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"Han, Sang-Woo"
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South Korea's 70-year endeavor for foreign policy, national defense, and unification
This book brings Korea's finest foreign policy minds together in contemplating the risks and rewards of finally ending the 70 year stalemate between North and South Korea through reunification. While North Korea is in conflict with the United States over denuclearization and regime security, the South Korean government is focusing on economic development preparing for the day when the two Koreas are unified. This book will help scholars, activists and policy-makers from all over the world systematically understand the current diplomatic and security issues in the Korean peninsula.
Book
Advancements in the Application of Ribosomally Synthesized and Post-Translationally Modified Peptides (RiPPs)
Ribosomally synthesized and post-translationally modified peptides (RiPPs) represent a significant potential for novel therapeutic applications because of their bioactive properties, stability, and specificity. RiPPs are synthesized on ribosomes, followed by intricate post-translational modifications (PTMs), crucial for their diverse structures and functions. PTMs, such as cyclization, methylation, and proteolysis, play crucial roles in enhancing RiPP stability and bioactivity. Advances in synthetic biology and bioinformatics have significantly advanced the field, introducing new methods for RiPP production and engineering. These methods encompass strategies for heterologous expression, genetic refactoring, and exploiting the substrate tolerance of tailoring enzymes to create novel RiPP analogs with improved or entirely new functions. Furthermore, the introduction and implementation of cutting-edge screening methods, including mRNA display, surface display, and two-hybrid systems, have expedited the identification of RiPPs with significant pharmaceutical potential. This comprehensive review not only discusses the current advancements in RiPP research but also the promising opportunities that leveraging these bioactive peptides for therapeutic applications presents, illustrating the synergy between traditional biochemistry and contemporary synthetic biology and genetic engineering approaches.
Journal Article
Reprogramming biocatalytic futile cycles through computational engineering of stereochemical promiscuity to create an amine racemase
Repurposing the intrinsic properties of natural enzymes can offer a viable solution to current synthetic challenges through the development of novel biocatalytic processes. Although amino acid racemases are ubiquitous in living organisms, an amine racemase (AR) has not yet been discovered despite its synthetic potential for producing chiral amines. Here, we report the creation of an AR based on the serendipitous discovery that amine transaminases (ATAs) can perform stereoinversion of 2-aminobutane. Kinetic modeling revealed that the unexpected off-pathway activity results from stereochemically promiscuous futile cycles due to incomplete stereoselectivity for 2-aminobutane. This finding motivated us to engineer an
S
-selective ATA through in silico alanine scanning and empirical combinatorial mutations, creating an AR with broad substrate specificity. The resulting AR, carrying double point mutations, enables the racemization of both enantiomers of diverse chiral amines in the presence of a cognate ketone. This strategy may be generally applicable to a wide range of transaminases, paving the way for the development of new-to-nature racemases.
Racemization of chiral amines poses a challenge, making dynamic kinetic resolution inaccessible for industrial applications. Here, the authors demonstrate in silico engineering of an amine transaminase to create a new-to-nature amine racemase.
Journal Article
Parameter Optimization of WAAM with Pulsed GMAW for Manufacturing Propeller-Shaped Blade
The experiments of wire arc additive manufacturing (WAAM) with general pulsed gas metal arc welding (GMAW) machine were performed to optimize process parameters. As optimized conditions, 140A welding current, 34 V welding voltage, 40 cm/min welding speed, and 15 mm contact tip to work distance (CTWD), were selected. Welding current and voltage affected heat input. Lower deposition, short circuit mode, and spatters were obtained by insufficient heat input. The melting of previous layer and lower height were obtained by excessive heat input. Welding speed and CTWD affected surface state and arc stability. Unsound surface and bead shape due to short circuit mode and spatters were observed in slow welding speed and short CTWD conditions. Arc instability were also observed in fast welding speed and long CTWD conditions. With the optimized process parameter, curved propeller shape blade was manufactured by the change of robot arm teaching point. It can be considered that the desired sufficient quality was obtained due to curved shape.
Journal Article
Numerical investigation of energy input characteristics for high-power fiber laser welding at different positions
Partial penetration welding with a fiber laser at a 9-kW laser power was carried out on 20-mm-thick plates at different positions and analyzed by both experimental and numerical methods. Experiments were carried out for four different angular positions at 1.5 m/min welding speed. All four cases showed a tail-like structure of molten pool on the top surface. The in-depth mechanisms of the energy input characteristics in fiber laser position welding for eight different positions were studied by numerical simulation using the volume-of-fluid (VOF) method. Experimental and numerical results were compared for four cases and showed fair agreement. Observation of flow pattern and bead shape revealed that gravity had little influence on bead shape and flow structure, but changed the pore structure considerably. Flow structure showed a periodic behavior which probably nullified the effect of gravity in position welding. Most of the laser rays reached the bottom of the keyhole without interruption and then multiple reflections started within the keyhole. The first five reflections inside the keyhole delivered around 70 % of the total energy.
Journal Article
Effect of Elevated Temperature and Internal Pressure due to Severe Accidents on the Internal Pressure Capacity of Prestressed Concrete Containment Vessel
The integrity of containment buildings in nuclear power plants is crucial for preventing the release of radioactive materials during severe accidents. This study investigates the effect of the uncertainty in temperature-dependent strength of concrete on the internal pressure capacity of prestressed concrete containment vessel (PCCV). To this end, a high-fidelity finite element model is developed and the uncertainty in concrete material properties due to temperature variations is taken into account for a finite element analysis with internal pressure and temperature histories. In addition, two limit states of PCCV, such as onset of leakage and functional failure, are defined to investigate internal pressure capacity depending on the different damages to PCCV. The results provide insights into the behavior of PCCV under severe accident conditions and the impact of the uncertainty in the concrete material due to temperature on their performance, in terms of the leakage of PCCV. This research enhances the understanding of PCCV’s response to internal pressure and temperature and contributes to the safety assessment of nuclear power plants.
Journal Article
Enhancement of sophorolipids production in Candida batistae, an unexplored sophorolipids producer, by fed-batch fermentation
Sophorolipids (SLs) from
Candida batistae
has a unique structure that contains ω–hydroxy fatty acids, which can be used as a building block in the polymer and fragrance industries. To improve the production of this industrially important SLs, we optimized the culture medium of
C. batistae
for the first time. Using an optimized culture medium composed of 50 g/L glucose, 50 g/L rapeseed oil, 5 g/L ammonium nitrate and 5 g/L yeast extract, SLs were produced at a concentration of 24.1 g/L in a flask culture. Sophorolipids production increased by about 19% (28.6 g/L) in a fed-batch fermentation using a 5 L fermentor. Sophorolipids production more increased by about 121% (53.2 g/L), compared with that in a flask culture, in a fed-batch fermentation using a 50 L fermentor, which was about 787% higher than that of the previously reported SLs production (6 g/L). These results indicate that a significant increase in
C. batistae
-derived SLs production can be achieved by optimization of the culture medium composition and fed-batch fermentation. Finally, we successfully separated and purified the SLs from the culture medium. The improved production of SLs from
C. batistae
in this study will help facilitate the successful development of applications for the SLs.
Journal Article
Investigation of phase evolution within ZnO–Bi2O3 varistors utilizing thin film prototypes
Varistors are technologically important for their large energy handling capabilities and highly nonlinear electrical behavior when voltages above a characteristic switch field are applied. It is generally accepted that the prototypical ZnO–Bi
2
O
3
varistor system forms electrostatic Schottky barriers at grain boundaries in response to residual Bi and other dopants left at grain surfaces during Bi
2
O
3
segregation. While barrier heights can be modulated with formulation and defect chemistry, mechanisms by which dopant locations, defect compensation, and local phases determine varistor behavior are not completely understood. Bulk studies are challenging due to random grain boundary formation and difficulties studying individual boundaries. To circumvent these challenges in the ZnO–Bi
2
O
3
varistor system, we use as-deposited and post-heat-treated thin film ZnO–Bi
2
O
3
prototypes to simulate bulk varistor grain boundary phase formation and investigate resulting defect chemistry. Characterizing interactions between Bi
2
O
3
films deposited on thin film and single-crystal ZnO by XRD and TEM-EDS revealed primarily Zn-out diffusion, resulting in two (Bi
2
O
3
)
1−
x
(ZnO)
x
, or BZO, phases. Using these results, we present a saturated front model correlating changes in Bi
2
O
3
thickness to phase evolution. We subsequently explore the influence of MnO doping leading to substantial changes in phase evolution for post-heat-treated (Mn:ZnO)–Bi
2
O
3
stacks. Dopant-controlled Bi
2
O
3
phase formations yield a 12 × difference, on average, between nonlinear coefficients for
γ*
- and
β*
-BZO.
Graphical Abstract
Journal Article
One-Pot Preparation of d-Amino Acids Through Biocatalytic Deracemization Using Alanine Dehydrogenase and ω-Transaminase
d
-Amino acids are pharmaceutically important building blocks, leading to a great deal of research efforts to develop cost-effective synthetic methods. Preparation of
d
-amino acids by deracemization has been conceptually attractive owing to facile synthesis of racemic amino acids by Strecker synthesis. Here, we demonstrated biocatalytic deracemization of aliphatic amino acids into
d
-enantiomers by running cascade reactions; (1) stereoinversion of
l
-amino acid to a
d
-form by amino acid dehydrogenase and ω-transaminase and (2) regeneration of NAD
+
by NADH oxidase. Under the cascade reaction conditions containing 100 mM isopropylamine and 1 mM NAD
+
, complete deracemization of 100 mM
dl
-alanine was achieved after 24 h with 95% reaction yield of
d
-alanine (> 99%
ee
D
, 52% isolation yield).
Graphical Abstract
Journal Article
Modification of Hybrid Receptor Model for Atmospheric Fine Particles (PM2.5) in 2020 Daejeon, Korea, Using an ACERWT Model
Hybrid receptor models overestimate the contribution of background areas (no specific emission sources), like the Yellow Sea in Korea. This study aimed to improve model performances using Advanced Concentration Emission and Retention Time Weighted Trajectory (ACERWT). ACERWT was combined with a positive matrix factorization (PMF), back trajectory, and Regional Emission Inventory in Asia (REAS). The PMF receptor model used one year of data from Korea’s Central Air Environment Research Center. In the PMF receptor model, eight sources (dust/soil, secondary nitrate, biomass burning, vehicles, secondary sulfate, industry, coal combustion and sea salt) influenced PM2.5 pollution at the receptor site (Daejeon, Korea). Secondary sulfate was the most dominant source, followed by secondary nitrate and vehicle sources. ACERWT results showed high contributions from China, Japan, and Korean regions, while the contribution from the Yellow Sea was significantly lower. Several regions, such as the eastern and south-eastern areas of China, the southern area of Taiwan, the western area of Tokyo, and the central area of Korea, showed high contributions due to large-scale emission facilities and industrial complexes. In this study, the ACERWT model significantly improved its performance regarding regional contributions to PM2.5 pollution at the receptor site.
Journal Article