Biomedical subjects

Yu-Jane Sheng

Publications and source records attributed to Yu-Jane Sheng.

9 recordsLinked to original sources

PubMed2004

Charge renormalization of charged spheres based on thermodynamic properties.

At strong electrostatic coupling, counterions are accumulated in the vicinity of the surface of the charged particle with intrinsic charge Z. In order to explain the behavior of highly charged particles, effective charge Z(*) is therefore invoked in the models based on Debye-Huckel approximation, such as the Derjaguin-Landau-Verwey-Overbeek potential. For a salt-free colloidal suspension, we perform Monte Carlo simulations to obtain various thermodynamic properties omega in a spherical Wigner-Seitz cell. The effect of dielectric discontinuity is examined. We show that at the same particle volume fraction, counterions around a highly charged sphere with Z may display the same value of omega as those around a weakly charged sphere with Z(*), i.e., omega(Z)=omega(Z(*)). There exists a maximally attainable value of omega at which Z=Z(*). Defining Z(*) as the effective charge, we find that the effective charge passes through a maximum and declines again due to ion-ion correlation as the number of counterions is increased. The effective charge is even smaller if one adopts the Debye-Huckel expression omega(DH). Our results suggest that charge renormalization can be performed by chemical potential, which may be observed in osmotic pressure measurements.

Journal Article↗

PubMed2004

Effect of solvent quality on the conformations of a model comb polymer.

The effect of solvent quality on the equilibrium structure of a densely branched comb polymer is investigated based on the structure factor analyses by off-lattice Monte Carlo simulations. First, theta temperature (theta(infinity)) must be determined to identify the solvent condition. We locate the characteristic temperature theta(A)(N) at which the second virial coefficient vanishes and the transition temperature theta(R)(N) at which radius of gyration R(g) of the chain varies most rapidly with temperature, i.e., d(2)R(g)/dT(2)|(theta(R)) = 0. N represents the total number of monomers of a comb. As N --> infinity, theta(A) and theta(R) coincide to a point that is identified as the true theta temperature (theta(infinity)). The structure factors of the main chain, the side chain, and the whole polymer are calculated, respectively. It is found that at T = theta(infinity), the structural factors S(qR(g)) for the overall comb polymers match quite well with those of their Gaussian counterparts. When T< theta(infinity), the overall comb polymer assumes collapsed conformations, similar to a homogeneous sphere. However, the structure factor of the side chain indicates that it always remains in an expanded state regardless of the solvent condition. It is attributed to the strong interactions between side chains. The same effect leads to enhanced rigidity of the main chain in comparison to the linear chain, as clearly observed from the rescaled Kratky plot.

Journal Article↗

PubMed2004

Monte Carlo simulations of antibody adsorption and orientation on charged surfaces.

Monte Carlo simulations were performed to study the adsorption and orientation of antibodies on charged surfaces based on both colloidal and all-atom models. The colloidal model antibody consists of 12 connected beads representing the 12 domains of an antibody molecule. The structure of the all-atom antibody model was taken from the protein databank. The effects of the surface charge sign and density, the solution pH and ionic strength on the adsorption and orientation of different colloidal model antibodies with different dipole moments were examined. Simulation results show that both the 12-bead and the all-atom models of the antibody, for which the dipole moment points from the Fc to (Fab)2 fragments, tend to have the desired "end-on" orientation on positively charged surfaces and undesired "head-on" orientation on negatively charged surfaces at high surface charge density and low solution ionic strength where electrostatic interactions dominate. At low surface charge density and high solution ionic strength where van der Waals interactions dominate, 12-bead model antibodies tend to have "lying-flat" orientation on surfaces. The orientation of adsorbed antibodies results from the compromise between electrostatic and van der Waals interactions. The dipole moment of an antibody is an important factor for antibody orientation on charged surfaces when electrostatic interactions dominate. This charge-driven protein orientation hypothesis was verified by our simulations results in this work. It was further confirmed by surface plasmon resonance biosensor and time-of-flight secondary ion mass spectrometry experiments reported elsewhere.

Adsorption↗

PubMed2004

Electrostatic attraction between neutral microdroplets by ion fluctuations.

The interaction between two aqueous droplets containing ions is investigated. The ion-fluctuation correlation gives rise to attraction between two neutral microdroplets, similar to the van der Waals interaction between neutral atoms. Electrostatic attraction consists of contributions from various induced multipole-multipole interactions, including dipole-dipole < P(2)(z) >(2) r(-6), dipole-quadrupole < P(2)(z) > < Q (2)(zz ) > r(-8), dipole-octupole < P(2)(z) > < O (2)(zzz ) > r(-10), and quadrupole-quadrupole interactions < Q (2)(zz ) >(2) r(-10). The mean-square multipole moments are determined analytically by linear response theory. The fluctuation-driven attraction is so strong at short distance that it may dominate over the Coulomb repulsion between like-charged droplets. These theoretical results are confirmed by Monte Carlo simulations.

Journal Article↗

PubMed2004

Surface tension increment due to solute addition.

Addition of solute into solvent may lead to an increase in surface tension, such as salt in water and water in alcohol, due to solute depletion at the interface. The repulsion of the solute from the interface may originate from electrostatic forces or solute-solvent attraction. On the basis of the square-well model for the interface-solute interaction, we derive the surface tension increment Deltagamma by both canonical and grand-canonical routes (Gibbs adsorption isotherm) for a spherical droplet. The surface tension is increased linearly with the bulk concentration of the solute c(b) and the interaction range lambda. The theoretical results are consistent with those obtained by experiments and Monte Carlo simulations up to a few molarity. For weak repulsion, the increment is internal energy driven. When the repulsion is large enough, the surface tension increment is entropy driven and approaches the asymptotic limit, Deltagamma approximately c(b)k(B)Tlambda, due to the nearly complete depletion of the solute at the interface. Our result may shed some light on the surface tension increment for electrolyte solutions with concentration above 0.2M.

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PubMed2002

Ion distributions within a microdroplet without surface charge: fluctuation-correlation effects.

The mean-field approach like the Poisson-Boltzmann theory predicts a trivial result that ions from a symmetric electrolyte are uniformly distributed within an aqueous microdroplet without surface charge. However, the phenomenon of an increase in surface tension due to salt addition indicates ion depletion near the air-water interface. It was believed that electrical polarization associated with dielectric differences is responsible for salt depletion. Using Monte Carlo simulations with the restrictive primitive model and Poisson equation, a depletion zone near the water-oil interface is observed. Nevertheless, without any polarization, ion depletion can still be attained near a hard interface and was theoretically predicted by the Debye-Hückel theory [Phys. Rev. E 60, 3174 (1999)]. The nonuniform ion distribution is caused by the fluctuation-correlation effect with broken symmetry, which is furnished by the boundary.

Journal Article↗

PubMed2002

Orientation of a Y-shaped biomolecule adsorbed on a charged surface.

The adsorption and orientation properties of a Y-shaped biomolecule, which models an immunoglobulin (Ig), on a charged surface are analyzed mesoscopically by Monte Carlo simulations. The orientation is a consequence of the interplay between van der Waals interactions and electrostatic interactions. For adsorption dominated by van der Waals attraction, the molecule prefers lying flat on the surface. For weak attraction, we observe a depletion zone in the concentration profile, which can result in a negative surface excess. A secondary peak is found for strong adsorption. For electrostatically dominated adsorption, the orientation is mainly determined by electric dipole and a vertically adsorbed molecule can be attained as it possesses strong electric dipole. Our study provides an explanation for experimental observations of preferential orientation.

Adsorption↗

PubMed2002

Electrostatic interaction between two aqueous microdroplets in an apolar medium.

In an apolar medium, the electrostatic interactions between two aqueous microdroplets coated with surfactants are investigated by the mean-field theory. The electric field and ion distributions within the aqueous core are described by the Poisson-Boltzmann equation. Under Debye-Hückel approximation, the interaction energy is obtained analytically. Due to the polarization effect, the Coulomb interaction is altered by the induced multipoles in the aqueous droplet. The interaction, however, is insensitive to the ion concentration. In comparison with the Coulomb interaction, the repulsion for a pair of similarly charged droplets is reduced and the attraction for a pair of oppositely charged droplets is enhanced. More importantly, the interaction between a neutral and charged droplets is attractive. The effect of interdroplet interactions on the collision frequency is also discussed.

Journal Article↗

PubMed2001

Polymer knot confined in a tube: statics and relaxation dynamics.

The statics and relaxation dynamics of a polymer knot confined in a cylindrical tube of diameter D are investigated by Monte Carlo simulations. The prime knots with crossings varying from 3(1) to 7(1) are considered. The equilibrium radius of gyration along the axial direction is scaled as R(parallel) approximately R(F)(R(F)/D)(m), where R(F) is the Flory radius of a knotted polymer. Our simulation results suggest that m=0.95, which is different from the value for a linear chain, m=2/3. The relaxation behavior of the knotted polymer is studied after the tubular constraint is removed. The relaxation time can be scaled as tau(parallel) approximately NR(1+1/m)(F)R(1-1/m)(parallel), which is confirmed by simulations. The effect of topological complexity on both properties is manifested through the Flory radius.

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