Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z4: Biological Polyelectrolytes |
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Sponsoring Units: DPOLY DBP Chair: Garegin Papoian, University of North Carolina Room: 306/307 |
Friday, March 20, 2009 11:15AM - 11:51AM |
Z4.00001: How mechanical properties of DNA relate to chromosome structure Invited Speaker: |
Friday, March 20, 2009 11:51AM - 12:27PM |
Z4.00002: Electrostatic Rigidity of Biological Polyelectrolytes Invited Speaker: Electrostatic persistence length is one of the most controversial subjects in polymer physics. In this talk I will present an overview of the history of the problem and a new results showing that the bending rigidity of the biological polyelectrolytes (semiflexible charged polymers) is a multiscale process. The existence of the different length scales in the bending process is manifested in change of the form of the correlation function, describing bond-bond orientational correlations, with salt concentration. At high salt concentrations when the electrostatic interactions are significantly screened these correlations may be approximated by a single exponential function indicating the existence of a dominant length scale. However, when the Debye screening length exceeds a critical value the orientational correlations between chain's bond vectors undergo a qualitative change resulting in appearance of two different bending rigidities (persistence lengths). One increases quadratically and the other changes logarithmically with the Debye screening length. This transition occurs when the chain's bare persistence length becomes on the order of so-called OSF electrostatic persistence length. Simulation results and theoretical model demonstrate good qualitative agreement. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 1:03PM |
Z4.00003: Deafness and espin-actin self-organization in stereocilia Invited Speaker: Espins are F-actin-bundling proteins associated with large parallel actin bundles found in hair cell stereocilia in the ear, as well as brush border microvilli and Sertoli cell junctions. We examine actin bundle structures formed by different wild-type espin isoforms, fragments, and naturally-occurring human espin mutants linked to deafness and/or vestibular dysfunction. The espin-actin bundle structure consisted of a hexagonal arrangement of parallel actin filaments in a non-native twist state. We delineate the structural consequences caused by mutations in espin's actin-bundling module. For espin mutation with a severely damaged actin-bundling module, which are implicated in deafness in mice and humans, oriented nematic-like actin filament structures, which strongly impinges on bundle mechanical stiffness. Finally, we examine what makes espin different, via a comparative study of bundles formed by espin and those formed by fascin, a prototypical bundling protein found in functionally different regions of the cell, such as filopodia. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:39PM |
Z4.00004: Mesoscale Modeling of Chromatin Folding Invited Speaker: Eukaryotic chromatin is the fundamental protein/nucleic acid unit that stores the genetic material. Understanding how chromatin fibers fold and unfold in physiological conditions is important for interpreting fundamental biological processes like DNA replication and transcription regulation. Using a mesoscopic model of oligonucleosome chains and tailored sampling protocols, we elucidate the energetics of oligonucleosome folding/unfolding and the role of each histone tail, linker histones, and divalent ions in regulating chromatin structure. The resulting compact topologies reconcile features of the zigzag model with straight linker DNAs with the solenoid model with bent linker DNAs for optimal fiber organization and reveal dynamic and energetic aspects involved. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 2:15PM |
Z4.00005: Complexation of oppositely charged polyelectrolytes in gene delivery and biology Invited Speaker: Charge inversion of a DNA double helix by a positively charged flexible polymer (polyelectrolyte) is widely used to facilitate DNA contact with negative cell membranes for gene delivery. Motivated by this application in the first part of the talk I study the phase diagram a solution of long polyanions (PA) with a shorter polycations (PC) as a function the ratio of total charges of PC and PA in the solution, $x$, and the concentration of monovalent salt. Each PA attracts many PCs to form a complex. When $x= 1$, the complexes are neutral and condense in a macroscopic drop. When $x$ is far away from 1, complexes are strongly charged and stable. PA are overcharged by PC at $x > 1$ and undercharged by PC at $x < 1$. As $x$ approaches 1, PCs attached to PA disproportionate between complexes. Some complexes become neutral and condensed in a macroscopic drop while others become even stronger charged and stay free. The second part of the talk deals with biological example of PA -PC complexes namely self-assembly of vegetable viruses from long ss-RNA molecule paying role of scaffold and identical capsid proteins with long positive tails. I show that optimization Coulomb energy of the virus leads to the charge of RNA twice larger than the total charge of the capsid, in agreement with the experimental data. Then I discuss kinetics of the Coulomb complexation driven virus self-assembly. Capsid proteins stick to unassembled chain of ss RNA (which we call ``antenna'') and slide on it towards the assembly site. I show that at excess of capsid proteins such one-dimensional diffusion accelerates self-assembly more than ten times. On the other hand at excess of ss-RNA, antenna slows self-assembly down. Several experiments are proposed to verify the role of ss-RNA antenna in self-assembly. [Preview Abstract] |
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