Helim Aranda-Espinoza

Interaction between inclusions embedded in membranes.

We calculate the membrane-induced interaction between inclusions, in terms of the membrane stretching and bending moduli and the spontaneous curvature. We find that the membrane-induced interaction between inclusions varies nonmonotonically as a function of the inclusion spacing. The location of the energy minimum depends on the spontaneous curvature and the membrane perturbation decay length, where the latter is set by the membrane moduli. The membrane perturbation energy increases with the inclusion radius. The Ornstein-Zernike theory, with the Percus-Yevick closure, is used to calculate the radial distribution function of inclusions. We find that when the spontaneous curvature is zero, the interaction between inclusions due to the membrane deformation is qualitatively similar to the hard-core interaction. However, in the case of finite spontaneous curvature, the effective interaction is dramatically modified.

Electromechanical Limits of Polymersomes

Self-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 V(c) 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 V(c)(2) 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.

Pore stability and dynamics in polymer membranes

Vesicles 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 complex manner than what is generally assumed. Further control over these systems would make them useful for numerous applications.

Domain unfolding in neurofilament sidearms: effects of phosphorylation and ATP

Lateral projections of neurofilaments (NF) called sidearms (SA) affect 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.

Elongation and Fluctuations of Semiflexible Polymers in a Nematic Solvent

We directly visualize single polymers with persistence lengths l(p), ranging from 0.05 to 16 microm, dissolved in the nematic phase of rodlike fd virus. Polymers with a 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 the semiflexible polymers and show that at long wavelengths they are driven by the fluctuating nematic background. We extract the Odijk deflection length and the elastic constant of the background nematic phase from the data.

STRUCTURE AND SELF-DIFFUSION IN A MODEL TWO-DIMENSIONAL BROWNIAN LIQUID

The static structure and the time‐dependent self‐diffusion motion of interacting Brownian particles in a model two‐dimensional suspension are discussed. For the static structure we report Brownian dynamics results assuming a hard disk plus Yukawa pair potential. The self‐diffusion properties of this model system are calculated from two independent theoretical approaches. In order to assess the accuracy of the predictions of these two theories, we also performed Brownian dynamics calculations of the time‐dependent self‐diffusion coefficient for a wide range of values of both the particle concentration and the pair potential coupling constant. We find that both theories reproduce very well the main features exhibited by the Brownian dynamics data. Quantitatively, there are some discrepancies between both theoretical predictions and the Brownian dynamics results, which are negligible at moderate couplings, but become larger for strongly coupled systems and long times.

Synthetic cell elements from block copolymers – hydrodynamic aspects

Amphiphilic block copolymers can self-assemble in water into various stable morphologies which resemble key cell structures, notably filaments and membranes. Filamentous ‘worms’ of copolymer, microns-long, are briefly introduced, and related dynamics of copolymer vesicle ‘polymersomes’ are reviewed. Fluorescence visualization of single worms stretched under flow demonstrates their stability as well as a means to control conformation. Polymersome membranes have been more thoroughly studied, especially copolymer molecular weight effects. We summarize results suggestive of a transition from Rouselike behavior to entangled chains. Viewed together, the results ask the question: what physics are needed next to mimic cell activities such as crawling? To cite this article: P. Dalhaimer et al., C. R. Physique 4 (2003). uf6d9 2003 Academie des sciences/Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Electrostatically Induced Undulations of Lamellar DNA-Lipid Complexes

Abstract:We consider DNA-cationic lipid complexes that form lamellar stacks of lipid bilayers with parallel DNA strands intercalated in between. We calculate the electrostatically induced elastic deformations of the lipid bilayers. It is found that the membranes undulate with a periodicity that is set by the DNA interaxial distance. As a consequence the lamellar repeat distance changes resulting in a swelling or compression of the lamellar stack. Such undulations may be responsible for the intermembrane coupling between DNA strands in different layers as it is observed experimentally.

Nuclear pores and membrane holes: generic models for confined chains and entropic barriers in pore stabilization

The lumen of the nuclear pore complex is increasingly understood to be lined by a polymer brush that entropically regulates transport in and out of the nucleus-and it seems likely that similar effects probably arise with glycocalyx-lined holes in cell membranes. Here we mimic such pore-confined brushes with self-assembled polymer membranes imbued with nano-holes. Experiment and theory help elucidate the entropic origin and stabilization of the pores, which appear to have a similar basis as steric stabilization of colloids bearing polymer brushes. Free energies of interacting brushes reveal stable minima at pore sizes smaller than the classical metastable point, with little effect of the particular pore geometry. Such entropic forces have potential implications for lock and key mechanisms of nuclear pore assembly as well as transient poration of cells and synthetic nano-pores with regulatory mechanisms for transport.

Shapes of Mixed Phospholipid Vesicles

We studied the shape of phospholipid vesicles prepared by hydration of a mixture of phosphatidylcholine (SOPC) and phosphatidylserine (SOPS) in different proportions. The aim of the work is to obtain some insight into the influence of the chemical composition of a biomembrane on its shape. The optical microscopy results show that the shape of the vesicles depend on the SOPC:SOPS composition. For low SOPS contents, coiled cylindrical vesicles are observed. The results suggest that specific compositions of the SOPC:SOPS vesicles produce some spontaneous curvature on the membrane and then a coiling instability.