Bulletin of the American Physical Society
75th Annual Meeting of the Southeastern Section of APS
Volume 53, Number 13
Thursday–Saturday, October 30–November 1 2008; Raleigh, North Carolina
Session BC: Biophysics I |
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Chair: David Hill, University of North Carolina Room: Holiday Inn Brownstone Lincoln |
Thursday, October 30, 2008 8:30AM - 9:00AM |
BC.00001: Magnetomotive optical coherence tomography for elastography of small biosamples Invited Speaker: Optical coherence tomography (OCT) is a 3D micron-resolution imaging modality using the low-coherence properties of near-infrared light to render depth-resolved images typically a few millimeters into biological tissue. Visco-elasticity is an important parameter for detecting and staging various human diseases. We report a method for analyzing the visco-elastic properties of small tissue samples using magnetomotive OCT. Superparamagnetic nanoparticles (MNPs, $\sim $20nm) are diffused into a tissue sample. Subsequently, an electromagnet is modulated with a chirped frequency waveform from 0-1kHz, providing a modulated force on the MNPs in the tissue. The mechanical response of the tissue is recorded using OCT at linerates of 1-10kHz. Because OCT is a coherence imaging technique, sub-wavelength displacements are detected in the phase of the interferogram. The mechanical frequency response and associated phase lag fit a model for a damped harmonic oscillator, and results in homogeneous agarose cylinders can be interpreted in terms of Love's solutions for longitudinal vibration modes. A rat mammary tumor biopsy was also analyzed with this technique during formaldehyde fixation, and a trend toward higher frequency correlates with stiffening of the tissue during the fixation process. In collaboration with Stephen Boppart, University of Illinois at Urbana-Champaign. [Preview Abstract] |
Thursday, October 30, 2008 9:00AM - 9:12AM |
BC.00002: Time-resolved Photoelectron Spectroscopy and the Photoprotective Properties of Adenine Nick Evans, William Potter, Amanda Briouillette, Susanne Ullrich A system for fs time-resolved photoelectron and photoion spectroscopy has recently been developed at the University of Georgia, Department of Physics and Astronomy, in order to study the photophysical properties of isolated biomolecular building blocks. Ultrafast electronic excited state deactivation processes are observed in these chromophores and contribute to their photostability under UV radiation. Time-resolved photoelectron spectroscopy (TRPES) provides a unique tool to investigate these processes as the two dimensional data comprises both spectral and dynamic information. The spectral data allows identification of participating electronically excited states while the dynamic data allows the state's associated lifetimes to be extracted. Details of the experimental setup and technique will be presented in this talk as well as our initial results on the deactivation pathways in the DNA base adenine following excitation by wavelengths between 245 - 266 nm. [Preview Abstract] |
Thursday, October 30, 2008 9:12AM - 9:24AM |
BC.00003: High Throughput Magnetic Force System for Experiments in Polymer and Biological Physics Richard Spero, Leandra Vicci, Jeremy Cribb, Vinay Swaminathan, R. Superfine While technologies for micro- and nano-scale manipulation have expanded the fields of nano-mechanical and biophysical experimentation, these manipulation techniques are typically low-throughput. Techniques using microbeads (particles $\sim 0.1 - 10\mu m$) show promise for enabling high throughput mechanical measurements of these systems. We demonstrate instrumentation to magnetically drive microbeads in a biocompatible, multi-well magnetic force system. It is based on commercial high throughput screening standards, and is scalable to 96 wells. The rheology of polymers and biomaterials can be studied, and cells can be cultured, in this Magnetic High Throughput System (MHTS). The MHTS can apply independently controlled forces to 16 specimen wells. Force calibrations demonstrate forces in excess of 1nN, predicted force saturation as a function of pole material, and powerlaw dependence of $F \sim r^{-2.7\pm 0.1}$. We also report our recent results in applying the MHTS to measure rheology of fibrin clots and cell mechanics. [Preview Abstract] |
Thursday, October 30, 2008 9:24AM - 9:36AM |
BC.00004: Observing ultrafast dynamics in gas-phase biomolecules Susanne Ullrich, N.L. Evans, William M. Potter The UV photostability of biomolecules is determined by their excited state electronic relaxation mechanisms. To be effective, these mechanisms must operate on ultrafast timescales in order to dominate over competing photochemical processes that potentially lead to destruction of the biomolecule. Femtosecond time-resolved photoelectron spectroscopy (TRPES) provides unique capabilities for studying photoinduced processes in small polyatomic molecules. Changes in the PES, observed as the delay between the pump and probe pulses is scanned, can be associated with electronic configurational changes during the relaxation process. Analysis based on ionization correlations allows us to extract the electronic character of the excited states in addition to their lifetimes. TRPES has successfully been applied to the study of small biomolecular building blocks, such as the DNA base Adenine, however many challenges are faced when the interest in slightly larger biomolecular subunits, e.g. Adenosine. In this talk I will provide details on our newly constructed photoelectron photoion spectrometer and discuss problems associated with evaporation of larger biomolecules. [Preview Abstract] |
Thursday, October 30, 2008 9:36AM - 9:48AM |
BC.00005: Novel Bioreactors to Study Forces on Bronchial Epithelial Cultures Jerome Carpenter, Mike Millard, Matthew Cozon, Richard Superfine Studying cells in a physiologically relevant environment is an important tool in understanding cell signaling and gene expression. Human bronchial epithelial cells (HBECs) are responsible for mucociliary clearance, which removes pathogens from the air we breathe. Recreating the in vivo conditions of HBECs is difficult; they are polarized and undergo a variety of forces. Polarization is required for organ-specific systems such as cilia motility and mucus regulation. We achieve polarization by growing cells on an electrospun nanoporous scaffold which we attach to a silastic annulus. Using this geometry we apply vacuum to the annulus and stretch the cells. This bioreactor allows us to study polarized HBECs as they experience cyclic strain similar to breathing. We've grown polarized cultures on the scaffold and are evaluating the scaffold's mechanical properties. In a second bioreactor, we place the scaffold into a microfluidics channel to study the affect of shear stress on polarized cells. We also reproduce the branching structure found in the lungs to investigate the regulation of mucus as it ascends the airway tree. [Preview Abstract] |
Thursday, October 30, 2008 9:48AM - 10:00AM |
BC.00006: Magnetically driven nanorod transport through Matrigel in vitro Lamar Mair, Richard Superfine The dense mesh of interwoven collagen and laminin sheets found in the extracellular matrix (ECM) presents a significant barrier for efficient and widespread delivery of particle-bound therapeutic drugs within the volume of a tumor. We use templated electrodeposition as an inexpensive means to creating highly uniform, calibrated, magnetic nanorods with process-defined dimensions. Specifically, we grow Cu/Ni multilayered rods and selectively etch Cu regions, successfully creating Ni nanorods with diameters ranging from 55 to 250nm. Novel to the field of micro- and nano-carrier research is our ability to observe single particles move through the ECM as AC or DC magnetic field gradients are applied. Using a microscope fitted with an in-house designed high throughput rheometer capable of applying magnetic field gradients to microliter-sized volumes we test the effect aspect ratio has on the efficacy of particle transport through Matrigel, as well as how the parameters of time-varying fields affect transport. [Preview Abstract] |
Thursday, October 30, 2008 10:00AM - 10:12AM |
BC.00007: Analysis of Folded Geometry for Organic Solar Cells Aric Meyer, Jaewook Seok, Harald Ade Organic solar cells offer promise of clean affordable energy due to their potential for low cost, ease of production, and flexibility. Unfortunately practical implementation is prevented by lower power conversion efficiencies and shorter device lifetimes compared to inorganic photovoltaics. Many techniques are being investigated to improve the efficiency of organic solar cells beyond current limits of 5-6{\%}. Recent work has demonstrated that folding a flat organic solar cell can be an effective way to improve power conversion efficiency; however, efficiency gains range from 20-100{\%} depending on the details of the system. This theoretical work details the parameters affecting potential gains from a folded geometry. Results include guidelines for predicting which materials and device structures stand to benefit most from folding, and showing that some materials which cannot reach high efficiencies in a planar geometry can be competitive when in a folded solar cell. [Preview Abstract] |
Thursday, October 30, 2008 10:12AM - 10:24AM |
BC.00008: Free energy calculations of short peptide chains using Adaptively Biased Molecular Dynamics Vadzim Karpusenka, Volodymyr Babin, Christopher Roland, Celeste Sagui We performed a computational study of monomer peptides composed of methionine, alanine, leucine, glutamate, lysine (all amino acids with a helix-forming propensities); and proline, glycine tyrosine, serine, arginine (which all have poor helix-forming propensities). The free energy landscapes as a function of the handedness and radius of gyration have been calculated using the recently introduced Adaptively Biased Molecular Dynamics (ABMD) method, combined with replica exchange, multiple walkers, and post-processing Umbrella Correction (UC). Minima that correspond to some of the left- and right-handed $3_{10}$-, $\alpha$- and $\pi$-helixes were identified by secondary structure assignment methods (DSSP, Stride). The resulting free energy surface (FES) and the subsequent steered molecular dynamics (SMD) simulation results are in agreement with the empirical evidence of preferred secondary structures for the peptide chains considered. [Preview Abstract] |
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