Bulletin of the American Physical Society
Joint Fall 2011 Meeting of the Texas Sections of the APS, AAPT, and Zone 13 of the SPS
Volume 56, Number 7
Thursday–Saturday, October 6–8, 2011; Commerce, Texas
Session F7: Biological & Chemical Physics I |
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Chair: Jennifer Steele, Rice University Room: Science Building 123 |
Friday, October 7, 2011 3:40PM - 3:52PM |
F7.00001: High-resolution architecture of Gram-negative bacterial cell wall Ahmed Touhami, John Dutcher The major structural component of bacterial cell walls is the peptidoglycan sacculus, which is one of nature's strongest and largest macromolecules that maintains the large internal pressure within the cell while allowing the transport of molecules into and out of the cell and cell growth. The three-dimensional structure of this unique biopolymer is controversial, and two models have been proposed: the planar model, in which the glycan strands lie in the plane of the cell surface, and the sca{\textregistered}old model, in which the glycan strands lie perpendicular to the cell surface. We have used atomic force microscopy to investigate the high resolution structure of isolated, intact sacculi of Escherichia coli K12 bacteria. Atomic force microscopy-single molecule force spectroscopy was performed on single sacculi exposed to the tAmiB enzyme which cleaves the peptide-glycan bonds. Surprisingly, the measurements revealed individual strands of up to 250 nm in length. This finding combined with high resolution AFM images recorded on hydrated sacculi provide evidence for the validity of the planar model for the peptidoglycan structure in Gram-negative bacteria. [Preview Abstract] |
Friday, October 7, 2011 3:52PM - 4:04PM |
F7.00002: Detection of superlattice domain formation in ternary lipid mixtures using fluorescence spectroscopy Burcin Mutlu, Stephanie Lopez, Mark Vaughn, Juyang Huang, K. Cheng Multicomponent lipid bilayers represent an important model system for studying the structures and functions of cell membranes. At present, the lateral organization of lipid components, particularly the formation of regular distribution, in lipid membranes containing charged lipid, e.g., phosphatidylserine, is not clear. Using a ternary phosphatidylcholine/phosphatidylserine/cholesterol lipid bilayer system, the presence of ordered domain formation was examined by measuring the fluorescence anisotropy of the embedded fluorescent probe, 22-($N$-(7-nitrobenz-2-oxa-1,3-diazol- 4-yl)amino)-23,24-bisnor-5-cholen-3$\beta $- ol (NBD-CHOL), with structure similar to that of a cholesterol, as a function of phospatidylserine composition. The plot of the anisotropy vs. phosphatidylserine revealed abrupt changes at certain critical compositions of phosphatidylserine. Some of these critical compositions agree favorably with those predicted by the headgroup superlattice model suggesting that the charged phosphatidylserine lipid molecules adopt a superlattice-like distribution in the lipid bilayer at some predicted compositions. The ordered distribution of charged lipids may play an important role in the regulation of the composition of the biological membranes. [Preview Abstract] |
Friday, October 7, 2011 4:04PM - 4:16PM |
F7.00003: Fundamentals of Charge Transport through DNA Jason Slinker, Natalie Muren, Sara Renfrew, Jackie Barton, Chris Wohlgamuth, Marc McWilliams Achieving charge transport (CT) through DNA comlements its inherent biological recognition capabilities and its unmatched capacity to be patterned into precise, nanoscale shapes. We have probed the length and temperature dependence of DNA charge transport with DNA-mediated electrochemistry. Cyclic voltammetry of 100-mer and 17-mer DNA monolayers on gold revealed sizable peaks from distally-bound Nile Blue redox probes for well matched duplexes, but highly attenuated redox peaks from monolayers containing a single base pair mismatch, demonstrating that the charge transfer is DNA-mediated. The similarity in electron transfer rates through 100-mer and 17-mer monolayers is consistent with fast transport through the DNA and rate-limiting tunneling injection. Temperature dependence studies of 17-mer and 34-mer duplexes showed CT is thermally activated and highly sensitive to the integrity of the DNA base pair pi stack. Activation energies are increased by the presence and identity of single base pair mismatches. Furthermore, the yield of DNA CT with temperature argues that this CT is conformationally gated. These results elucidate the mechanism of DNA CT and direct the use of DNA as molecular wires in electronic applications. [Preview Abstract] |
Friday, October 7, 2011 4:16PM - 4:28PM |
F7.00004: Fiber optic spanner Bryan Black, Samarendra Mohanty Rotation is a fundamental function in nano/biotechnology and is being useful in a host of applications such as pumping of fluid flow in microfluidic channels for transport of micro/nano samples. Further, controlled rotation of single cell or microscopic object is useful for tomographic imaging. Though conventional microscope objective based laser spanners (based on transfer of spin or orbital angular momentum) have been used in the past, they are limited by the short working distance of the microscope objective. Here, we demonstrate development of a fiber optic spanner for rotation of microscopic objects using single-mode fiber optics. Fiber-optic trapping and simultaneous rotation of pin-wheel structure around axis perpendicular to fiber-optic axis was achieved using the fiber optic spanner. By adjusting the laser beam power, rotation speed of the trapped object and thus the microfluidic flow could be controlled. Since this method does not require special optical or structural properties of the sample to be rotated, three-dimensional rotation of a spherical cell could also be controlled. Further, using the fiber optic spanner, array of red blood cells could be assembled and actuated to generate vortex motion. Fiber optical trapping and spinning will enable physical and spectroscopic analysis of microscopic objects in solution and also find potential applications in lab- on-a-chip devices. [Preview Abstract] |
Friday, October 7, 2011 4:28PM - 4:40PM |
F7.00005: A new Monte Carlo method for investigating geometrical structures of lipid membranes with atomistic detail Sara Cheng, Liming Qiu, K. Cheng, Mark Vaughn The distribution statistics of the surface area, volume and voids of lipid molecules are important parameters to characterize the structures of self-assembling lipid membranes. Traditional methods are mostly based on various assumptions of the thickness of the lipid membrane and the volumes of certain types of lipid molecules. However, those methods usually lead to an over- or underestimation of the average surface area of lipid molecules when compared to the experimental results of the pure lipid systems. We developed a new Monte Carlo method that is able to estimate the distributions and averages of surface area, volume and void space of the lipid molecules in the absence and presence of proteins of the MD simulation results of lipid membranes at the atomistic scale. We successfully validated our new method on an ordered hard-sphere system and on a phospholipid/cholesterol binary lipid system, all with known structural parameters. Using this new method, the structural perturbation of the conformal annular lipids in close proximity to the embedded protein in a lipid/protein system will also be presented. [Preview Abstract] |
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