Biomedical subjects

M N Tamashiro

Publications and source records attributed to M N Tamashiro.

5 recordsLinked to original sources

PubMed2006

Rayleigh instability of charged aggregates: Role of the dimensionality, ionic strength, and dielectric contrast.

We extended a previous analysis of the classical Rayleigh instability of spherical charged droplets in the presence of neutralizing monovalent counterions [M. Deserno, Eur. Phys. J. E 6, 163 (2001)], by generalizing the problem for suspensions of aggregates with D-dimensional symmetry, corresponding for D = 2 to infinite (rodlike) cylindrical charged bundles and for D = 3 to spherical charged droplets. In addition, we include the effects of added monovalent salt and of dielectric contrast between the charged aggregate and the surrounding solvent. The electrostatic energy taking the microion screening into account is estimated using uniform profiles within the framework of the cell model. We verify the robustness of these results by also considering Debye-Hückel-type microion profiles that are obtained by the minimization of a linearized Poisson-Boltzmann free-energy functional. In the case when the microions can enter inside the charged aggregates, we confirm the occurrence of a discontinuous phase change between aggregates of finite size and an infinite aggregate, which takes place at a collapse temperature that depends on their volume fraction phi and on the salt content. Decrease of phi shifts the phase-change temperature toward higher values, while salt addition has an opposite effect. We obtain analytical expressions for the phase-separation line in the asymptotic limit of infinite dilution (phi-->0), showing that the collapse temperature depends logarithmically on phi . As an application for D = 3 we discuss the stability of the pearl-necklace structures of flexible polyelectrolytes in poor solvents. The case D = 2 is applied to the problem of finite bundle sizes of stiff polyelectrolytes that attract each other-via, e.g., multivalent counterions-leading to an effective surface tension.

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PubMed2005

Aqueous suspensions of charged spherical colloids: dependence of the surface charge on ionic strength, acidity, and colloid concentration.

We theoretically investigate the dependence of the surface charge developed on charged spherical colloids upon several environmental parameters: the ionic strength of the monovalent added electrolyte, acidity (stabilized by a pH buffer solution), and colloid concentration. In the framework of the mean-field Poisson-Boltzmann spherical cell model, we include the charged colloid-microion correlations into the buffer equation, and we allow for the specific binding of ions to the ionizable groups on the colloid surface. Theoretical predictions are compared to the results obtained under the planar-symmetry Gouy-Chapman approximation and analyzed for the experimental conditions of an aqueous dispersion of the phospholipid dimyristoyl phosphatidylglycerol (DMPG). Experimental measurements of the partition ratio of an aqueous soluble cationic spin label on buffered dispersions of polyanionic unilamellar vesicles of DMPG in the presence of added monovalent salt are theoretically interpreted in terms of ion partition due to electrostatic interactions. We show that the specific binding of the probe must be admitted to explain the experimental results.

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PubMed2003

Where the linearized Poisson-Boltzmann cell model fails: the planar case as a prototype study.

The linearized Poisson-Boltzmann (PB) approximation is investigated for the classical problem of two infinite, uniformly charged planes in electrochemical equilibrium with an infinite monovalent salt reservoir. At the nonlinear level, we obtain an explicit expression of the associated electrostatic contribution to the semi-grand-canonical potential. The linearized osmotic-pressure difference between the interplane region and the salt reservoir becomes negative in the low-temperature, large-separation, or high-surface charge limits, in disagreement with the exact (at mean-field level) nonlinear PB solution. We show that these artifacts--although thermodynamically consistent with quadratic expansions of the nonlinear functional--can be traced back to the nonfulfillment of the underlying assumptions of the linearization. Explicit comparison between the analytical expressions of the exact nonlinear solution and the corresponding linearized equations allows us to show that the linearized results are asymptotically exact in the weak-coupling and counterionic ideal-gas limits, but always fail otherwise, predicting negative osmotic-pressure differences. By taking appropriate limits of the full nonlinear PB solution, we provide asymptotic expressions for the semi-grand-canonical potential and the osmotic-pressure difference that involve only elementary functions, which cover the complementary region where the linearized theory breaks down.

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PubMed2001

Helix-coil transition in homopolypeptides under stretching.

We consider the effect of an external applied force on the alpha-helix-coil transition of a single-stranded homopolypeptide chain. An annealed scenario is assumed, where the building amino acid monomers may interconvert between random-coiled and ordered alpha-helical configurations. By exact evaluation of the partition function of the freely jointed chain with helix-coil internal degrees of freedom in the thermodynamic limit, we obtain the result that the stress-strain characteristic has an asymmetrical sigmoid shape with a prominent pseudoplateau. Because of the one-dimensional nature of this system, fluctuations dominate over the mean-field approximation, which incorrectly predicts a second-order phase transition.

Amino Acids↗

PubMed1999

Electrolytic depletion interactions.

We consider the interactions between two uncharged planar macroscopic surfaces, immersed in an electrolyte solution, which are induced by interfacial selectivity. These forces are taken into account by introducing a depletion free-energy density functional, in addition to the usual mean-field Poisson-Boltzmann functional. The minimization of the total free-energy functional yields the density profiles of the microions and the electrostatic potential. The disjoining pressure is obtained by differentiation of the total free energy with respect to the separation of the surfaces, holding the range and strength of the depletion forces constant. We find that the induced interaction between the two surfaces is always repulsive for sufficiently large separations, and becomes attractive at shorter separations. The nature of the induced interactions changes from attractive to repulsive at a distance corresponding to the range of the depletion forces.

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