Biomedical subjects

Ramin Golestanian

Publications and source records attributed to Ramin Golestanian.

16 recordsLinked to original sources

PubMed2006

Counterions between charged polymers exhibit liquid-like organization and dynamics.

Current understanding of electrostatics in water is based on mean-field theories like the Poisson-Boltzmann formalism and its approximations, which are routinely used in colloid science and computational biology. This approach, however, breaks down for highly charged systems, which exhibit counterintuitive phenomena such as overcharging and like-charge attraction. Models of counterion correlations have been proposed as possible explanations, but no experimental comparisons are available. Here, collective dynamics of counterions that mediate like-charge attraction between F-actin filaments have been directly observed in aqueous solution using high-resolution inelastic x-ray scattering down to molecular length-scales. We find a previously undescribed acoustic-like phonon mode associated with correlated counterions. The excitation spectra at high wave-vector Q reveal unexpected dynamics due to ions interacting with their "cages" of nearest neighbors. We examine this behavior in the context of intrinsic charge density variations on F-actin. The measured speed of sound and collective relaxation rates in this liquid agree surprisingly well with simple model calculations.

Actins↗

PubMed2006

Efficient in-depth trapping with an oil-immersion objective lens.

Maximum trapping efficiency in optical tweezers occurs close to the coverslip because spherical aberration owing to a mismatch in the refractive indices of the specimen (water) and the immersion oil dramatically decreases the trap efficiency as the trap depth increases. Measuring the axial trap efficiency at various tube lengths by use of an oil-immersion objective has shown that such an aberration can be balanced by another source of spherical aberration, leading to a shift in the position of the maximum efficiency in the Z direction. For a 1.1 microm polystyrene bead we could achieve the maximal efficiency at a depth of 70 microm, whereas the trap was stable up to a depth of 100 microm.

Journal Article↗

PubMed2006

Condensation of DNA-actin polyelectrolyte mixtures driven by ions of different valences.

Multivalent ions can induce condensation of like-charged polyelectrolytes into compact states, a process that requires different ion valences for different polyelectrolyte species. In this work we examine the condensation behavior in binary anionic polyelectrolyte mixtures consisting of DNA coils and F-actin rods in the presence of monovalent, divalent, and trivalent ions. As expected, monovalent ions do not condense either component and divalent ions selectively condense F-actin rods out of the polyelectrolyte mixture. For trivalent ions, however, we observe a microphase separation between the two polyelectrolytes into coexisting finite-sized F-actin bundles and DNA toroids. Further, by increasing the DNA volume fraction in the mixture, condensed F-actin bundles can be completely destabilized, leading to only DNA condensation within the mixture. We examine a number of possible causes and propose a model based on polyelectrolyte competition for ions.

Actins↗

PubMed2006

Can nonlinear elasticity explain contact-line roughness at depinning?

We examine whether cubic nonlinearities, allowed by symmetry in the elastic energy of a contact line, may result in a different universality class at depinning. Standard linear elasticity predicts a roughness exponent zeta = 1/3 (one loop), zeta = 0.388 +/- 0.002 (numerics) while experiments give zeta approximately = 0.5. Within functional renormalization group methods we find that a nonlocal Kardar-Parisi-Zhang-type term is generated at depinning and grows under coarse graining. A fixed point with zeta approximately = 0.45 (one loop) is identified, showing that large enough cubic terms increase the roughness. This fixed point is unstable, revealing a rough strong-coupling phase. Experimental study of contact angles theta near pi/2, where cubic terms in the energy vanish, is suggested.

Journal Article↗

PubMed2005

Lifshitz interaction between dielectric bodies of arbitrary geometry.

A formulation is developed for the calculation of the electromagnetic-fluctuation forces for dielectric objects of arbitrary geometry at small separations, as a perturbative expansion in the dielectric contrast. The resulting Lifshitz energy automatically takes on the form of a series expansion of the different many-body contributions. The formulation has the advantage that the divergent contributions can be readily determined and subtracted off, and thus makes a convenient scheme for realistic numerical calculations, which could be useful in designing nanoscale mechanical devices.

Journal Article↗

PubMed2005

Orientational ordering and dynamics of rodlike polyelectrolytes.

The interplay between electrostatic interactions and orientational correlations is studied for a model system of charged rods positioned on a chain, using Monte Carlo simulation techniques. It is shown that the coupling brings about the notion of electrostatic frustration, which in turn results in: (i) a rich variety of orientational orderings such as chiral phases, and (ii) an inherently slow dynamics characterized by stretched-exponential behavior in the relaxation functions of the system.

Journal Article↗

PubMed2005

Elastic correlations in nucleosomal DNA structure.

The structure of DNA in the nucleosome core particle is studied using an elastic model that incorporates anisotropy in the bending energetics and twist-bend coupling. Using the experimentally determined structure of nucleosomal DNA [T. J. Richmond and C. A. Davey, Nature (London) 423, 145 (2003)], it is shown that elastic correlations exist between twist, roll, tilt, and stretching of DNA, as well as the distance between phosphate groups. The twist-bend coupling term is shown to be able to capture these correlations to a large extent, and a fit to the experimental data yields a new estimate of G = 25 nm for the value of the twist-bend coupling constant.

Computer Simulation↗

PubMed2005

Propulsion of a molecular machine by asymmetric distribution of reaction products.

A simple model for the reaction-driven propulsion of a small device is proposed as a model for (part of) a molecular machine in aqueous media. The motion of the device is driven by an asymmetric distribution of reaction products. The propulsive velocity of the device is calculated as well as the scale of the velocity fluctuations. The effects of hydrodynamic flow as well as a number of different scenarios for the kinetics of the reaction are addressed.

Colloids↗

PubMed2004

Moving contact lines on heterogeneous substrates.

The dynamics of the deformations of a moving contact line on a disordered substrate are formulated, taking a proper account of the various interfacial forces as well as the dissipation mechanisms. Prompted by the results from dynamical renormalization group calculations, it is suggested that the coating transition in contact lines receding at relatively high velocities can be understood as a roughening transition in the contact line. A phase diagram is proposed for the system in which the phase boundaries corresponding to the coating transition and the pinning transition meet at a junction point, and suggest that for sufficiently strong disorder a receding contact line will leave a Landau-Levich film immediately after de-pinning.

Journal Article↗

PubMed2004

Electrostatic contribution to twist rigidity of DNA.

The electrostatic contribution to the twist rigidity of DNA is studied, and it is shown that the Coulomb self-energy of the double-helical sugar-phosphate backbone makes a considerable contribution-the electrostatic twist rigidity of DNA is found to be C(elec) approximately 5 nm, which makes up about 7% of its total twist rigidity ( C(DNA) approximately 75 nm). The electrostatic twist rigidity is found, however, to depend only weakly on the salt concentration, because of a competition between two different screening mechanisms: (1) Debye screening by the salt ions in the bulk, and (2) structural screening by the periodic charge distribution along the backbone of the helical polyelectrolyte. It is found that, depending on the parameters, the electrostatic contribution to the twist rigidity could stabilize or destabilize the structure of a helical polyelectrolyte.

Computer Simulation↗

PubMed2004

Simple swimmer at low Reynolds number: three linked spheres.

We propose a very simple one-dimensional swimmer consisting of three spheres that are linked by rigid rods whose lengths can change between two values. With a periodic motion in a nonreciprocal fashion, which breaks the time-reversal symmetry as well as the translational symmetry, we show that the model device can swim at low Reynolds number. This model system could be used in constructing molecular-sized machines.

Animals↗

PubMed2003

Anomalous bending of a polyelectrolyte.

We report on a study of the shape of a stiff, charged rod that is subjected to equal and opposite force couples at its two ends. Unlike a neutral elastic rod, which forms a constant curvature configuration under such influences, the charged rod tends to flatten in the interior and accumulate the curvature in the end points, to maximally reduce the electrostatic self-repulsion. The effect of this nonuniform bending on the effective elasticity and on the statistical conformations of a fluctuating charged rod is discussed. An alternative definition for the electrostatic persistence length is suggested. This definition is found to be consistent with a corresponding length that can be deduced from the end-to-end distribution function of a fluctuating polyelectrolyte.

Journal Article↗

PubMed2003

Roughening transition in a moving contact line.

The dynamics of the deformations of a moving contact line on a disordered substrate is formulated, taking into account both local and hydrodynamic dissipation mechanisms. It is shown that both the coating transition in contact lines receding at relatively high velocities and the pinning transition for slowly moving contact lines can be understood in a unified framework as roughening transitions in the contact line. We propose a phase diagram for the system in which the phase boundaries corresponding to the coating transition and the pinning transition meet at a junction point, and suggest that for sufficiently strong disorder a receding contact line will leave a Landau-Levich film immediately after depinning.

Journal Article↗

PubMed2003

Probing polyelectrolyte elasticity using radial distribution function.

We study the effect of electrostatic interactions on the distribution function of the end-to-end distance of a single polyelectrolyte chain in the rodlike limit. The extent to which the radial distribution function of a polyelectrolyte is reproduced by that of a wormlike chain with an adjusted persistence length is investigated. Strong evidence is found for a universal scaling formula connecting the effective persistence length of a polyelectrolyte with its linear charge density and the Debye screening of its self-interaction. An alternative definition of the electrostatic persistence length is proposed based on matching of the maximum of the distribution with that of an effective wormlike chain, as opposed to the traditional matching of the first or the second moments of the distributions. It is shown that this definition provides a more accurate probe of the affinity of the distribution to that of the wormlike chains, as compared to the traditional definition. It is also found that the length of a polyelectrolyte segment can act as a crucial parameter in determining its elastic properties.

Journal Article↗

PubMed2002

Fluctuations of fluctuation-induced casimir-like forces.

The force experienced by objects embedded in a correlated medium undergoing thermal fluctuations-the so-called fluctuation-induced force-is actually itself a fluctuating quantity. Using a scalar field model, we compute the corresponding probability distribution and show that it is a Gaussian centered on the well-known Casimir force, with a nonuniversal standard deviation that can be typically as large as the mean force itself. The relevance of these results to the experimental measurement of fluctuation-induced forces in soft condensed matter is discussed, as well as the influence of the finite temporal resolution of the measuring apparatus.

Journal Article↗

PubMed2002

Conformational instability of rodlike polyelectrolytes due to counterion fluctuations.

The effective elasticity of highly charged stiff polyelectrolytes is studied in the presence of counterions, with and without added salt. The rigid polymer conformations may become unstable due to an effective attraction induced by counterion density fluctuations. Instabilities at the longest, or intermediate length scales, may signal collapse to globule, or necklace states, respectively. In the presence of added salt, a generalized electrostatic persistence length is obtained, which has a nontrivial dependence on the Debye screening length. It is also found that the onset of conformational instability is a reentrant phenomenon as a function of polyelectrolyte length for the unscreened case, and the Debye length or salt concentration for the screened case. This may be relevant in understanding the experimentally observed reentrant condensation of DNA.

Journal Article↗