Biomedical subjects

M Olvera de la Cruz

Publications and source records attributed to M Olvera de la Cruz.

10 recordsLinked to original sources

PubMed2006

Electrostatic attraction between cationic-anionic assemblies with surface compositional heterogeneities.

Electrostatics play a key role in biomolecular assembly. Oppositely charged biomolecules, for instance, can be coassembled into functional units, such as DNA and histone proteins into nucleosomes and actin-binding protein complexes into cytoskeleton components, at appropriate ionic conditions. These cationic-anionic coassemblies often have surface charge heterogeneities that result from the delicate balance between electrostatics and packing constraints. Despite their importance, the precise role of surface charge heterogeneities in the organization of cationic-anionic coassemblies is not well understood. We show here that coassemblies with charge heterogeneities strongly interact through polarization of the domains. We find that this leads to symmetry breaking, which is important for functional capabilities, and structural changes, which is crucial in the organization of coassemblies. We determine the range and strength of the attraction as a function of the competition between the steric and hydrophobic constraints and electrostatic interactions.

Anions↗

PubMed2005

Pattern formation on the surface of cationic-anionic cylindrical aggregates.

Charged pattern formation on the surfaces of self-assembled cylindrical micelles formed from oppositely charged heterogeneous molecules such as cationic and anionic peptide amphiphiles is investigated. The net incompatibility chi among different components results in the formation of segregated domains, whose growth is inhibited by electrostatics. The transition to striped phases proceeds through an intermediate structure governed by fluctuations, followed by states with various lamellar orientations, which depend on cylinder radius Rc and chi. We analyze the specific heat, susceptibility S(q*), domain size Lambda = 2pi/q*, and morphology as a function of Rc and chi.

Anions↗

PubMed2005

Molecular multivalent electrolytes: microstructure and screening lengths.

We study small rod-like molecular electrolytes solutions with their corresponding atomic counterions. The asymptotic length scales (decay length and wavelength) of the structural correlations are analyzed using the formalism of the dressed interaction site theory (DIST). The correlation functions are determined using the reference interaction site model equation complemented with a mixed approach in which the hypernetted-chain closure is used for the repulsive interactions, and the mean spherical approximation is used for the attractive interactions. The results from this scheme are in good agreement with the Monte Carlo computer simulations reported here. The asymptotic properties of the correlation functions of this molecular system are compared against those corresponding to two related simple (atomic) electrolyte models. The main conclusion is that the molecular structure of the ions lowers by two orders of magnitude the concentration at which the transition from monotonic to oscillatory decay occurs.

Electrolytes↗

PubMed2005

Charge induced pattern formation on surfaces: segregation in cylindrical micelles of cationic-anionic peptide-amphiphiles.

Micelles, vesicles, and films composed of two species of incompatible heterogeneous molecules exhibit full internal segregation of the component species. This macroscopic segregation can be inhibited by oppositely charging the two different molecular species. The degree of compatibility achieved by the charges leads to either fully homogenous mixtures or to local segregation and the possible formation of regular patterns. We investigate the induction of periodic surface patterns by the presence of opposite charges in flat films and cylindrical micelles. In the strong segregation limit the incompatibility between species can be described by a line tension parameter gamma. The size of the patterns formed is of the order of a characteristic size L approximately (gamma/sigma(2))(1/2), where sigma is the surface charge density. The pattern symmetry on flat surfaces is function only of the fraction of area covered by the components, f: lamellar for 0.34<f<0.66 and hexagonal otherwise. In cationic and anionic mixtures aggregated into cylindrical micelles the symmetry and size of the surface charged domains depends also on the radius R of the cylinder. For large radii the patterns are similar to those in the plane, but for small radii only lamellar patterns are formed. At high salt concentration the surface domain size jumps discontinuously to an infinite value, resulting in macroscopic phase segregation.

Anions↗

PubMed2004

Encapsulated drop breakup in shear flow.

We investigate the deformation and breakup in shear flow of an encapsulated drop in which both the core and shell are Newtonian fluids. The equations of motion are solved numerically using a level set method to track interface motion. We consider the case of a drop stretched to a given length in constant shear and then allowed to relax. A range of morphologies is produced, and novel kinematics occur, due to the interaction of the core and outer interfaces. A phase diagram is presented to describe the morphologies produced over a range of capillary numbers and core interfacial tensions.

Journal Article↗

PubMed2004

Thermoreversible crosslinking of polyelectrolyte chains.

Thermoreversible crosslinking of polyelectrolyte chains via short-range attractions such as hydrogen bonding induced by uncharged or charged particles is studied within the Flory model of ideal association. Electrostatic interactions between the charges at different linking fractions are taken into account by using a generalized random phase approximation approach which includes the network connectivity. We find that at certain concentration of linking agents an infinitely large polymer network is formed. We calculate the structural gelation lines for linkers of different charges and functionalities.

Cross-Linking Reagents↗

PubMed2004

Dynamics of a drop at a fluid interface under shear.

We analyze the dynamics of a two-dimensional drop lying on a fluid interface, sometimes called a liquid lens, subjected to simple shear flow. The three fluids, the drop and the two external fluids, meet at a triple point (or a triple line in three dimensions). A requirement for steady drop shapes is that the triple points are stationary. This leads to a flow topology different than that of a freely suspended drop. Results are substantiated with numerical results using a level set method for interface evolution and treatment of triple points. Possible implications for new drop instabilities are also discussed.

Journal Article↗

PubMed2003

Polyelectrolytes in the presence of multivalent ions: gelation versus segregation.

We analyze solutions of strongly charged chains bridged by linkers such as multivalent ions. The gelation induced by the strong short range electrostatic attractions is dramatically suppressed by the long range electrostatic correlations due to the charge along the non-cross-linked monomers and ions. A modified Debye-Hückel approach of cross-linked clusters of charged chains is used to determine the mean field gelation transition self-consistently. Highly dilute polyelectrolyte solutions tend to segregate macroscopically. Semidilute solutions can form gels if the Bjerrum length l(B) and the distance between neighboring charged monomers along the chain b are both greater than the ion size a.

Electrolytes↗

PubMed1999

Attractive interactions between rodlike polyelectrolytes: polarization, crystallization, and packing.

We study the attractive interactions between rodlike charged polymers in solution that appear in the presence of multivalence counterions. The counterions condensed to the rods exhibit both a strong transversal polarization and a longitudinal crystalline arrangement. At short distances between the rods, the fraction of condensed counterions increases, and the majority of these occupy the region between the rods, where they minimize their repulsive interactions by arranging themselves into packing structures. The attractive interaction is strongest for multivalent counterions. Our model takes into account the hard-core volume of the condensed counterions, and their angular distribution around the rods. The hard-core constraint strongly suppresses longitudinal charge fluctuations.

Journal Article↗

PubMed1998

Precipitation of DNA by polyamines: a polyelectrolyte behavior.

Conditions of double-stranded DNA precipitation by the polyamines spermidine and spermine have been determined experimentally and compared to theoretical predictions. The influence of the concentrations of DNA and added monovalent salt, and of the DNA length has been investigated in a systematic manner. Three regimes of DNA concentrations are observed. We clarify the dependence of these regimes on the monovalent salt concentration and on the DNA length. Our observations make possible a rationalization of the experimental results reported in the literature. A comparison of the precipitation conditions of different kinds of polyelectrolytes suggests a general process. Our experimental data are compared to the "ion-bridging" model based on short-range electrostatic attractions. By starting from the spinodal equation, predicted by this model, and using the limiting form of Manning's fractions of condensed counterions, analytical expressions of the precipitation conditions have been found in the three regimes. Experimental and theoretical results are in good agreement.

Animals↗