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Information Center for Mathematical Science

세미나

Information Center for Mathematical Science

세미나

서울대학교 지구환경과학부 세미나

Dansgaard Oeschger (DO) events are one of the most fascinating phenomena in our climate system. They are characterized by rapid [ (10 years)] warming of the Northern Hemisphere reaching values of about 4oC and a gradual temperature decline that can last up to several thousand years. These events preferably occurred during glacial periods, when North America and Eurasia were partly covered by massive ice sheets. Unraveling the underlying mechanisms of DO events has been a major challenge and it requires a concerted multi-disciplinary effort, which has involved glaciologists, oceanographers, climate modelers and mathematicians. In this presentation, I will report on my personal 15-year long journey to understand these peculiar climatic events. It started with a Dynamical Systems’ approach and the discovery that climate oscillations, that bear quite some similarity to the observed DO events, can be simulated in a simplified climate model through a mechanism referred to as “Coherence Resonance”. According to this scenario, DO-type events originate from the interaction of the Atlantic Ocean circulation and polar sea ice. 5 years ago, my principal understanding of these events was shattered when new paleoceanographic data became available that demonstrate that in fact instabilities of the Scandinavian ice-sheet may have contributed to the generation of these climatic transitions. Using an earth system model of intermediate complexity my team and I developed a paleo-climate model simulation that can reproduce all observed DO events during one of the most active periods of global abrupt climate change 50-10,000 years ago, if we apply the reconstructed Eurasian and Laurentide ice-sheet variations as a freshwater forcing to the North Atlantic. I will discuss this new solution in the light of new paleo-climate data from the North Atlantic region, the deep Southern Ocean, Monsoon regions and the North Pacific.

서울대학교 지구환경과학부 세미나

서울대학교 지구환경과학부 세미나

In the southwestern part of the Korean Peninsula, Cretaceous volcanic rocks occur along a NE-SW-trending, sinistral strike-slip fault (Hamyeol Fault). Based on their spatial distribution, the volcanic rocks are classified into three lithostratigraphic units: the Buan, the Seonunsan, and the Beopseongpo Volcanics. These volcanics are composed of various types of pyroclastic, sedimentary, and lavas/intrusive rocks. SHRIMP U-Pb ages of zircon crystals were isolated from each unit. For the Buan Volcanics, Cheonmasan Tuff = 87.23 ±0.92 Ma; Udongje Tuff = 86.79 ±0.71 Ma; Seokpo Tuff = 87.30 ±0.99 Ma; and Yujeongje Tuff = 86.66 ±0.93 Ma. For the Seonunsan Volcanics, Gyeongsusan Tuff = 84.9 ±1.1 Ma and Yeongije Tuff = 86.61 ±0.67 Ma. For the Beopseongpo Volcanics, Seongsan Tuff = 87.73 ±0.89 Ma and Gyema Rhyolite = 86.2 ±1.7 Ma. The ages correspond well to recent mapping results of volcanic rocks in the southwest and south parts of the Korean Peninsula. In addition, the age data are comparable to those of the Aioi and Arima volcanic rock groups in southwestern Japan. This suggests that regional volcanic activity in the Japanese Islands and the southwestern part of the Korean Peninsula is related, and results from magmatism together with crustal deformation due to slab rollback of the Izanagi Oceanic Plate underneath the Eurasian Plate.

서울대학교 지구환경과학부 세미나

서울대학교 지구환경과학부 세미나

To better understand the behavior of estuarine labile organic matter, stable carbon isotope values of amino acids were investigated from the particles and sediment collected along a salinity transect in the Pearl River Estuary in the winter of 2016. Variation in amino acids 13C values was observed from the upstream stations to those adjacent to the shelf. A varied isotopic difference between amino acids and bulk organic carbon was found across the salinity transect, suggesting a mismatch in the carbon source between labile organic matter and the rest of bulk OM with a particular consideration of refractory terrestrial OM input to the estuary. A microbial degradation incubation provides further evidence that the changing isotope values in bulk organic carbon during phytoplankton decomposition mainly result from the change in relative abundance of amino acids and other organic components, rather than the isotopic changes in amino acids themselves. With an assumption of constant isotope difference between amino acids and other organic components from the same carbon source, a lability model was established to differentiate the relative contributions of three major portions of estuarine organic carbon: 1) amino acids and 2) other organic carbon originated from the estuarine phytoplankton as well as 3) terrestrial organic carbon. The model suggests a highly variable terrestrial OM contribution to the studied estuary. The model also manages to evaluate the changing lability of organic matter from the estuarine phytoplankton growth without the terrestrial OM interference.

서울대학교 지구환경과학부 세미나

Atmospheric aerosol particles play an important role in atmospheric chemistry, global radiative forcing, cloud formation and precipitation, thus the climate. Marine is one of the largest sources for atmospheric particles. There are different types of particles in marine atmospheric environments including sea salt, secondary organic aerosol, or marine biogenic particles. These particles participate in different heterogeneous reactions which change the physicochemical properties of particles, and thus their interactions with light and water vapor. Marine particles can serve as cloud condensation and ice nuclei to form clouds, thus may change the radiative balance and precipitation over ocean. I will present our work on the potential heterogeneous reactions between sea salt and organic acids and their ice nucleation ability to act as ice nuclei from field and laboratory studies.

서울대학교 지구환경과학부 세미나

서울대학교 지구환경과학부 세미나

With the appearance of new diseases and drug resistant strains, there is an urgent need to look for unconventional new sources of bioactive natural products for drug development. Many structurally diverse natural products with an astounding array of bioactivities, including antimicrobial, anticancer, antiviral, insecticidal and enzyme inhibitory activities together with other likely uses (e.g. in therapeutics), have been discovered from marine resources. Marine resources within complex submarine environments are typically encrusting and highly cryptic, and also highly diverse. Natural-product structures derived from them have the characteristics of high chemical diversity, biochemical specificity and other molecular properties. The isolation of these biogenic compounds and determination of their structure could provide leads for the future development of marine compounds as potential drugs.

한국천문연구원 콜로퀴움

Galaxy evolution is driven by a complex combination of internal (nature) and external (nurture) processes. Gas stripping due to ram pressure arises as a galaxy falls into the dense intracluster medium of a galaxy cluster, and is among the most violent environmental experiences a galaxy can have. The most spectacular examples of ram-pressure stripping in action are the so-called "jellyfish galaxies", which display extended tails of optically bright stripped material. I will review several theoretical and observational studies that aim to characterize the effect of gas stripping in galaxy evolution, including the latest results of the large MUSE program GASP, dedicated to studying jellyfish galaxies. Finally, I will, present the recent discovery of a previously unknown connection between ram-pressure stripping and nuclear black hole activity.

한국천문연구원 콜로퀴움

One of the biggest challenges in modern cosmology is to understand the first generation of stars and galaxies that formed during the cosmic Dark Ages. Since they reside in the observationally unexplored territory, we need to predict the properties of the first galaxies by pushing numerical simulations to new levels of physical realism and detail. In this talk, I will present the results of our highly-resolved cosmological ab-initio simulations to understand the assembly process of first galaxies under the feedback from the first generation of stars, the so-called Population III. Also, I will illustrate how first galaxies can be connected with their local descendants in terms of chemical abundances in the local ultra-faint dwarf galaxies.

한국천문연구원 콜로퀴움

The recent discoveries of gravitational waves from the advanced LIGO have already been critical cosmological resources. Here, I will present cosmological implications of gravitational wave detection, and show how current and future gravitational observatories can advance our knowledge on the nature of dark matter and dark energy.

한국고등과학원 세미나

The observation of Gravitational Waves (GWs) by the LIGO/Virgo Collaborations provides new opportunities to explore the fundamental laws of the Universe. GW physics is indeed not only relevant for our understanding of gravity but also for probing the interplay between particle physics and cosmology. Therefore, I discuss on one side how the waveforms observed by LIGO/Virgo allow us to test General Relativity and to put constraints on modified theories of gravity, in particular on the scale of a hypothetical noncommutative space-time. On the other side, I emphasise how the potential detection of GWs produced from first-order phase transitions in the early Universe is able to provide valuable information for particle physics beyond the Standard Model.

고려대학교 화학과 세미나

Hydrogen bonds are ubiquitous and have major role in many situations involving biomolecules and materials interacting with water. In this seminar I present two topics in which hydrogen bonds are the main actor. The first part is about a molecular dynamics study on solvation dynamics of formylperylene dissolved in methanol-acetonitrile liquid mixture. I present results from equilibrium and non-equilibrium molecular dynamics simulations that relate the slowdown of the solvation dynamics to the formation of a hydrogen-bonded methanol oligomer, which is hydrogen–bonded to the carbonyl group of formylperylene. In the second part, the presentation is about molecular dynamics simulations to characterize the structure of water confined in between two MoS2 sheets. Water spontaneously fills the region sandwiched by two MoS2 sheets in ambient conditions to form a specific pattern in which a square ring structure is formed by four diamonds via H-bonds, while each diamond shares a corner in a perpendicular manner, yielding an intriguing isogonal tiling structure.

고려대학교 화학과 세미나

This presentation will discuss several photophysical and spectroscopic properties of zero- and two-dimensional colloidal nanostructure focusing on two groups of materials: (1) perovskite quantum dots and Mn-doped perovskite quantum dots and (2) quantum dots of monolayer transition metal dichalcogenides. First half of the presentation will focus on recent development on the synthesis of cesium lead halide perovskite quantum dots with high level of control on their quantum confinement and ensemble uniformity as well as their photophysical properties. New photophysical properties resulting from the isoelectronic doping of paramagnetic Mn2+ ions in these QDs will also discussed. The second part of the presentation will discuss the solution-phase chemical synthesis and photophysical properties of quantum dots derived from single and multi-layered 2D transition metal dichalcogenides. The nature of the optical transition and photoluminescence in single-layer QD as well as the strong interaction with surrounding solvent environment will be discussed

서울대학교 화학과 세미나

Functional materials composed of p-stacked dyes and organic semiconductor molecules have gained increasing popularity during the last two decades. However, our understanding is still rather limited with regard to the impact of particular packing arrangements in the solid state on the optical and electronic properties of dye-based materials. Studies on dye assemblies in solution[1] constitute the missing link, providing many insights into the coupling among chromophores upon p-p-stacking, in particular with regard to absorption and fluorescence properties. Within our research on dye assemblies of dipolar merocyanine dyes[2] and quadrupolar perylene bisimide dyes[3] we became, however, aware of severe limitations: Thus, it is very challenging for the conventional self-assembly approach to establish a particular dye-dye packing arrangement and to limit the size of an aggregate to a desired supramolecular species. Further, it is almost impossible to properly assemble different dyes into structurally defined hetero-aggregate architectures. As a consequence of these limitations many interesting scientific questions could not be addressed in the past.

서울대학교 화학과 세미나

I will present my work on molecular engineering of three-dimensional scaffolds—specifically, enzyme active sites and pores of metal–organic frameworks (MOFs)—for biological and environmental applications. In the first part, I will describe the development of heme peroxidase enzymes as genetically encoded probes for cell biology. Fluorescent proteins such as GFP have been revolutionary in allowing specific proteins to be tracked within living cells, but the resolution of fluorescence microscopy (~200 nm) is often inadequate for determining precise subcellular localization. Using rational enzyme engineering and directed evolution, I developed APEX (enhanced ascorbate peroxidase), a versatile genetic tag for electron microscopy (which provides spatial resolution <10 nm) and several additional applications in cell biology. In the second part, I will present the concept of engineering nanopores within MOFs to mimic protein active sites, thus enabling capture of specific guest molecules in a robust and recyclable material. I will demonstrate this concept in diamine-appended MOFs that capture carbon dioxide (CO2) by an unprecedented cooperative mechanism. The CO2 adsorption properties can be tuned through systematic variation of the diamine and framework, making these materials suitable for a variety of CO2 capture applications. I will also describe the development of a chiral MOF with high stability, permanent porosity, and a large pore diameter (1.8 nm)—promising characteristics for the separation of chiral pharmaceuticals. I discovered that this MOF is highly enantioselective toward chiral ammonium carbamates and elucidated the structural basis for this enantioselectivity, which involves numerous non-covalent guest–framework interactions.

POSTECH 화학과 세미나

KAIST 화학과 세미나

Dioxygen activation by non-heme Fe-enzymes, such as cysteine dioxygenase (CDO), isopenicillin N-Synthase (IPNS), and ergothioneine (EgtB), has been proposed to proceed through several intermediates, including Fe-superoxo(O2 – ), -hydroperoxo( - OOH), and/or high-valent oxo species. Thiolate (RS– ) ligands have been shown to lower the activation barrier to O2 binding, and facilitate peroxo O-O bond cleavage, and HAT reactions. Although they proceed via similar intermediates, CDO catalyzes S-O bond formation, whereas IPNS and EgtB catalyze C-S bond formation. There are few reported examples of well-characterized RS-Fe-(O2, OOH) intermediates. This talk will show that reduced bis-thiolate ligated [FeII(S2 Me2N3(Pr,Pr)] reacts with O2 at low temperatures (≤ –70 ˚C) to afford two metastable intermediates, en route to a singly oxygenated sulfenate (RSO– ) complex. The first of these intermediates is also obtained when KO2 is added to the oxidized derivative, [FeIII(S2 Me2N3(Pr,Pr)]+ . Oxo atom donors, ArIO, react with the latter to afford a metastable intermediate with properties dependent on the ArIO, which convert to an identical FeIII-S(R)O– product. The crystal structure of an oxo atom donor adduct, PyN-O-FeIII, containing stronger XO bonds will also be described. Aryl iodides (ArI) inhibit this reaction providing kinetic evidence for the involvement of an Fe(V)-oxo. HAT reactivity, TD-DFT calculations, and spectroscopic characterization of the intermediates formed in the O2 and KO2 reactions support the formation of a reactive Fe-O2 – . Thiolates are shown to facilitate the activation of strong (92 kcal/mol) C-H bonds

KAIST 화학과 세미나

Molecular assembly forms diverse supramolecular architectures through non-covalent interactions which can be changed reversibly by external stimuli such as temperature, light, salt, and pH. I present the switchable assembly by adjusting non-covalent interactions for 1D tubules and 2D porous sheets. For example, dynamic tubular pores undergo rapid switching between open and closed states in response to a thermal signal in water. Notably, this pore switching mediates a controlled water-pumping catalytic action for the dehydration reaction (Fig. 1a). A virus-like hierarchical assembly with the native DNA and a synthetic coat shows repeated collective helicity switching triggered by pH change. This collective helicity inversion occurs during translocation of the DNA–coat assembly into intracellular compartments. Translating DNA conformational dynamics into a higher level of hierarchical dynamics may provide an approach to create DNA-based nanomachine (Fig. 1b). Homochiral porous nanosheets are presented with open-closed pore switching. The porous 2D structures can serve as enantiomer sieving membranes which exclusively capture a single enantiomer in a racemic mixture solution with high uptake capacity. The entrapped guests inside the pores can be pumped out by pore closing triggered by salt (Fig. 1c). Moreover, I also present supramolecular concepts to translate the adaptive nature of biological systems into synthetic self-assembly.

KAIST 화학과 세미나

The transcriptional pausing plays pivotal roles in transcription initiation, elongation, termination, RNA folding, translation, and genome stability. RNA polymerase (RNAP) pauses at every ~100 base pairs during transcription in the model bacterium, E. coli. In bacteria, secondary structure of nascent RNA can increase pause lifetimes ten-fold or more. Despite its central life-maintaining function, the underlying molecular mechanism of transcriptional pausing has remained elusive. To probe this, we determined a 3.8 Å cryo-electron microscopy (cryo-EM) structure of an E. coli his pause elongation complex (hisPEC), which is stabilized by a nascent RNA hairpin derived from the leader sequence of the histidine synthesis operon. We found that i) the double-stranded A-form RNA hairpin stem forms within the RNA exit channel of RNAP, ii) the DNA:RNA hybrid is trapped in a distinctive half-translocated conformation, and iii) mobile domains of the RNAP (dock, clamp, jaw, shelf and SI3) rearrange in a concerted manner to maintain the RNA hairpin-stabilized paused state. Although the transcriptional pause is dynamically controlled by transcription factors for the gene expression regulation, how the factors can adjust the pausing at the atomic level remains unknown. To address this question, we focused on the two prominent anti-pausing proteins, NusG and RfaH, which belong to the only conserved transcription factor family in all domains of life. Cryo-EM structures of elongation complexes bound with NusG and RfaH, at 3.7 Å and 3.5 Å resolution, respectively, revealed that NusG and RfaH bind to the β protrusion, β gate loop, and β’ clamp helices of RNAP, stabilizing the RNAP in a conformation of active elongation state. While NusG is a ubiquitous pausing inhibitor, RfaH associates with an RNAP in a sequence-specific manner. In the RfaH-elongation complex structure, RfaH interacts with ops (operon polarity repressor) sequence forming a short hairpin on the non-template DNA. While both NusG and RfaH reduce backtrack pausing, which is transient transcription stalling by RNAP’s backward movement, only RfaH blocks RNA hairpin-stabilized pausing by binding to the elongation complex tightly enough to resist RNAP swiveling. Based on our results, we propose that the pause RNA hairpin stabilizes global conformational changes in the RNAP that secures the paused state and NusG and RfaH enhance transcription elongation by maintaining the active conformation of the elongation complex.