Bulletin of the American Physical Society
Joint Fall 2012 Meeting of the Texas Sections of the APS, AAPT, and Zone 13 of the SPS
Volume 57, Number 10
Thursday–Saturday, October 25–27, 2012; Lubbock, Texas
Session B3: Biological & Chemical Physics |
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Chair: Kelvin Cheng, Trinity University and Texas Tech University Room: Holiday Inn Towers Heritage |
Friday, October 26, 2012 10:30AM - 10:42AM |
B3.00001: Ergosterol and Stigmasterol Interact with Phosphatidylcholine Lipid Bilayers Less Favorably Than Cholesterol Serkan Balyimez, Juyang Huang The maximum solubility of sterol in a lipid bilayer is the highest mole fraction of sterol that can be incorporated into a lipid bilayer before sterol precipitates from the bilayer to form crystals. A higher maximum solubility indicates more favorable interactions between the sterol and lipid bilayer. In this study, the maximum solubilities of ergosterol and stigmasterol in DOPC and DSPC lipid bilayers were measured using light scattering and further confirmed using optical microscopy. We found that correlation function of scattering intensities from two independent detectors can be used to sensitively determine the solubility limits of sterols. The validity of our new technique was confirmed by measuring the solubility limit of cholesterol in DOPC and DPPC lipid bilayers. We found that the maximum solubilities of ergosterol and stigmasterol are higher in PC lipid bilayers with saturated chains (DSPC) than that in PC bilayers with unsaturated chains (DOPC). Compared with cholesterol, ergosterol and stigmasterol both have much lower solubility limits in PC lipid bilayers. Our results suggest that minor differences in sterol structure could result in large differences in sterol-PC interactions. [Preview Abstract] |
Friday, October 26, 2012 10:42AM - 10:54AM |
B3.00002: Effect of Hybrid Lipid on Line Tension in Lipid Membranes Eda Baykal-Caglar, Juyang Huang Giant unilamellar vesicles (GUVs) made of at least three components, one being cholesterol, one a high-melting-point lipid and one a low-melting-point lipid, can exhibit coexisting \textit{liquid}$_{ ordered}$\textit{ -liquid}$_{disordered}$ ($l_{o }- l_{d})$ phases. The energy per unit length of boundary is called line tension and it depends on the temperature of the system according to power law and vanishes at critical temperature. Universal scaling behavior can help us to understand the phase behavior of many different systems. Systems in the same universality class represent similar collective behavior in phase transitions apart from their physicals details. Critical exponents characterize the continuous phase transition of systems, and all systems belonging to a universality class will have the same critical exponents. In this work, we measured the critical exponent related to line tension using fluorescence microscopy and image processing. We investigated the effects of hybrid lipid on line tension and critical exponent. One chain of hybrid lipid is saturated and the other one is unsaturated, because of which they behave as linactants and can reduce the line tension. We prepared GUVs with three different compositions: DOPC/DSPC/Cholesterol 30:45:25, DOPC/DSPC/POPC/Cholesterol 22.5:45:25:7.5, DOPC/DSPC/POPC/Cholesterol 15:45:55:25. Our results show that the critical exponent associated with line tension gradually increase with hybrid lipid concentration. Having different values of critical exponent in different mixtures indicate that lipid bilayers cannot be classified as a universality class. [Preview Abstract] |
Friday, October 26, 2012 10:54AM - 11:06AM |
B3.00003: Gramicidin Alters the Lipid Compositions of Liquid-Ordered and Liquid-Disordered Membrane Domains Ebrahim Hassan-Zadeh, Juyang Huang The effects of adding 1 mol {\%} of gramicidin A to the well-known DOPC/DSPC/cholesterol lipid mixtures were investigated. 4-component giant unilamellar vesicles (GUV) were prepared using our recently developed Wet-Film method. The phase boundary of liquid-ordered and liquid-disordered (\textit{Lo-Ld}) coexisting region was determined using video fluorescence microscopy. We found that if cares were not taken, light-induced domain artifacts could significantly distort the measured phase boundary. After testing several fluorescence dyes, we found that the emission spectrum of Nile Red is quite sensitive to membrane composition. By fitting the Nile Red emission spectra at the phase boundary to the spectra in the \textit{Lo-Ld} coexisting region, the thermodynamic tie-lines were determined. As an active component of lipid membranes, gramicidin not only partitions favorably into the liquid-disordered (\textit{Ld}) phase, it also alters the phase boundary and thermodynamic tie-lines. Even at as low as 1 mol {\%}, gramicidin decreases the cholesterol mole fraction of \textit{Ld} phase and increases the area of \textit{Lo} phase. [Preview Abstract] |
Friday, October 26, 2012 11:06AM - 11:18AM |
B3.00004: Dynamic Heterogeneity in Ionic Liquids near and below the Glass Transition: Rotational Diffusion of Probes in 1-Butyl-3-methylimidazolium Hexafluorophosphate Fehmi Bardak, Justin R. Rajian, Larry G. Hines, Richard A. Bartsch, Edward L. Quitevis Dynamic heterogeneity in the ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF$_{6}$]) ($T_{\mathrm{g}}=$ 196 K) near glass transition is investigated by conducting probe rotational diffusion experiments on rubrene and tetracene in [Bmim][PF$_{6}$] using the fluorescence recovery after photobleaching (FRAP) technique. Rotational anisotropy decays for the probes in the temperature range $T_{\mathrm{g}}$--6 to $T_{\mathrm{g}}+$4 are well described by the stretched exponential function. The stretching parameter is found to be constant for both probes with $\beta _{\mathrm{tetracene}}=$0.71 and $\beta_{\mathrm{rubrene}}=$ 0.88. The viscosity of [Bmim][PF$_{6}$] at high temperatures from 353 K to 283 K and near $T_{\mathrm{g}}$ from 203 K to 196 K can be fit by a single VFT equation. In the temperature range of our measurements, the rotational diffusion of these probes in [Bmim][PF$_{6}$] is decoupled from structural relaxation with a rotational correlation time following a fractional Debye-Stokes-Einstein (DSE) relation. [Preview Abstract] |
Friday, October 26, 2012 11:18AM - 11:30AM |
B3.00005: The effects of bariatric surgeries on type 2 diabetes mellitus Jia Lerd Ng, Roberto Ortiz, Tyler Hughes, Michel Abou Ghantous, Othmane Bouhali, Abdelilah Arredouani, Roland Allen We consider a scientific mystery which is of central importance in treating the most rapidly emerging national and global health threat: type 2 diabetes mellitus. The mystery involves a surprising effect of certain bariatric surgeries, and specifically Roux-en-Y gastric bypass (RYGB), a procedure which bypasses most of the stomach and upper intestine. An unanticipated result is that RYGB is usually found to contribute within only a few days to glucose homeostasis. This means the surgery can immediately cure patients even before they start losing weight. We are investigating this wondrous biochemical response with a quantitative model which includes the most important mechanisms. One of the major contributors is glucagon-like peptide 1 (GLP-1), an incretin whose concentration is found to increase by a large amount right after the RYGB surgical procedure. However, our results, in conjunction with the experimental and medical data, indicate that other substances must also contribute. If these substances can be definitively identified, it may be possible to replace the surgery with pharmaceuticals as the preferred treatment for type 2 diabetes. [Preview Abstract] |
Friday, October 26, 2012 11:30AM - 11:42AM |
B3.00006: A general method for modeling biochemical and biomedical response Roberto Ortiz, Jia Lerd Ng, Tyler Hughes, Michel Abou Ghantous, Othmane Bouhali, Abdelilah Arredouani, Roland Allen The impressive achievements of biomedical science have come mostly from experimental research with human subjects, animal models, and sophisticated laboratory techniques. Additionally, theoretical chemistry has been a major aid in designing new drugs. Here we introduce a method which is similar to others already well known in theoretical systems biology, but which specifically addresses biochemical changes as the human body responds to medical interventions. It is common in systems biology to use first-order differential equations to model the time evolution of various chemical concentrations, and we as physicists can make a significant impact through designing realistic models and then solving the resulting equations. Biomedical research is rapidly advancing, and the technique presented in this talk can be applied in arbitrarily large models containing tens, hundreds, or even thousands of interacting species, to determine what beneficial effects and side effects may result from pharmaceuticals or other medical interventions. [Preview Abstract] |
Friday, October 26, 2012 11:42AM - 11:54AM |
B3.00007: Efficient Stochastic Model Simulation by Using Zassenhaus Formula Approximation and Kronecker Product Analysis Mehmet Umut Caglar, Ranadip Pal Biological systems are inherently stochastic such that they require the use of probabilistic models to understand and simulate their behaviors. However, stochastic models are extremely complex and computationally expensive which restricts their application to smaller order systems. Probabilistic modeling of larger systems can help to recognize the underlying mechanisms of complex diseases, including cancer. The fine-scale stochastic behavior of genetic regulatory networks is often modeled using stochastic master equations. The inherently high computational complexity of the stochastic master equation simulation presents a challenge in its application to biological system modeling even when the model parameters can be properly estimated. In this article, we present a new approach to stochastic model simulation based on Kronecker product analysis and approximation of Zassenhaus formula for matrix exponentials. Simulation results illustrate the comparative performance of our modeling approach to stochastic master equations with significantly lower computational complexity. We also provide a stochastic upper bound on the deviation of the steady state distribution of our model from the steady state distribution of the stochastic master equation. [Preview Abstract] |
Friday, October 26, 2012 11:54AM - 12:06PM |
B3.00008: Bacterial Growth in Weak Magnetic Field Samina Masood We study the effect of the weak magnetic field on the growth of bacteria. We found that the magnetic field has tremendous effects on the growth of bacteria. They follow the same growth curve however the increased growth rate let the growth curve follow through its path, quickly. We also notice that different magnetic fields affect differently on different types of bacteria. [Preview Abstract] |
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