Double twist liquid crystal model of collagen structure
Aidan Brown Dalhousie University
PIRSA:13120023
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Thermodynamics of pore formation in lipid bilayers
Drew Bennett University of Waterloo
PIRSA:13120022 -
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How are cell concentrations implicated in activity and selectivity of antimicrobial peptides?
Azadeh Bagheri University of Waterloo
PIRSA:13120020 -
Dynamic Force Patterns of an Undulatory Microswimmer
Rafael Shulman McMaster University
PIRSA:13120038 -
The viscoelastic properties of the nematode C. elegans
Matilda Backholm McMaster University
PIRSA:13120019 -
Dynamics of the magnetic disc in nematic liquid crystal under the action of magnetic field
Alena Antipova Western University
PIRSA:13120018 -
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Slow Melting and Fast Crystals
PIRSA:13120015 -
Circumferential gap propagation in an anisotropic elastic bacterial sacculus
Andrew Rutenberg Dalhousie University
PIRSA:13120014 -
Predicting plasticity with soft vibrational modes: from dislocations to glasses
Joerg Rottler University of British Columbia
PIRSA:13120013
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Double twist liquid crystal model of collagen structure
Aidan Brown Dalhousie University
PIRSA:13120023Collagen is the main component of connective tissue and the most abundant protein in mammals. The structure of collagen is hierarchical with the triple-helical molecules organizing into fibrils and fibrils contained in higher-order arrangements. A fibril may be considered as a liquid crystal of individual triple helices. Their chiral molecular structure can lead to a macroscopic helical arrangement known as the cholesteric phase which has been observed in fragments of collagen fibrils. The cholesteric orientation can vary with radial distance in the fibril as a double twist. We numerically minimize mean-field Frank free energy in the bulk to solve for the liquid crystal orientation as a function of radial distance (r). By also considering surface terms the overall energy per fibril area is minimized to find the optimal fibril radius R and molecule orientation on the fibril surface (R) both of which may be compared to experimental measurements. -
Thermodynamics of pore formation in lipid bilayers
Drew Bennett University of Waterloo
PIRSA:13120022Lipid bilayers form the basic structure of cellular membranes creating a semi-permeable barrier necessary for separating distinct chemical environments. Hydrophilic pores can form in bilayers that breach the barrier potentially causing cell death or enhance the uptake of hydrophilic molecules. We use molecular dynamics simulations and free energy calculations to investigate pore formation in model bilayers. The free energy barrier for pore formation is much lower in thinner phosphatidylcholine bilayers compared to thicker bilayers. The free energy barrier is due to an unfavourable change in entropy while the enthalpy of pore formation is favourable. These results have implications on a wide range of biological and biotechnological applications such as drug delivery antimicrobial peptides and transmembrane structure and stability. -
Transport in molecular scale
Behnaz Bagheri Varnousfaderani University of Waterloo
PIRSA:13120021Recently there has been a large growth of research effort for nanoelectronic devices.Investigations of quantumly coherent nano-meter scale systems whose fabrication has been made possible by recent advances in experimental and sample preparation techniques have revealed that transport properties could be non-Ohmic and G could be quantized. Understanding electron conduction in such devices is an extremely active research topic. Our theoretical goal is to predict quantum transport properties of molecular nanodevices including their I-V characteristics from first principle theoretical method. -
How are cell concentrations implicated in activity and selectivity of antimicrobial peptides?
Azadeh Bagheri University of Waterloo
PIRSA:13120020Antimicrobial peptides (AMPs) are known to be active against a wide range of microbes. Cell selectivity is an important quality of AMPs which enables them to preferentially bind to and kill the microbes over host cells. Despite its significance in determining the cell selectivity however the cell-concentration dependence of AMP activity has not been criticality examined. Here we present a coarse-grained model for describing how cell concentrations are implicated in AMP's membrane-perturbing activity and selectivity. -
Dynamic Force Patterns of an Undulatory Microswimmer
Rafael Shulman McMaster University
PIRSA:13120038C. elegans is a millimeter-sized nematode which has served as a model organism in biology for several decades primarily due to its simple anatomy. Using an undulatory form of locomotion this worm is capable of propelling itself through various media. Due to the small length scales involved swimming in this regime is qualitatively different from macroscopic locomotion because the swimmers can be considered to have no inertia. In order to understand the microswimming that this worm exhibits it is crucial to determine the viscous forces experienced during its motion. Using a micropipette deflection technique in conjunction with high speed imaging we have directly measured the time-varying forces generated by C elegans during swimming. Furthermore by analyzing the bodys kinematics over time and applying a simple model of locomotion we can compute the theoretical force curves. We observe excellent agreement between the measured and calculated forces. The success of this simple model has important implications in the understanding of microswimming in general. -
The viscoelastic properties of the nematode C. elegans
Matilda Backholm McMaster University
PIRSA:13120019Undulatory motion is utilized by crawlers and swimmers such as snakes and sperm at length scales spanning more than seven orders of magnitude. The understanding of this highly efficient form of locomotion requires an experimental characterisation of the passive material properties of the organism as well as of its active force output on the surrounding medium. The millimeter-sized nematode Caenorhabditis elegans provides an excellent biophysical system for both static and dynamic biomechanical studies. Here we present a novel experimental technique where the deflection of a force-calibrated micropipette is used to directly probe the viscoelastic material properties of C. elegans. The worm was shown to have a self-similar elastic structure as well as a surprising shear-thinning viscous component (M. Backholm W. S. Ryu and K. Dalnoki- Veress PNAS 110 (2013)). The excellent force (pN) and time (ms) resolution provided by the micropipette deflection technique also enables measurements of the drag forces experienced by the microswimmer as it moves through a liquid. This direct experimental characterisation of C. elegans provides guidance for theoretical treatments of undulatory locomotion in general. -
Dynamics of the magnetic disc in nematic liquid crystal under the action of magnetic field
Alena Antipova Western University
PIRSA:13120018We simulated Ni disc immersed in a liquid crystal using a lattice Boltzmann algorithm for liquid crystals. In the absence of external torques discs with homeotropic anchoring align with their surface normal parallel to the director of the nematic liquid crystal. In the presence of a weak magnetic field (<10G) the disc will rotate to equilibrate the magnetic torque and the elastic torque due to the distortion of the nematic director. When the magnetic field rotates the disc so that the angle between normal to the surface of the disc and director of the liquid crystal becomes greater than the disc goes through the transition in which . The analysis of this behavior was performed. -
Inferring the spatiotemporal DNA replication program from noisy data
PIRSA:13120017In eukaryotic organisms, DNA replication is initiated at “origins,” launching “forks” that spread bidirectionally to replicate the genome. The distribution and firing rate of these origins and the fork progression velocity form the “replication program.” With Antoine Baker, I generalize a stochastic model of DNA replication to allow for space and time variations in origin-initiation rates, characterized by a function I(x,t). We then address the inverse problem of inferring I(x,t) from experimental data concerning replication in cell populations. Previous work based on curve fitting depended on arbitrarily chosen functional forms for I(x,t), with free parameters that were constrained by the data. We introduce a model-free, non-parametric method of inference that is based on Gaussian process regression, a well-known inference technique from the machine-learning community. The method replaces specific assumptions about the functional form of the initiation rate with more general prior expectations about the smoothness of variation of this rate, along the genome and in time. Using this inference method, we can recover simulated replication schemes with data that are typical of current experiments without having to know or guess the functional form for the initiation rate I(x,t). I will argue that Gaussian process regression has many other potential applications to physics. -
Polymer translocation : alternative driving forces
Gary Slater University of Ottawa
PIRSA:13120016TBA -
Slow Melting and Fast Crystals
PIRSA:13120015This talk will focus on the behavior of colloidal crystals, and will describe both the nucleation and growth of crystals and their melting. The nucleation and growth of colloidal crystals is experimentally observed to be much faster than expected theoretically or through simulation. The discrepancy can be as much as 10150! I will describe some new experiments that suggest a possible reason for this. I will also describe the melting of colloidal crystals formed with highly charged particles that form a Wigner lattice. I will show that this melting resembles a second-order phase transition, and follows the prediction of Born for a catastrophic collapse of the elastic constant. -
Circumferential gap propagation in an anisotropic elastic bacterial sacculus
Andrew Rutenberg Dalhousie University
PIRSA:13120014We have modelled stress concentration around small gaps in anisotropic elastic sheets, corresponding to the peptidoglycan sacculus of bacterial cells, under loading corresponding to the effects of turgor pressure in rod-shaped bacteria. We find that under normal conditions the stress concentration is insufficient to mechanically rupture bacteria, even for gaps up to a micron in length. We then explored the effects of stress-dependent smart-autolysins, as hypothesised by Arthur L Koch. We show that the measured anisotropic elasticity of the PG sacculus can lead to stable circumferential propagation of small gaps in the sacculus. This is consistent with the recent observation of circumferential propagation of PG-associated MreB patches in rod-shaped bacteria. We also find a bistable regime of both circumferential and axial gap propagation, which agrees with behavior reported in cytoskeletal mutants of B. subtilis. We conclude that the elastic anisotropies of a bacterial sacculus, as characterised experimentally, may be relevant for maintaining rod-shaped bacterial growth. -
Predicting plasticity with soft vibrational modes: from dislocations to glasses
Joerg Rottler University of British Columbia
PIRSA:13120013We show how to utilize soft modes in the vibrational spectrum as a universal tool for the identification of defects in solids. Perfect crystals with isolated dislocations exhibit single phonon modes that localize at the dislocation core, and their polarization pattern predicts the motion of atoms during elementary dislocation glide in two and three dimensions in great detail. A superposition of soft modes can be used to construct a population of soft spots that predict the location of local plastic rearrangements at the grain boundaries of polycrystals and in amorphous solids. Additionally, we find a significant correlation between the soft directions of the polarization fields and the atomic displacements that result from elementary shear events.