Biomedical subjects

Yong Woon Kim

Publications and source records attributed to Yong Woon Kim.

6 recordsLinked to original sources

PubMed2006

Static and dynamic correlations in a charged membrane.

We study static and dynamic correlations of two fluctuations, the charge density fluctuation and height fluctuation (undulation), on a fluid membrane with a finite excess charge in a viscous fluid. For a planar and symmetrical membrane, we consider a model Hamiltonian inclusive of the fluctuations at the Gaussian level, and construct their equations of motion. Within the model, there exists no coupling, either static or dynamic, between the two fluctuations. The correlation function of the charge density has a short-range damped oscillation over the size of lipid heads due to Coulomb attraction between unlike-charged lipids. Its dynamic correlation function is shown to decay much faster in time than that in simple diffusion. The correlation function of height undulation, on the other hand, has a long-range damped oscillation (bud) over the membrane size, due to Coulomb repulsion among the excess charges. As the excess charge density increases to a critical value, a bending instability sets in, where a minute perturbation on the membrane can cause a large bud to form. Due to the excess charge, the dynamic correlation of the undulation decays slowly in time; at the critical density of the instability, the decay becomes infinitely slow.

Lipid Bilayers↗

PubMed2006

Pumping fluids with periodically beating grafted elastic filaments.

Using Brownian dynamics simulations, we investigate the pumping efficiency of an array of periodically beating semiflexible filaments that are grafted to a surface. Full hydrodynamic interactions among and within filaments and no slip at the surface are considered. Optimal pumping is obtained for a characteristic ratio of applied forward-backward torques and filament persistence length. For independently driven filaments, phase locking between neighboring filaments occurs autonomously via hydrodynamic coupling, giving rise to significantly enhanced pumping efficiency.

Biological Clocks↗

PubMed2006

Electro-osmosis at inhomogeneous charged surfaces: hydrodynamic versus electric friction.

Electrokinetic methods are efficient in probing the electrostatic surface properties of charged systems. However, anomalies observed in experiments indicate that the classical electrokinetic theory should be reconsidered. Using Green's function methods and hydrodynamic simulations, we investigate electro-osmosis driven by electric-field-induced ion motion near a charged planar substrate with smooth or rough boundary. First, a reformulation of electro-osmotic theory for planar charged surfaces employing Green's functions shows that the Helmholtz-Smoluchowski (HS) relation between electrostatic potential and solvent velocity is exact for smooth surfaces, even in the presence of ion correlations. Deviations from HS theory are caused by combined hydrodynamic and electric surface friction, as our hydrodynamic simulations of ions at smooth and corrugated charged surfaces in lateral electric fields demonstrate. Within the simulations, hydrodynamic interactions are treated in the continuum limit and the presence of a no-slip boundary condition at the surface is taken into account. While electrofriction is relevant in highly charged system and/or for multivalent ions, hydrodynamic friction is dominant in systems with moderate surface charge density and/or low ionic valency. We also derive the effective electrokinetic surface charge from the electro-osmotic solvent profiles, which is substantially reduced when compared with the bare value and shows qualitative agreement with the experimental tendency.

Journal Article↗

PubMed2006

Dissimilar kinetic behavior of electrically manipulated single- and double-stranded DNA tethered to a gold surface.

We report on the electrical manipulation of single- and double-stranded oligodeoxynucleotides that are end tethered to gold surfaces in electrolyte solution. The response to alternating repulsive and attractive electric surface fields is studied by time-resolved fluorescence measurements, revealing markedly distinct dynamics for the flexible single-stranded and stiff double-stranded DNA, respectively. Hydrodynamic simulations rationalize this finding and disclose two different kinetic mechanisms: stiff polymers undergo rotation around the anchoring pivot point; flexible polymers, on the other hand, are pulled onto the attracting surface segment by segment.

Computer Simulation↗

PubMed2006

A protective role for heme oxygenase-1 in INS-1 cells and rat islets that are exposed to high glucose conditions.

Heme oxygenase-1 (HO-1) has been described as an inducible protein that is capable of cytoprotection via radical scavenging and the prevention of apoptosis. Chronic exposure to hyperglycemia can lead to cellular dysfunction that may become irreversible over time, and this process has been termed glucose toxicity. Yet little is known about the relation between glucose toxicity and HO-1 in the islets. The purposes of the present study were to determine whether prolonged exposure of pancreatic islets to a supraphysiologic glucose concentration disrupts the intracellular balance between reactive oxygen species (ROS) and HO-1, and so this causes defective insulin secretion; we also wanted to evaluate a protective role for HO-1 in pancreatic islets against high glucose levels. The intracellular peroxide levels of the pancreatic islets (INS-1 cell, rat islet) were increased in the high glucose media (30 mM glucose or 50 mM ribose). The HO-1 expression was induced in the INS-1 cells by the high glucose levels. Both the HO-1 expression and glucose stimulated insulin secretion (GSIS) was decreased simultaneously in the islets by treatment of the HO-1 antisense. The HO-1 was upregulated in the INS-1 cells by hemin, an inducer of HO-1. And, HO-1 upregulation induced by hemin reversed the GSIS in the islets at a high glucose condition. These results suggest HO-1 seems to mediate the protective response of pancreatic islets against the oxidative stress that is due to high glucose conditions.

Animals↗

PubMed2003

Effects of charge fluctuation on two-membrane instability and fusion.

Membrane fusion is fundamental to diverse biological processes ranging from intercellular and intracellular transport to egg fertilization. We study the effects of coupling between membrane undulation and charge fluctuation on its fusion. We find that, at concentrations of millimolar range, multivalent cations such as calcium in solution induce a strong correlated-charge fluctuation on each membrane, leading to inversion and overcondensation of surface charges. When the charge fluctuation is cooperatively coupled to undulation, two apposing membranes undergo a dynamic instability to spontaneous growth of in-phase undulation with submicron wavelengths, thereby greatly reducing fusion barrier.

Calcium↗