Joonil Seog - Academia.edu (original) (raw)

Papers by Joonil Seog

We studied the nucleation mechanism of silk-elastin-like peptide (SELP) nanofibers using lateral ... more We studied the nucleation mechanism of silk-elastin-like peptide (SELP) nanofibers using lateral force microscopy. When a single line was repeatedly scanned on SELP coated mica surface, a sudden height increase was observed, indicating that the nucleus of amyloid fiber was formed during lateral scanning. The detailed analysis of frictional force profiles revealed that increase of frictional force was followed by a nucleus formation. The profile of increased frictional force was well fitted with exponential function, suggesting that AFM tip stretches multiple SELP molecules to the scanning direction. The probability of nucleus formation was highly dependent on the maximum level of increased frictional force, implying that the highly stretched SELPs are more likely to form nucleus for nanofiber growth.

Polymer, 1996

The excessively brittle diallyl isophthalate (DAIP) resin was toughened using polyarylate (PAr). ... more The excessively brittle diallyl isophthalate (DAIP) resin was toughened using polyarylate (PAr). The cure kinetics and appropriate cure condition were investigated with differential scanning calorimetry. Fracture surfaces showed that phase separation occurred, resulting in a two-phase morphology. At 2 and 5 phr of PAr, spherical particles of PAr containing DAIP inclusions were obtained. Observation of tails at the rear of particles on the fracture surfaces provides some evidence for crack pinning, which resulted in a slight increase in fracture toughness. At 10phr of PAr a co-continuous connected globular morphology was obtained, leading to a step increase in the fracture toughness by ductile drawing of the PAr. Co-continuous structure was also observed at 15 phr of PAr, and fracture toughness was increased by 1.5 times with little sacrifice in other mechanical properties. From optical and scanning electron microscopy studies on fracture surfaces, the toughening mechanism is considered to be crack path deflection and ductile drawing of PAr.

The European Physical Journal D, 2016

Plasma Processes and Polymers, 2015

Aps March Meeting Abstracts, Mar 1, 2003

In this study, we employed a Poisson-Boltzmann continuum approach in order to investigate how non... more In this study, we employed a Poisson-Boltzmann continuum approach in order to investigate how nonuniform molecular level charge distributions affect the nanoscale electrostatic forces between opposing oppositely charged polyelectrolyte brushes. The time-averaged space taken up by each macromolecule was represented as a finite-length cylindrical rod of uniform volume charge density. Finite difference methods were used to solve for the potential, ion concentrations, and interaction force in a variety of solution conditions (i.e. ionic strength, pH, brush height, and brush density). Depending on whether the brush layers compress (exclude each other) or whether they can interdigitate, there are significant differences in the magnitude and form of the predicted force, even when the total charge remains the same. The compressed model is equivalent to doubling the compressive force of one brush while in the interdigitated model there are additional brush-brush repulsions as the rods from the different opposing brushes are interspersed and closer to each other.

Integrins are cell adhesion receptors that link to the cytoskeleton and the extracellular matrix,... more Integrins are cell adhesion receptors that link to the cytoskeleton and the extracellular matrix, and transmit signals bidirectionally across the cell membrane. Signal transduction between the cytoplasmic domain and the ligand-binding domain (the "Inserted" or I-domain) occurs by a series of conformational changes that lead to allosteric transitions in the I-domain. Different allosteries of the I-domain have markedly different binding affinities for ligands and are critical for cell motility and adhesion. In addition, the I-domain bears the Rossmann fold, a common motif found in nucleic-acid binding proteins and von Willebrand factor A domains, which has a characteristic of inner beta strands being connected by peripheral alpha helices in an alternating matter. We have performed single molecule pulling experiments on the integrin LFA-1 I-domain using mini optical tweezers. In pulling experiments, mechanical unfolding and refolding revealed 2 or more transitions for the nat...

It is thought that the negatively charged, disaccharide macromolecules, glycosaminoglycans (GAGs)... more It is thought that the negatively charged, disaccharide macromolecules, glycosaminoglycans (GAGs), are the major determinant of the outstanding biomechanical properties of cartilage; i.e. its ability to resist compressive loading in vivo, responsible for 50-75 percent of the equilibrium compressive elastic modulus, as well as directly related to disease (i.e. osteoarthritis). The underlying molecular mechanism responsible for the outstanding compressive properties of cartilage arises from the unique nanomechanical properties of the constituent GAGs. In particular, intermolecular electrostatic repulsion due to the counterion electrical double layer of the charged polymers, as well as steric repulsion originating from configurational entropy. The technique of high-resolution force spectroscopy was employed to directly quanitfy these intermolecular interactions between biomimetic model end-grafted GAG "brush" surfaces with packing density 6nmx6nm-area per chain and probe tips...

Repulsive electrostatic interactions between negatively charged glycosaminoglycans (GAGs) are a m... more Repulsive electrostatic interactions between negatively charged glycosaminoglycans (GAGs) are a major determinant of compressive strength of cartilage. To probe the molecular origin of cartilage biomechanical properties, the interactions between two opposing end-grafted GAG brushes were measured using the technique of high-resolution force spectroscopy. A nanosized probe tip was functionalized with GAG using electric field chemisorption and the parking density was determined using a "reverse calibration" curve obtained from HRFS experiments on GAG-functionalized planar substrates of various parking densities (3.6-10 nm intermolecular separation distance) measured by DMMB assay (dimethylmethylene blue). The GAG-GAG interaction forces were measured as a function of ionic strength, 0.0001M-1M NaCl at pH ˜5.6, and pH=3 & 7 at 0.015 M NaCl, and compared to the predictions of a newly developed Poisson-Boltzmann model that represents the GAGs as rods of a uniform charge density, ...

Plasma Processes and Polymers, 2012

ABSTRACT It is widely accepted that plasma-generated energetic and reactive species are responsib... more ABSTRACT It is widely accepted that plasma-generated energetic and reactive species are responsible for plasma-induced sterilization; however, how these species act alone or synergistically to deactivate endotoxic biomolecules is not completely understood. Using a vacuum beam system, we study the effects of vacuum ultraviolet (VUV) radiation, oxygen and deuterium radicals on lipid A, the immune-stimulating region of lipopolysaccharide. VUV-induced photolysis causes bulk modification of exposed lipid A film up to the penetration depth of VUV photons, ≈200 nm. Although radical-induced etch yield of lipid A is lower than VUV-induced photolysis, secondary ion mass spectrometry and human whole blood-based assay suggest that radicals render a higher degree of modification at the film surface. This study contributes to the fundamental understanding of plasma effects on biomolecules for a better deactivation scheme and applications.

Biointerphases, 2015

An atmospheric pressure plasma jet (APPJ) was used to treat polystyrene (PS) films under remote c... more An atmospheric pressure plasma jet (APPJ) was used to treat polystyrene (PS) films under remote conditions where neither the plume nor visible afterglow interacts with the film surface. Carefully controlled conditions were achieved by mounting the APPJ inside a vacuum chamber interfaced to a UHV surface analysis system. PS was chosen as a model system as it contains neither oxygen nor nitrogen, has been extensively studied, and provides insight into how the aromatic structures widespread in biological systems are modified by atmospheric plasma. These remote treatments cause negligible etching and surface roughening, which is promising for treatment of sensitive materials. The surface chemistry was measured by X-ray photoelectron spectroscopy to evaluate how ambient chemistry, feed gas chemistry, and plasma-ambient interaction impact the formation of specific moieties. A variety of oxidized carbon species and low concentrations of NOx species were measured after APPJ treatment. In th...

Nonviral carriers have been developed to deliver nucleic acids by forming nanoscale complexes; ho... more Nonviral carriers have been developed to deliver nucleic acids by forming nanoscale complexes; however, there has been limited success in achieving high transfection efficiency. Our hypothesis is that a factor affecting gene delivery efficiency is the mechanical response of the condensed complex. To begin to test this hypothesis, we directly measured the mechanical properties of DNA-carrier complexes using optical tweezers. Histidine-lysine (HK) polymer, Asparagine-lysine (NK) polymer and poly-L-lysine were used to form complexes with a single DNA molecule. As carriers were introduced, a sudden decrease in DNA extension occurrs at a force level which is defined as critical force (Fc). Fc is carrier and concentration dependent. Pulling revealed reduction in DNA extension length for HK-DNA complexes. The characteristics of force profiles vary by agent and can be dynamically manipulated by changes in environmental conditions such as ionic strength of the buffer as well as pH. Heparin c...

Branched peptides containing histidines and lysines (HK) have been determined to be effective car... more Branched peptides containing histidines and lysines (HK) have been determined to be effective carriers for DNA or siRNA. We anticipate that elucidation of the binding mechanism of HK with siRNA will provide greater insight into the self-assembly and delivery of the polyplex. In siRNA silencing experiments with mammalian cells, a four-branched HK peptide siRNA polyplex maintained silencing activity even with prolonged pre-incubation with serum. In contrast, siRNA in complex with four-branched N3K4b peptide, in which histidines were substituted with asparagines, showed a marked decreased in silencing activity with pre-exposure to serum. Consequently, we explored the hypothesis that histidines might form non-covalent bonds with nucleic acids to enhance the stability of siRNA polyplexes. To accomplish this, we initially compared the biophysical properties of H3K(+H)4b with N3K4b and polylysine. Consistent with siRNA silencing experiments, gel electrophoresis analysis demonstrated that t...

ABSTRACT Repulsive forces between end-grafted chondroitin-4-sulfate glycosaminoglycan (GAG) polym... more ABSTRACT Repulsive forces between end-grafted chondroitin-4-sulfate glycosaminoglycan (GAG) polymer brushes have been measured and reported by us using the technique of high-resolution force microscopy as a function of pH and ionic strength. To understand the nature of these forces, we have compared our experimental data to three electrostatic double layer models based on the Poisson-Boltzmann equation. The first two models are formulated in the literature and represent the GAG brush as a planar surface of constant charge and as a smoothed volume of known fixed charge density, while the third model is newly developed and represents the GAGs as rods of uniform charge density, which more accurately describes the molecular morphology and nonuniform charge distributions. The GAG brush could not be modeled accurately using a planar charge model, which severely underestimated the magnitude of the intersurface force throughout the distance range measured. The volume charge model fit better to experimental data for reasonable values of the brush height suggesting that the volume distribution of charge on the GAGs leads to a significant increase in repulsive force.

ABSTRACT In this study, we employed a Poisson-Boltzmann continuum approach in order to investigat... more ABSTRACT In this study, we employed a Poisson-Boltzmann continuum approach in order to investigate how nonuniform molecular level charge distributions affect the nanoscale electrostatic forces between opposing oppositely charged polyelectrolyte brushes. The time-averaged space taken up by each macromolecule was represented as a finite-length cylindrical rod of uniform volume charge density. Finite difference methods were used to solve for the potential, ion concentrations, and interaction force in a variety of solution conditions (i.e. ionic strength, pH, brush height, and brush density). Depending on whether the brush layers compress (exclude each other) or whether they can interdigitate, there are significant differences in the magnitude and form of the predicted force, even when the total charge remains the same. The compressed model is equivalent to doubling the compressive force of one brush while in the interdigitated model there are additional brush-brush repulsions as the rods from the different opposing brushes are interspersed and closer to each other.

Journal of Physics D: Applied Physics, 2014

ABSTRACT This paper reports the influence of gas plasma flux on endotoxin lipid A film deactivati... more ABSTRACT This paper reports the influence of gas plasma flux on endotoxin lipid A film deactivation. To study the effect of the flux magnitude of reactive species, a modified low-pressure inductively coupled plasma (ICP) with O radical flux ~1016 cm−2 s−1 was used. After ICP exposures, it was observed that while the Fourier transform infrared absorbance of fatty chains responsible for the toxicity drops by 80% through the film, no obvious film endotoxin deactivation is seen. This is in contrast to that previously observed under low flux exposure conducted in a vacuum beam system: near-surface only loss of fatty chains led to significant film deactivation. Secondary ion mass spectrometry characterization of changes at the film surface did not appear to correlate with the degree of deactivation. Lipid A films need to be nearly completely removed in order to detect significant deactivation under high flux conditions. Additional high reactive species flux experiments were conducted using an atmospheric pressure helium plasma jet and a UV/ozone device. Exposure of lipid A films to reactive species with these devices showed similar deactivation behaviour. The causes for the difference between low and high flux exposures may be due to the nature of near-surface structural modifications as a function of the rate of film removal.

Nano Energy, 2013

The solid electrolyte interphase (SEI) is a layer that forms at the anode surface for all alkali ... more The solid electrolyte interphase (SEI) is a layer that forms at the anode surface for all alkali metal ion batteries which utilize liquid electrolytes. It acts as an electronic insulator and ion conductor, but the formation of the SEI results in an irreversible capacity loss. For high capacity anodes in sodium ion batteries (NIBs), continued cycling of the battery ruptures the SEI and exposes new areas of the anode surface where the electrolyte will reduce. In this work, we utilize colloidal probe microscopy to investigate the mechanical properties of the SEI layer in NIBs for use in future anode designs. Our results indicate a lateral inhomogeneity in the Young's Modulus on the micron scale spanning 3 orders of magnitude. This technique can be used to investigate the differences in the SEI formed by different electrolytes and electrolyte additives and ultimately to determine the best liquid electrolyte for NIBs.

Angewandte Chemie, 2014

Gene delivery is a promising way to treat hereditary diseases and cancer; however, there is littl... more Gene delivery is a promising way to treat hereditary diseases and cancer; however, there is little understanding of DNA:carrier complex mechanical properties, which may be critical for the protection and release of nucleic acids. We applied optical tweezers to directly measure single-molecule mechanical properties of DNA condensed using 19-mer poly-L-lysine (PLL) or branched histidine-lysine (HK) peptides. Force-extension profiles indicate that both carriers condense DNA actively, showing force plateaus during stretching and relaxation cycles. As the environment such as carrier concentration, pH, and the presence of zinc ions changes, DNA:HK complexes showed dynamically regulated mechanical properties at multiple force levels. The fundamental knowledge from this study can be applied to design a mechanically tailored complex which may enhance transfection efficiency by controlling the stability of the complex temporally and spatially.

Macromolecules, 2002

Intermolecular repulsion forces between negatively charged glycosaminoglycan (CS-GAG) macromolecu... more Intermolecular repulsion forces between negatively charged glycosaminoglycan (CS-GAG) macromolecules are a major determinant of cartilage biomechanical properties. It is thought that the electrostatic component of the total intermolecular interaction is responsible for 50-75% of the equilibrium elastic modulus of cartilage in compression, while other forces (e.g., steric, hydration, van der Waals, etc.) may also play a role. To investigate these forces, radiolabeled CS-GAG polymer chains, with a fully extended contour length of 35 nm, were chemically end-grafted to a planar surface to form model biomimetic polyelectrolyte "brush" layers whose environment (e.g., ionic strength, pH) was varied to mimic physiological conditions. The total intersurface force (enN) between the CS-GAG brushes and chemically modified probe tips (SO 3and OH) was measured as a function of tip-substrate separation distance in aqueous solution using the technique of high-resolution force spectroscopy (HRFS). These experiments showed long-range, nonlinear, purely repulsive forces that decreased in magnitude and range with increasing ionic strength and decreasing pH. To estimate the contribution of the electrostatic component to the total intersurface force, the data were compared to a theoretical model of electrical double layer repulsion based on the Poisson-Boltzmann formulation. The CS-GAG brush layer was approximated as either a flat surface charge density or a smoothed volume of known fixed charge density and the probe tip was modeled as a smooth hemisphere of constant surface charge density. Modeling the CS-GAG brush as a volume charge yielded theoretical fits much closer to the experimental data and is a good first step toward deconvolution of the force components.