Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J10: Focus Session: Interactions in Biological Systems |
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Sponsoring Units: DBIO Chair: Todd Sulchek Room: 201 |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J10.00001: Mechanotransduction at the Immunological Synapse Invited Speaker: Cheng Zhu |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J10.00002: Immune cell activation from multivalent interactions with liquid-crystalline polycation-DNA complexes Nathan Schmidt, Fan Jin, Roberto Lande, Tine Curk, Wujing Xian, Loredana Frasca, Jure Dobnikar, Daan Frenkel, Michel Gilliet, Gerard Wong Microbial DNA can trigger type I interferon (IFN) production in plasmacytoid cells (pDCs) by binding to endosomal toll-like receptor 9 (TLR9). TLR9 in pDCs do not normally respond to self-DNA, but in certain autoimmune diseases self-DNA can complex with the polycationic antimicrobial peptide LL37 into condensed structures which allow DNA to access endosomal compartments and stimulate TLR9 in pDCs. We use x-ray studies and cell measurements of IFN secretion by pDCs to show that a broad range of polycation-DNA complexes stimulate pDCs and elucidate the criterion for high IFN production. Furthermore, we show via experiments and computer simulations that the distinguishing factor for why certain complexes activate pDCs while others do not is the self-assembled structure of the liquid-crystalline polycation-DNA complex. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J10.00003: High Fc Density Particles Result in Binary Complement Activation but Tunable Macrophage Phagocytosis Todd Sulchek, Patricia Pacheco, David White Macrophage phagocytosis and complement system activation represent two key components of the immune system and both can be activated through the presentation of multiple Fc domains of IgG antibodies. We have created functionalized micro- and nanoparticles with various densities of Fc domains to understand the modulation of the immune system for eventual use as a novel immunomodulation platform. Phagocytosis assays were carried out by adding functionalized particles to macrophage cells and quantitatively determined using fluorescent microscopy and flow cytometry. Complement system activation by the functionalized particles in human serum was quantified with an enzyme immunoassay. Our phagocytosis assay revealed a strong dependence on particle size and Fc density. For small particles, as the Fc density increased, the number of particles phagocytosed also increased. Large particles were phagocytosed at significantly lower levels and showed no dependency on Fc density. Complement was successfully activated at levels comparable to positive controls for small particles at high Fc densities. However at low Fc densities, there is a significant decrease in complement activation. This result suggests a binary response for complement system activation with a threshold density for successful activation. Therefore, varying the Fc density on micro/nanoparticles resulted in a tunable response in macrophage phagocytosis while a more binary response for complement activation. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J10.00004: Can correlations among receptors affect the information about the stimulus? Vijay Singh, Martin Tchernookov, Ilya Nemenman In the context of neural information processing, it has been observed that, compared to the case of independent receptors, correlated receptors can often carry more information about the stimulus. We explore similar ideas in the context of molecular information processing, analyzing a cell with receptors whose activity is intrinsically negatively correlated because they compete for the same ligand molecules. We show analytically that, in case the involved biochemical interactions are linear, the information between the number of molecules captured by the receptors and the ligand concentration does not depend on correlations among the receptors. For a nonlinear kinetic network, correlations similarly do not change the amount of information for observation times much shorter or much longer than the characteristic time scale of ligand molecule binding and unbinding. However, at intermediate times, correlations can increase the amount of available information. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 4:18PM |
J10.00005: Mechanisms of Cellular De-adhesion Invited Speaker: James Hone |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J10.00006: Three-component homeostasis control Jin Xu, Hyunsuk Hong, Junghyo Jo Two reciprocal components seem to be sufficient to maintain a control variable constant. However, pancreatic islets adapt three components to control glucose homeostasis. They are $\alpha$ (secreting glucagon), $\beta$ (insulin), and $\delta$ (somatostatin) cells. Glucagon and insulin are the reciprocal hormones for increasing and decreasing blood glucose levels, while the role of somatostatin is unknown. However, it has been known how each hormone affects other cell types. Based on the pulsatile hormone secretion and the cellular interactions, this system can be described as coupled oscillators. In particular, we used the Landau-Stuart model to consider both amplitudes and phases of hormone oscillations. We found that the presence of the third component, $\delta$ cell, was effective to resist under glucose perturbations, and to quickly return to the normal glucose level once perturbed. Our analysis suggested that three components are necessary for advanced homeostasis control. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J10.00007: Surface binding of polymer coated nanoparticles: Coupling of physical interactions, molecular organization, and chemical state Rikkert Nap, Igal Szleifer A key challenge in nanomedicine is to design carrier system for drug delivery that selectively binds to target cells without binding to healthy cells. A common strategy is to end-functionalize the polymers coating of the delivery device with specific ligands that bind strongly to overexpressed receptors. Such devices are usually unable to discriminate between receptors found on benign and malignant cells. We demonstrate, theoretically, how one can achieve selective binding to target cells by using multiple physical and chemical interactions. We study the effective interactions between a polymer decorated nanosized micelle or solid nanoparticle with model lipid layers. The polymer coating contains a mixture of two polymers, one neutral for protection and the other a polybase with a functional end-group to optimize specific binding and electrostatic interactions with the charged lipid head-groups found on the lipid surface. The strength of the binding for the combined system is much larger than the sum of the independent electrostatic or specific ligand-receptor binding. The search for optimal binding conditions lead to the finding of a non-additive coupling that exists in systems where chemical equilibrium, molecular organization, and physical interactions are coupled together. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J10.00008: Coherence and energy transport in molecular aggregates: stochastic approach Darius Abramavicius, Vytautas Abramavicius, Vladimir Chorosajev Recent spectroscopy studies of various molecular assemblies have shown that optically induced quantum coherences in these systems survive much longer than predicted from standard rate equations. This result sparked numerous debates whether the coherent system dynamics are related to the efficiency and/or speed of the excitation energy transfer in such systems. The problem could be addressed by studying coherent excitation dynamics and its relaxation due to interaction with the bath. The reduced density matrix propagation theories provide the reduced/averaged information on the dynamics. Stochastic approaches allow accessing more detailed microscopic picture. Two types of stochastic equations have been derived for a system coupled to the bath of an arbitrary spectral density. The stochastic wave functions allowed to define excitation coherent dynamics, polaron formation dynamics and energy relaxation times together with energy transport pathways in molecular aggregates. Simulations of the energy transport in few model molecular aggregates were performed using both approaches for the system wavefunction. It was demonstrated that the quantum coherences in the system appearing from mixture of vibrational and excitonic resonances significantly affect the energy transport process. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J10.00009: Radical-pair Based Avian Magnetoreception: Robustness and Optimality Maria Procopio, Thorsten Ritz Behavioural experiments suggest that migratory birds possess a magnetic compass sensor able to detect the direction of the geomagnetic. One hypothesis for the basis of this remarkable sensory ability is that the coherent quantum spin dynamics of photoinduced radical pair reactions transduces directional magnetic information from the geomagnetic field into changes of reaction yields, possibly involving the photoreceptor cryptochrome in the birds retina. The suggested radical-pair based avian magnetoreception has attracted attention in the field of quantum biology as an example of a biological sensor which might exploit quantum coherences for its biological function. Investigations on such a spin-based sensor have focussed on uncovering the design features for the design of a biomimetic magnetic field sensor. We study the effects of slow fluctuations in the nuclear spin environment on the directional signal. We quantitatively evaluate the robustness of signals under fluctuations on a timescale longer than the lifetime of a radical pair, utilizing two models of radical pairs. Our results suggest design principles for building a radical-pair based compass sensor that is both robust and highly directional sensitive. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J10.00010: Influence of thermal light correlations on photosynthetic structures Adriana De Mendoza, Pedro Manrique, Felipe Caycedo-Soler, Neil F. Johnson, Ferney J. Rodr\'iguez, Luis Quiroga The thermal light from the sun is characterized by both classical and quantum mechanical correlations. These correlations have left a fingerprint on the natural harvesting structures developed through five billion years of evolutionary pressure, specially in photosynthetic organisms [1]. In this work, based upon previous extensive studies of spatio-temporal correlations of light fields, we hypothesize that structures involving photosensitive pigments like those present in purple bacteria vesicles emerge as an evolutionary response to the different properties of incident light. By using burstiness and memory as measures that quantify higher moments of the photon arrival statistics, we generate photon-time traces. They are used to simulate absorption on detectors spatially extended over regions comparable to these light fields coherence length. Finally, we provide some insights into the connection between these photo-statistical features with the photosynthetic membrane architecture and the lights' spatial correlation.\\[4pt] [1] N. Johnson et al. ``Extreme alien light allows survival of terrestrial bacteria,'' Nature Scientific Reports 3, 2198 (2013) doi:10.1038/srep02198. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J10.00011: Radical-Ion-Pair Spin Decoherence and the Quantum Efficiency of Photosynthetic Charge Separation Iannis Kominis, A.T. Dellis We have pioneered the fundamental quantum dynamics of radical-ion-pair reactions, elucidating the basic spin-decoherence mechanism pertaining to these biochemical reactions. Radical-ion pair reactions appear in the avian magnetic compass, but more importantly, they participate in the cascade of electron-transfer reactions taking place in photosynthetic reaction centers. We will here present new insights on how the fundamental quantum dynamics of radical-ion pair reactions affect the quantum efficiency of charge separation in photosynthetic reaction centers. [Preview Abstract] |
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