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Publication Elongation and Fluctuations of Semi-flexible Polymers in a Nematic Solvent(2004-03-26) Dogic, Z.; Zhang, J.; Discher, Dennis E; Lau, A. W.C.; Janmey, Paul; Aranda-Espinoza, Helim; Kamien, Randall; Dalhaimer, Paul M; Lubensky, Thomas C.; Yodh, ArjunWe directly visualize single polymers with persistence lengths ranging from lp = 0:05 to 16 µm, dissolved in the nematic phase of rod-like fd virus. Polymers with sufficiently large persistence length undergo a coil-rod transition at the isotropic-nematic transition of the background solvent. We quantitatively analyze the transverse fluctuations of semi-flexible polymers and show that at long wavelengths they are driven by the fluctuating nematic background. We extract both the Odijk deflection length and the elastic constant of the background nematic phase from the data.Publication Cholesterol Depletion Increases Membrane Stiffness of Aortic Endothelial Cells(2004-11-01) Byfield, Fitzroy H; Aranda-Espinoza, Helim; Romanenko, Victor G.; Rothblat, George H.; Levitan, IrenaThis study has investigated the effect of cellular cholesterol on membrane deformability of bovine aortic endothelial cells. Cellular cholesterol content was depleted by exposing the cells to methyl-ß-cyclodextrin or enriched by exposing the cells to methyl-ß-cyclodextrin saturated with cholesterol. Control cells were treated with methyl-ß-cyclodextrincholesterol at a molar ratio that had no effect on the level of cellular cholesterol. Mechanical properties of the cells with different cholesterol contents were compared by measuring the degree of membrane deformation in response to a step in negative pressure applied to the membrane by a micropipette. The experiments were performed on substrate-attached cells that maintained normal morphology. The data were analyzed using a standard linear elastic half-space model to calculate Young elastic modulus. Our observations show that, in contrast to the known effect of cholesterol on membrane stiffness of lipid bilayers, cholesterol depletion of bovine aortic endothelial cells resulted in a significant decrease in membrane deformability and a corresponding increase in the value of the elastic coefficient of the membrane, indicating that cholesterol-depleted cells are stiffer than control cells. Repleting the cells with cholesterol reversed the effect. An increase in cellular cholesterol to a level higher than that of normal cells, however, had no effect on the elastic properties of bovine aortic endothelial cells. We also show that although cholesterol depletion had no apparent effect on the intensity of F-actin-specific fluorescence, disrupting F-actin with latrunculin A abrogated the stiffening effect. We suggest that cholesterol depletion increases the stiffness of the membrane by altering the properties of the submembrane F-actin and/or its attachment to the membrane.Publication Pore Stability and Dynamics in Polymer Membranes(2003-11-15) Bermudez, Harry; Hammer, Daniel A; Aranda-Espinoza, Helim; Discher, Dennis EVesicles self-assembled from amphiphilic diblock copolymers exhibit a wide diversity of behavior upon poration, due to competitions between edge, surface and bending energies, while viscous dissipation mechanisms determine the time scales. The copolymers are essentially chemically identical, only varying in chain length (related to the membrane thickness d). For small d, we find large unstable pores and the resulting membrane fragments reassemble into vesicles within minutes. For large d, however, submicron pores form and are extremely long-lived. The results show that pore behavior depends strongly on d, suggesting that the relevant energies depend on d and pore size r in a more complexmanner than what is generally assumed. Further control over these systems would make them useful for numerous applications.Publication Domain unfolding in neurofilament sidearms: effects of phosphorylation and ATP(2002-10-10) Aranda-Espinoza, Helim; Carl, Philippe; Janmey, Paul; Leterrier, Jean-François; Discher, Dennis ELateral projections of neuro¢laments (NF) called sidearms (SA) a¡ect axon stability and caliber. SA phosphorylation is thought to modulate inter-NF distance and interactions between NF and other subcellular organelles. SA were probed by atomic force microscopy (AFM) and dynamic light scattering (DLS) as a function of phosphorylation and ATP content. DLS shows SA are larger when phosphorylated, and AFM shows four unfoldable domains in SA regardless of phosphorylation state or the presence of ATP. However, the native phosphorylated SA requires three-fold higher force to unfold by AFM than dephosphorylated SA, suggesting a less pliant as well as larger structure when phosphorylated.Publication Electrostatic Repulsion of Positively Charged Vesicles and Negatively Charged Objects(1999-07-16) Aranda-Espinoza, Helim; Chen, Yi; Lubensky, Thomas C.; Dan, Nily; Nelson, Philip; Ramos, Laurence; Weitz, David AA positively charged, mixed bilayer vesicle in the presence of negatively charged surfaces (for example, colloidal particles) can spontaneously partition into an adhesion zone of definite area, and another zone that repels additional negative objects. Although the membrane itself has nonnegative charge in the repulsive zone, negative counterions on the interior of the vesicle spontaneously aggregate there, and present a net negative charge to the exterior. Beyond the fundamental result that oppositely charged objects can repel, our mechanism helps explain recent experiments on surfactant vesicles.Publication Electromechanical Limits of Polymersomes(2001-11-12) Aranda-Espinoza, Helim; Bermudez, Harry; Bates, Frank S; Discher, Dennis ESelf-assembled membranes of amphiphilic diblock copolymers enable comparisons of cohesiveness with lipid membranes over the range of hydrophobic thicknesses d = 3-15 nm. At zero mechanical tension the breakdown potential Vc for polymersomes with d = 15 nm is 9 V, compared to 1 V for liposomes with d = 3 nm. Nonetheless, electromechanical stresses at breakdown universally exhibit a V2 c dependence, and membrane capacitance shows the expected strong d dependence, conforming to simple thermodynamic models. The viscous nature of the diblock membranes is apparent in the protracted postporation dynamics.