis behavior given lighting conditions. This work has implication for taxis strategies of other natural and synthetic chiral micro-swimmers. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y51.00009: Swimming Bacteria Guided by Modulated Nematic Director Taras Turiv, Runa Koizumi, Chenhui Peng, Hao Yu, Yubing Guo, Qihuo Wei, O Lavrentovich Micro-swimmers of biological and artificial nature exhibit remarkable collective behavior. This behavior can be controlled by placing the micro-swimmers in a nematic liquid crystal (NLC) environment with orientational order. The NLC can control local concentration, trajectories and net flows of active bacteria at low activity level, in particular, trigger unidirectional circulation of swimming bacteria around topological defects†. In this work, we explore the collective motion of motile Bacillus subtilis dispersed in a water-based lyotropic chromonic NLC with spatially-varying director. The director field represents an alternating one-dimensional system of splay and bend stripes imposed through surface photoalignment. The bacteria exhibit threshold-less unidirectional collective motion along the splay bands. If a bacterium enters the patterned field with a “wrong” direction of swimming, the patterned director realigns it by 180° and the bacterium continue to swim along the same direction as other bacteria. The demonstrated unidirectional linear motion of bacteria can be used for micro-cargo transport. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y51.00010: Temperature-dependent crystal growth of D2O hydrating model cell membranes determined by neutron diffraction1 Zachary Buck, James Torres, Joe Schaeperkoetter, Helmut Kaiser, Haskell Taub, Andrew Miskowiec, Madhusudan Tyagi, Flemming Hansen Our previous quasielastic neutron scattering measurements have revealed a series of freezing transitions of water in proximity to a model zwitterionic (DMPC) and an anionic (DMPG) membrane down to temperatures of 255 K and 200 K, respectively. We have interpreted these freezing/melting transitions as evidence of different water types defined by their local environment [2]. Here we use neutron diffraction to determine the structure of the ice formed in the freezing transitions of both membranes. In all cases, our diffraction patterns reveal the formation of hexagonal ice oriented with its basal plane parallel to the membrane surface. The temperature dependence of D2O Bragg peak intensities measured at the University of Missouri Research Reactor is in excellent agreement with that of previous incoherent elastic scans conducted on different DMPC and DMPG samples, using the backscattering spectrometer HFBS at the NIST Center for Neutron Research. Moreover, in the case of the DMPG membrane, the diffraction patterns allow us to differentiate between the structure of hexagonal ice formed from bulk-like water and that from confined water by the degree of lattice strain present. |
Friday, March 9, 2018 2:03PM - 2:15PM |
Y51.00011: Theoretical Analysis of Allosteric and Operator Binding for Cyclic-AMP Receptor Protein Mutants Tal Einav, Julia Duque, Rob Phillips Allosteric transcription factors undergo binding events both at their operator binding sites as well as at distinct allosteric sites, and it is often difficult to disentangle the structural and functional consequences of the two types of binding. In this work, we compare the ability of two statistical mechanical models - the Monod-Wyman-Changeux (MWC) and the Koshland-Némethy-Filmer (KNF) models of allostery - to characterize the multi-step activation mechanism of the cyclic-AMP receptor protein (CRP). We examine data from a recent experiment that created a single-chain version of the CRP homodimer, enabling each subunit to be mutated separately. Using this construct, six mutants were created using the wild type subunit, a D53H mutant subunit, and an S62F mutant subunit. We show that both the MWC and KNF models can simultaneously characterize the cyclic-AMP and DNA binding of all six CRP constructs based solely on their subunit compositions, thereby tying together the behavior of the mutants to a small, self-consistent set of parameters. |
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