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Ryu, K.,Kim, K.,Kim, T. i. THE POLYMER SOCIETY OF KOREA 2014 Macromolecular Research Vol.22 No.3
In this work, guanidinylated polyethylenimine-conjugated chitosan (CS-PEI-G) was synthesized, and characterized for efficient gene delivery systems. Low molecular weight PEI (0.8 kDa) was conjugated to chitosan as a low cytotoxic template for introducing the integrated bio-functionalities and guanidine groups, which are important components of cellular penetrating peptides. Synthesized CS-PEI-G could form positively charged and nano-sized complexes with pDNA. CS-PEI-G showed enhanced transfection efficiency in comparison with unmodified CSPEI, and high transfection efficiency comparable to or higher than PEI25k, due to guanidine moieties and the serum stability of chitosan backbone. Flow cytometry results suggest that the improved transfection efficiency of CS-PEIG may be not induced by cellular uptake of polyplexes but by other factors, such as the nuclear localization ability of guanidine moieties. These results showed that the simple strategy of introducing the guanidine groups of high density to chitosan via small PEI linkers could be effective for developing efficient gene delivery carriers.
Ryu, K.,Hwang, S.Y.,Kim, K.H.,Kang, J.H.,Lee, E.K. Elsevier Science Publishers 2008 Journal of biotechnology Vol.133 No.1
One of the major problems of wild-type lignin peroxidase (LiP) is its inactivity at the presence of excess H<SUB>2</SUB>O<SUB>2</SUB> and high concentration of aromatic compounds. Little is known about the substrate-binding site of LiP, and functionality improvement of LiP was not actively tried by genetic engineering and directed evolution. In order to improve LiPs functionality, we performed directed evolution with a colorimetric screening method. Finally, three types of LiP mutants were screened. The catalytic efficiency of the variants toward 2,4-dichlorophenol (DCP) degradation activity and the stability against H<SUB>2</SUB>O<SUB>2</SUB> was increased over the wild type. The K<SUB>m</SUB> value of the variants toward H<SUB>2</SUB>O<SUB>2</SUB> was increased, but K<SUB>m</SUB> value toward 2,4-DCP degradation was reduced. Overall, The K<SUB>cat</SUB>/K<SUB>m</SUB> values of the mutants toward 2,4-DCP was increased ca. 4-fold, and that toward H<SUB>2</SUB>O<SUB>2</SUB> was increased ca. 89-fold. Amino acid sequence analysis indicated that the most of the mutations were located on the enzyme surface. We expect that these results coupled with recombining mutation can be successfully applied to the molecular evolution cycles for screening of LiPs and other oxidative enzymes with improved functionality and stability.
Ryu, K.,Hwang, S.D.,Kim, S.W.,Hahn, S. IEEE 2005 IEEE transactions on applied superconductivity Vol.15 No.2
AC loss is an important issue to design high temperature superconductor power cables, which consist of a number of Bi-2223 tapes wound on a former. In the cables, the tapes have intrinsically different critical currents, and they are electrically connected to each other and current leads. This makes loss measurements considerably complex, especially for short samples. In this work, we have prepared a multi-tape conductor composed of Bi-2223 tapes. The ac losses of the conductor have been experimentally investigated. The loss tests indicate that the effect of critical currents of each tape on AC loss measurement in the multi-tape conductor is negligible only if currents in the tapes flow uniformly. Moreover, the measured loss of the conductor is in good agreement with the sum of the transport loss in each tape. However, in the case of nonuniform current distribution, the measured AC loss considerably depends on the current distribution parameter γ, and the positioning of a voltage lead. Thus it should be very careful to measure the AC losses in the short power cable samples.
Transport Loss Characteristics of the Bi-2223 Tapes in an External AC Magnetic Field
Ryu, K.,Song, H.J.,Kim, H.J.,Seong, K.C. IEEE 2006 IEEE transactions on applied superconductivity Vol.16 No.2
<P>A Bi-2223 tape has been developed for power applications such as a fault current limiter, a power cable and a superconducting magnetic energy storage system (SMES). In such applications, the Bi-2223 tape carries time varying transport current and in addition experiences time varying external magnetic field. It is well known that the external magnetic field not only causes magnetization loss in the Bi-2223 tape, but also drastically increases transport loss due to a so-called 'dynamic resistance'. We developed an evaluation setup, which can measure transport loss in an external AC magnetic field. Using this equipment, we measured the dynamic resistances for various amplitudes and frequencies of external AC magnetic field perpendicular to the face in the tape. Simultaneously we investigated the effect of external AC field on transport loss with different experimental conditions. This paper describes test results and discussions on correlation between the dynamic resistance and the transport loss for various technical Bi-2223 tapes</P>
AC losses of the 5m BSCCO cables with shield
Ryu, K.,Ma, Y.H.,Li, Z.Y.,Hwang, S.D.,Song, H.J. North-Holland 2010 Physica. C, Superconductivity Vol.470 No.20
In order to research the AC loss characteristics of a multi-layered conductor and a shield in a high temperature superconductor (HTS) cable, we prepared two short cable samples, which are the same as the 22.9kV/50MVA HTS-cable installed at Gochang test yard of Korea Electric Power Corporation, and attached voltage-leads to both the conductor and the shield. To investigate the effect of transport period on their AC losses, we also applied current with the same magnitude and opposite direction to the conductor and the shield from a few cycles to several minutes. The tests show that the AC loss measured from the lead attached to the shield (shield-lead) is constant regardless of transport period. But the measured loss from the lead attached to the conductor (conductor-lead) is greatly dependent on transport period. It seems to be caused by difficulty in heat transfer to the surrounding coolant due to thick insulator around the conductor. As transport period becomes longer, the conductor's temperature rises and thus the AC loss measured from the conductor-lead increases. In addition, the measured loss from the conductor-lead is 1.5 times larger than that from the shield-lead, particularly for the transport period of a few cycles.
Ryu, K.,Chae, J.-S.,Lee, E.,Parrot, M. Elsevier Science Ltd ; Pergamon 2014 Journal of Atmospheric and Solar-Terrestrial Physi Vol.121 No.1
While investigating possible precursory signatures of large earthquakes in the ionospheric data measured by the DEMETER and CHAMP satellites, we found ionospheric disturbances related to large earthquakes (M=7.2 and 7.4) that occurred on September 2004 near the south coast of Honshu, Japan. The satellite data were statistically compared with an empirical model and local averages of the large set of data in the study period. A fluctuation in the electron density above the epicenter was observed roughly 2weeks before the main earthquakes. Surveys of the space weather and geomagnetic activities suggest that these fluctuations were not caused by changes in space conditions or by a geomagnetic storm. The features were also distinct from well-known natural ionospheric anomalies. In addition, a peak-like profile in the ion temperature and lowered O<SUP>+</SUP> density around the region of the epicenter was observed a week before the main earthquakes along the satellite passes whose longitudes are close to the epicenter. The features are more apparent when they are compared with the data more distant from the epicenter, suggesting that the disturbances occur along the geomagnetic field lines. The concurrent measurements of the ion drift velocity suggest the fluctuations were triggered by the vertical plasma drift. The observed anomalies disappeared ~2 weeks after the quakes. According to the current theories on the seismo-ionospheric coupling, the horizontal electric field at the lower boundary of the ionosphere should have been strengthened by the seismic activity in order for the ionospheric plasma movements above the epicenter and its geomagnetic conjugate regions to trigger the observed ionospheric anomalies.