Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R26: Focus Session: Counterion Dynamics in Charged Biopolymer Systems |
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Sponsoring Units: DBP DPOLY Chair: Alex Travesset, Ames Laboratory Room: Baltimore Convention Center 323 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R26.00001: Counterion liquids between biological polyelectrolytes Invited Speaker: Electrostatic interactions between charged surfaces in water are dominated by counterion behavior. For example, repulsion between like-charged objects are due to the osmotic pressure of compressed counterions, and attraction between oppositely-charged objects are due to the entropy gain of counterion release. In systems with strong electrostatic interactions, mean field theories break down and counterion correlations and dynamics become important. To explain counterintuitive phenomena such as like-charge attraction between polyelectrolytes, exotic models such as counterion Wigner lattices were proposed, but no experimental comparisons at sufficient spatial resolution exist. Using inelastic x-ray scattering, the spatiotemporal correlations of counterions that mediate binding between charged polymers have been measured in aqueous solution down to molecular length-scales. We find that these ions are hierarchically organized into a dense, strongly correlated liquid that exhibits an acoustic-like phonon mode. The excitation spectra at high wave-vector Q reveal unexpected dynamics due to ions interacting with their `cages' of nearest neighbors. The measured speed of sound and collective relaxation rates in this liquid agree well with simple model calculations. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R26.00002: Bjerrum Pairing Correlations at Charged Interfaces Alex Travesset, David Vaknin Electrostatic correlations play a fundamental role in aqueous solutions. In this letter, we identify transverse and lateral correlations as two mutually exclusive regimes. We show that the transverse regime leads to binding by generalization of the Bjerrum pair formation theory and point out that this purely electrostatic correlation becomes dominant for strongly charged interfaces. We compare our theoretical predictions with different experiments on charged membranes and Langmuir monolayers and find good agreements. We contrast our approach with existing theories in the strong coupling limit and on charged modulated interfaces and discuss different scenarios leading to charge reversal and equal-charge attraction by macro-ions. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:54PM |
R26.00003: Charged Polymers in Electric Fields Invited Speaker: Charged polymers in solution are considered under the action of external electric fields. An electric field acts simultaneously on charged groups of the polyelectrolyte and on mobile counterions. As a result, charges are displaced and a dipole moment is produced, which leads to stretching and orientation of the polymer parallel to the external field. On the the other hand, hydrodynamic effects favor a perpendicular orientation of stiff polymers with respect to the external field, which is related to the phenomenon of anomalous birefringence. By performing hydrodynamic simulations, the relation between the electrophoretic mobility and the non-equilibrium perturbation of the polymer structure is investigated which results from a competition between electro-friction effects when counterions glide along the polymer backbone and coupling between hydrodynamic and elastic forces. Friction effects are more pronounced when polymers are confined, for example when they move through pores or when they adsorbed at surfaces. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R26.00004: Migration of DNA on electrically inhomogeneous surfaces Eric Petersen, Bingquan Li, Vladimir Samuilov, Xiaohua Fang, Jonathan Sokolov, Miriam Rafailovich Surface electrophoresis of kilo-bp DNA on electrically inhomogeneous surfaces was studied. The surfaces were produced by stamping micropatterns of gold strips onto silicon wafers using the Whitesides microcontact printing technique. Simulations of the field distribution on the surface revealed regions of very high field strength localized at the Au/Si interface. As a result, both electrophoretic and dielectrophoretic forces contribute to the movement of DNA on the surface. The mobility of DNA across the patterned surface was measured via laser induced fluorescence detection with an electric field oriented normal to the strips in the plane of the surface. The interaction of DNA with the surface was imaged with a confocal microscope in laser scanning mode with a CCD camera. Results indicate the measured mobility of DNA on a chemically micropatterned surface depends on the period size of the gold striped pattern relative to the contour length of the DNA molecule and the persistence length of the chain relative to the size of the electric field disturbance at the Au/Si interface. Consequentially, surface separation of DNA of different size and flexibility is possible. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R26.00005: Evidence for High Conductivity in the Pili of \textit{Geobacter sulfurreducens}: ``Nano-wires'' in a Prokaryotic Electron Transport Chain? Kevin D. McCarthy, Gemma Reguera, Teena Mehta, Julie S. Nicoll, Xinyu Wang, Mark T. Tuominen, Derek R. Lovley We discuss results of Conducting-Probe Atomic Force Microscopy (CP-AFM) applied to the nanoscopic filamentary pili of the prokaryote \textit{Geobacter sulfurreducens.} (G. Reguera, et al., Nature \textbf{435}, 1038, 2005) The apparently high cross-axis conductivity of this bacterial membrane protein complex, along with evidence of its necessity for respiratory reduction of insoluble Fe(III) deposits in nature, points to the possibility of a novel role for the pili protein complex in the electron transport chain of a prokaryote: as a kind of ``nano-wire'' for conduction of electrons to Fe(III) oxides. CP-AFM and \textit{in vivo} genetic engineering experiments supporting the ``nano-wire'' hypothesis are presented. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R26.00006: Dynamical Transition of the Protein Observed in Terahertz Dielectric Response Jing-Yin Chen, Joseph Knab, Andrea Markelz Temperature dependent measurements (10 K -- 296 K) were made of the terahertz dielectric response of oxidized and reduced cytochrome C (CytC) buffer solutions using THz time domain spectroscopy (THzTDS) (0.12-2.0 THz) . The imaginary part of the index ($\kappa )$ for both ferri and ferro CytC solutions increases linearly with temperature at low temperatures and then strongly increases near 200 K with a plateau at 250 K. The change of $\kappa $ for ferroCytC between 200 K and 250 K is nearly half that of ferriCytC. The result is consistent with Mossbauer measurements; however THzTDS measures motions in the significantly shorter time range, 0.5 -- 8 ps. These are the first measurements of the transition in the THz range and put constraints on allowed mechanisms. The agreement with polarizability insensitive Mossbauer supports that the dramatic increase in the THz dielectric response with oxidation we reported for CytC films (Phys Rev E 72, 040901 (2005)) results mainly from a large increase in the low frequency vibrational density of states. Work was supported by NSF Career PHY-0349256, and NSF IGERT DGE0114330. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R26.00007: Terahertz dielectric response dependence on protein melting and hydration Yunfen He, J.R. Knab, B. Shah, A.G. Markelz Hydration is critical for protein structure stability and flexibility. The hydration dynamics of denatured proteins necessarily plays a significant role in protein folding/unfolding and aggregation. We previously reported the observation of a transition in the THz dielectric response for native state hen egg white lysozyme (HEWL) with hydration. As hydration increases the response slowly increases until at 0.25h (gm water/gm protein) the absorbance and index sharply increase. The frequency range is associated with structural vibrational modes and the hydration level coincides with the filling of the first solvation shell. We present recent THz hydration dependence results as a function of denaturing. The THz absorption coefficient is nearly independent of denaturation while the index of refraction significantly decreases compared to native state and has a stronger frequency dependence with a peak appearing near 0.5 THz. Most significantly the hydration transition at 0.25h is still present for the denatured samples, suggesting the net \textit{bound} water content is nearly identical to the native state. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R26.00008: Inorganic Surface and Structure Adhesion of Amino Acids and Peptides LaRue Dunkleberger, Robert Willett, Loren Pfeiffer Interactions at the interface of biological molecules and inorganic materials are an open question in materials science; understanding these hybrid interfaces at the molecular level can have extensive basic and practical implications. In an extensive set of measurements we have systematically examined the adhesion of amino acids to a series of inorganic surfaces used in semiconductor devices. Peptides comprised of each of the twenty amino acids were exposed in solution to surfaces including metals, insulators, and semiconductors. Significant differential adhesion to the various surfaces is observed over the complement of amino acids, with adhesion determined largely by the amino acid side-chain charge. Mapping of adhesion findings for the amino acids versus materials in multiple solutions has been accomplished, in addition to examination of concentration and pH dependence. These results provide an empirical basis for building peptide to inorganic surface structures. In this vein, we have designed inorganic nano-structures using molecular beam epitaxy that are shown to selectively bind to prescribed primary peptide sequences. The inorganic structures fabricated here are shown to be able to discriminate between peptides with differences of only two to four amino acids. This surprising specific differential adhesion in both open surfaces to varied amino acids and in nanoscale structures to peptides is examined for the physical processes at play. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R26.00009: Interactions between the HIV-TAT transduction domain and cell membranes Abhijit Mishra, Gerard Wong Biologically active molecules such as proteins and oligonucleotides can be transduced into cells with high efficiency when covalently linked to a Protein Transduction Domain (PTD), such as the TAT domain in the HIV virus. All PTDs have a high content of basic amino acids resulting in a net positive charge. Electrostatic interactions between cationic PTDs and the negatively charged phospholipids that constitute the plasma membrane are likely to be responsible for peptide uptake, but no detailed structural studies exist. We compare membrane structures induced by the cationic TAT domain and those induced by other cationic polypeptides as a function of membrane composition using synchrotron x-ray scattering, and examine possible mechanisms of the anomalous transduction. [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R26.00010: Temperature Effects on Threshold Counterion Concentration to Induce Aggregation of fd Virus Qi Wen, Jay Tang We seek to determine the mechanism of like-charge attraction by measuring the temperature dependence of critical divalent counterion concentration ($C_{c}$) for the aggregation of fd viruses. We find that an increase in temperature leads to a decrease in the dielectric constant ($\varepsilon$) of the solvent, thus causing $C_{c}$ to decrease. At a constant $\varepsilon$, $C_{c}$ is found to increase as temperature increases. The effect of $T$ and $\varepsilon$ on $C_{c}$ is combined to that of one parameter: Bjerrum length ($l_{B}$). $ C_{c}$ decreases exponentially as $l_{B}$ increases. The exponential decay of $C_{c}$ suggests that entropic effect of counterions plays an important role at the onset of bundle formation. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R26.00011: Ion Distribution around DNA: Can Transitions Be Observed? Kurt Andresen, Lisa Kwok, Xiangyun Qiu, Hye Yoon Park, Jessica Lamb, Lois Pollack The spatial distribution of counterions that reduce or neutralize the charge of biopolymers is fundamentally important in determining how these polymers interact with each other and with their surroundings (i.e. with charged surfaces). We will discuss recent anomalous small-angle x-ray scattering (ASAXS) experiments that quantify the distribution of ions around DNA strands in solution. This counterion atmosphere has been studied as a function of a variety of experimental conditions, including those known to lead to structural transitions of the biopolymer. [Preview Abstract] |
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