Kyung Seok Oh

Crystal structure of Delta(5)-3-ketosteroid isomerase from Pseudomonas testosteroni in complex with equilenin settles the correct hydrogen bonding scheme for transition state stabilization

Δ5-3-Ketosteroid isomerase from Pseudomonas testosteroni has been intensively studied as a prototype to understand an enzyme-catalyzed allylic isomerization. Asp38 (pK a ∼4.7) was identified as the general base abstracting the steroid C4β proton (pK a ∼12.7) to form a dienolate intermediate. A key and common enigmatic issue involved in the proton abstraction is the question of how the energy required for the unfavorable proton transfer can be provided at the active site of the enzyme and/or how the thermodynamic barrier can be drastically reduced. Answering this question has been hindered by the existence of two differently proposed enzyme reaction mechanisms. The 2.26 Å crystal structure of the enzyme in complex with a reaction intermediate analogue equilenin reveals clearly that both the Tyr14 OH and Asp99 COOH provide direct hydrogen bonds to the oxyanion of equilenin. The result negates the catalytic dyad mechanism in which Asp99 donates the hydrogen bond to Tyr14, which in turn is hydrogen bonded to the steroid. A theoretical calculation also favors the doubly hydrogen-bonded system over the dyad system. Proton nuclear magnetic resonance analyses of several mutant enzymes indicate that the Tyr14 OH forms a low barrier hydrogen bond with the dienolic oxyanion of the intermediate.

STRUCTURE, VERTICAL ELECTRON-DETACHMENT ENERGY, AND O-H STRETCHING FREQUENCIES OF E+(H2O)12

The first comprehensive ab initio study is performed on an excess electron bound to the water dodecamer to find out if this wet electron can be regarded as a precursor of the fully solvated electron. Various structures of the wet electron are explored using ab initio calculations. Among a number of possible geometries categorized as unbounded, surface, internal, and partially internal excess-electron states, the lowest-energy conformer is predicted to be a structure of a partially internal state. The predicted vertical electron-detachment energy of this structure is in good agreement with the experimental value of Coe et al. [J. Chem. Phys. 92, 3980 (1990)]. This indicates that in the experiment the partially internal excess-electron state structure would have been detected. The electronic structure, interactions between the excess electron and dangling H atoms (e⋯ H interaction), and red-shifts of the O–H stretching frequencies with strong IR intensities are discussed.

Influence of particle size and shape on electrical and mechanical properties of graphite reinforced conductive polymer composites for the bipolar plate of PEM fuel cells

Graphite reinforced conductive polymer composites (CPCs) with high filler loadings were fabricated by compression molding technique. Various sizes and shapes of graphite particles were mixed with phenol resin to impart the electrical conductivity in composites. Fabricated CPCs showed good electrical conductivity (>100 S/cm) and flexural strength (>40 MPa) for the bipolar plate of polymer electrolyte membrane (PEM) fuel cells. The electrical conductivity of CPCs was affected by the formation of conductive networks among graphite particles. CPCs made of sphere-type particles (SG-CPCs) had the same physical density regardless of particle size; and they also showed the same bulk electrical conductivity. This means that there is a close correlation between the electrical conductivity and the densification level, or density, of graphite/phenol compounds. The particle shape was also a principal factor in influencing electrical conductivity. In this study, the electrical conductivity of CPCs made of flake-type graphite particles (FG-CPCs) was higher than that of SG-CPCs due to the difference of the densification characteristic. The flexural strength of SG-CPCs tended to increase with decreasing graphite particle size because the interfacial coherence between graphite particle and phenol resin increased as graphite particle size decreased. This influence of interfacial coherence was also founded in the variation of particle shape. FG-CPCs have higher flexural strength than SG-CPCs because a flake-type particle has larger specific area than a sphere-type particle.

Ab initio investigations on the HOSO2+O2→SO3+HO2 reaction

HOSO2 radical is the key intermediate for the oxidation SO2 to SO3 by OH radical in the atmosphere. The structural aspects and the energetics of the reaction HOSO2+O2→SO3+HO2 have been studied using Mo/ller–Plesset (MP2) and density functional (DFT) techniques with 6-31G** and triple-ζ, quadruple-ζ, and quintuple-ζ quality basis sets including diffuse basis functions. The detailed theoretical analyses have further revealed that this reaction could proceed through the formation of intermediate complexes and an intramolecular proton transfer like transition state. The energetics of these intermediate reactions has been studied in detail. The use of MP2 methods to study such radical mechanisms had some characteristic symmetry-breaking problem with larger basis sets. This unphysical situation with larger basis set MP2 calculations in this hypervalent system has been explained through the interpretation of the relevant energy surface.

Fatigue life modeling of short fiber reinforced metal matrix composites using mechanical and acoustic emission responses

The cyclic fatigue behavior of short fiber reinforced metal matrix composites was studied. Three fatigue life prediction models were developed by monitoring the resultant maximum strain and counts of acoustic emission during cyclic fatigue. Their increasing trends with the number of fatigue cycles were studied using the assumption that they can be expressed as a power-law function of the fatigue cycle number. Post-fatigue static tension tests were conducted to examine the degradation of the damaged materials. The acoustic emission count accumulated during tension testing decreased as the applied fatigue cycles increased; this change was also used to formulate life prediction models. All the new models show a better agreement to experimental data than do the existing equations.

Characterization of Expanded Graphite/Flake-Type Graphite Filled Conductive Polymer Composites

Conductive polymer composites (CPCs) consisting of expanded graphite (EG), flake-type graphite (FG) and thermalsetting resin were fabricated by means of a preform molding technique. Conductive fillers, EG and FG, were mechanically mixed with the phenol resin to provide an electrical property to composites. The filler loadings were fixed at 75wt.% to obtain a high electrical conductivity. The mechanical and electrical properties of CPCs were optimized according to the weight ratio and the particle size of FG. As the weight ratio increased, the flexural strength increased, however, the electrical conductivity decreased for both cases of CPCs using different sizes of FG. The particle size was an important parameter to change the mechanical and electrical behaviors. The flexural strength was sensitive to the particle size due to the different level of densification. The electrical conductivity also showed size-dependent behavior because of the different contribution to the conductive networking.

Development of Universal Test Method to Evaluate the Plastic Deformation of Sheet Metal

Universal test method to evaluate sheet metal formability was developed using finite element method based on axiomatic design. The newly developed formability test intended to generate the various modes of deformation and to control the onset of failure independently under each mode of deformation. The functional requirements (FRs) and the design parameters (DPs) of the test system were defined on axiomatic design approach and decomposed until the design reaches final stage. The independence axiom was applied throughout the design process to maintain the hierarchical independence of the formability test system. The flow diagram representing the system architecture was introduced after decomposition to give a help to establish the systematic design procedures and to determine the design parameters. Numerical simulation was carried out to determine the specific value of DPs which satisfies the FRs. Numerical results showed that modes of deformation varies accompanying various strain paths and good controllability of sheet forming is obtained for different kinds of materials. Experimental work was finally conducted to validate the proposed design. Stamping results represented that the outcome of the deformed geometry and strains are in good agreement with the numerical results.

Characteristics of Expanded Graphite Filled Conductive Polymer Composites for PEM Fuel Cell Bipolar Plates

This study aims to optimize the mechanical and electrical properties of electrically conductive polymer composites (CPCs) for use as a material of bipolar plates for PEM fuel cells. The thin CPCs consisting of conductive fillers and polymer resin were fabricated by a preform molding technique. Expanded graphite (EG), flake-type graphite (FG) and carbon fiber (CF) were used as conductive fillers. This study tested two types of CPCs, EG/FG filled CPCs and EG/CF filled CPCs, to optimize the material properties. First, the characteristics of EG/FG filled CPCs were investigated according to the FG ratio for 7 and 100 µm sized FG. CPCs using 100 µm FG showed optimal material properties at 60 wt% FG ratio, which were an electrical conductivity of 390 S/cm and flexural strength of 51 MPa. The particle size was an important parameter to change the mechanical and electrical behaviors. The flexural strength was sensitive to the particle size due to the different levels of densification. The electrical conductivity also showed size-dependent behavior because of the different contributions to the conductive network. Meanwhile, the material properties of EG/CF filled CPCs was also optimized according to the CF ratio, and the optimized electrical conductivity and flexural strength were 290 S/cm and 58 MPa, respectively. The electrical conductivity of this case decreased similarly to the EG/FG filled case. On the other hand, the behavior of the flexural strength was more complicated than the EG/FG filled case, and the reason was attributed to the interaction between the strengthening effect of CF and the deterioration of voids.