Showing papers by "Pusan National University published in 1997"

Studies on thermally stimulated shape memory effect of segmented polyurethanes

Abstract: The shape memory behavior of a series of polycaprolactone/diphenylmethane diisocyanate/butanediol (PCL/MDI/BDO) segmented polyurethanes of different composition was studied. The molecular weight of PCL diols was in the range of 1600-7000, and the hard segment content varied from 7.8 to 27% by weight. Film specimens for shape memory measurements were prepared by drawing at temperatures above the melting temperature of the soft segment crystals and subsequent quick cooling to room temperature under constrained conditions. The shape memory process was observed and recorded in a heating process. Parameters describing the recovery temperature, ability, and speed were used to study the influence of structure and processing conditions on the shape memory behavior of the sample. It was found that the high crystallinity of the soft segment regions at room temperature and the formation of stable hard segment domains acting as physical crosslinks in the temperature range above the melting temperature of the soft segment crystals are the two necessary conditions for a segmented copolymer with shape memory behavior. The response temperature of shape memory is dependent on the melting temperature of the soft segment crystals. The final recovery rate and the recovery speed are mainly related to the stability of the hard segment domains under stretching and are dependent on the hard segment content of the copolymers.

Effects of Voids on the Mechanical Strength and Ultrasonic Attenuation of Laminated Composites

Abstract: The effects of voids on the mechanical strength and ultrasonic attenuation have been investigated on a number of laminated composite systems containing a wide range of void content. Graphite fiber ...

Xanthine dehydrogenase/xanthine oxidase and oxidative stress

Abstract: Xanthine dehydrogenase (XDH) and xanthine oxidase (XOD) are single-gene products that exist in separate but interconvertible forms. XOD utilizes hypoxanthine or xanthine as a substrate and O2 as a cofactor to produce superoxide (·O2−) and uric acid. XDH acts on these same substrates but utilizes NAD as a cofactor to produce NADH instead of ·O2− and uric acid. XOD has been proposed as a source of oxygen radicals in polymorphonuclear, endothelial, epithelial, and connective tissue cells. However, several questions remain about the physiological significance and functions of XOD on aging and oxidative stress. XOD is reported to play an important role in cellular oxidative status, detoxification of aldehydes, oxidative injury in ischemia-reperfusion, and neutrophil mediation. For example, XOD may serve as a messenger or mediator in the activation of neutrophil, T cell, cytokines, or transcription in defense mechanisms rather than as a free radical generator of tissue damage. Emerging evidence on the synergistic interactions of ·O2−, a toxic product of XOD and nitric oxide, may be another illustration of XOD involvement in tissue injury and cytotoxicity in an emergent condition such as ischemia or inflammation.

Why ethnic minorities want to develop their heritage language: The case of Korean‐Americans

Abstract: Heritage language (HL) education has gained attention in recent years, although little is known about the students who enrol in such programmes. This study examines one group of adults attempting to develop their HL proficiency in order to discover their motivation for studying the language and the challenges they encounter. Twenty‐four Korean‐American students enrolled in beginning to advanced levels of Korean in one heritage language programme were surveyed. The results indicated that the respondents have both family‐ and career‐related reasons for developing their HL, including the desire to improve communication with parents and relatives, to develop closer association with the Korean‐American community, and to expand career options. These adults also reported a number of obstacles to achieving higher levels of HL proficiency, including low levels of language confidence and unrealistic expectations of other HL speakers. The results are discussed and implications are presented for heritage language edu...

The Cdc42 GTPase-associated proteins Gic1 and Gic2 are required for polarized cell growth in Saccharomyces cerevisiae

Abstract: BEM2 of Saccharomyces cerevisiae encodes a Rho-type GTPase-activating protein that is required for proper bud site selection at 26°C and for bud emergence at elevated temperatures. We show here that the temperature-sensitive growth phenotype of bem2 mutant cells can be suppressed by increased dosage of the GIC1 gene. The Gic1 protein, together with its structural homolog Gic2, are required for cell size and shape control, bud site selection, bud emergence, actin cytoskeletal organization, mitotic spindle orientation/positioning, and mating projection formation in response to mating pheromone. Each protein contains a CRIB (Cdc42/Rac-interactive binding) motif and each interacts in the two-hybrid assay with the GTP-bound form of the Rho-type Cdc42 GTPase, a key regulator of polarized growth in yeast. The CRIB motif of Gic1 and the effector domain of Cdc42 are required for this association. Genetic experiments indicate that Gic1 and Gic2 play positive roles in the Cdc42 signal transduction pathway, probably as effectors of Cdc42. Subcellular localization studies with a functional green fluorescent protein–Gic1 fusion protein indicate that this protein is concentrated at the incipient bud site of unbudded cells, at the bud tip and mother-bud neck of budded cells, and at cortical sites on large-budded cells that may delimit future bud sites in the two progeny cells. The ability of Gic1 to associate with Cdc42 is important for its function but is apparently not essential for its subcellular localization.

Characterization of Guanylate Kinase-Associated Protein, a Postsynaptic Density Protein at Excitatory Synapses That Interacts Directly with Postsynaptic Density-95/Synapse-Associated Protein 90

Abstract: The structure of central synapses is poorly understood at the molecular level. A recent advance came with the identification of the postsynaptic density-95 (PSD-95)/synapse-associated protein 90 family of proteins as important mediators of the synaptic clustering of certain classes of ion channels. By yeast two-hybrid screening, a novel protein termed guanylate kinase-associated protein (GKAP) has been isolated that binds to the GK-like domain of PSD-95 (Kim et al., 1997). Here we present a detailed characterization of GKAP expression in the rat brain and report the cloning of a novel GKAP splice variant. By Northern blot, GKAP mRNAs (4, 6.5, and 8 kB) are expressed predominantly in the rat brain. By in situ hybridization, GKAP is expressed widely in neurons of cortex and hippocampus and in the Purkinje and granule cells of the cerebellum. On brain immunoblots, two prominent bands of 95 and 130 kDa are detected that correspond to products of short and long N-terminal splice variants of GKAP. Two independent GKAP antibodies label somatodendritic puncta in neocortical and hippocampal neurons in a pattern consistent with synaptic elements. Immunogold electron microscopy reveals GKAP to be predominantly postsynaptic and present at asymmetric synapses and in dendritic spines. The distribution of GKAP immunogold particles is uniform in the lateral plane of the PSD but peaks in the perpendicular axis ∼20 nm from the postsynaptic membrane. In cultured hippocampal neurons GKAP immunoreactive puncta colocalize with the AMPA receptor subunit Glu receptor 1 but not with the GABA A receptor subunits β2 and β3. Thus GKAP is a widely expressed neuronal protein localized specifically in the PSD of glutamatergic synapses, consistent with its direct interaction with PSD-95 family proteins.

Numerical Study of Heat Transfer and Flow of Natural Convection in AN Enclosure with a Heat-Generating Conducting Body

Abstract: A numerical study has been conducted to investigate steady state heat transfer and flow characteristics of natural convection in a vertical square enclosure when a temperature difference exists across an enclosure and, at the same time, a conducting body generates heat within the enclosure. Dimensionless governing equations indicate that the heat transfer and flow characteristics of this system are governed by the Rayleigh and Prandtl numbers, the area ratio, the conductivity ratio, and the temperature-difference ratio. Here the temperature-difference ratio is defined as the ratio of a temperature difference across the enclosure to that caused by the heat source. In the present study, the Rayleigh number ranges from 103 to 104, and the temperature-difference ratio from 0 to 50, while the Prandtl number, the area ratio, and the conductivity ratio are kept constant at 0.71, 0.25, 1, respectively. The analysis is performed by observing variations of streamlines, isotherms, heatlines, and the average Nusselt ...

Crystal structure of zeolite x exchanged with pb(ii) at ph 6.0 and dehydrated : (pb4+)14(pb2+)18(pb4o4)8si100al92o384

Abstract: A single crystal of zeolite X was 39% overexchanged with aqueous Pb2+ at pH 6.0. After dehydration at 420 °C and 2 × 10-6 Torr, its crystal structure was determined by X-ray diffraction techniques in the cubic space group Fd3 at 21 °C (a = 25.238(5) A). The structure, Pb64O32−Si100Al92O384 per unit cell, was refined to the final error indices R1 = 0.080 and R2 = 0.070 with 486 reflections for which I > 3σ(I). Pb4+ formed upon evacuation at elevated temperature by a reaction which may be written, per unit cell, as 64 Pb2+ + 36 OH- → 18 Pb2+ + 8 Pb4O4 + 14 Pb4+ + 14 H2 + 4 H2O. Pb ions occupy three nonequivalent sites. Each of 14 lead ions at site II is nearly in a six-ring plane where it coordinates to three framework oxygens at 2.32(2) A and to a nonframework oxygen at 2.24(2) A; these short distances indicate that these are Pb4+. Eighteen Pb2+ ions, also at site II, are 2.54(2) A from three framework oxygens and 2.95(2) A from a nonframework oxygen. Each sodalite cage is filled by a nearly cubic Pb4O4 c...

Contributions to the cosmic ray flux above the ankle : clusters of galaxies

Abstract: Motivated by the suggestion of Kang, Ryu \& Jones (1996) that particles can be accelerated to high energies via diffusive shock acceleration process at the accretion shocks formed by the infalling flow toward the clusters of galaxies, we have calculated the expected particle flux from a cosmological ensemble of clusters. We use the observed temperature distribution of local clusters and assume a simple power-law evolutionary model for the comoving density of the clusters. The shock parameters such as the shock radius and velocity are deduced from the ICM temperature using the self-similar solutions for secondary infall onto the clusters. The magnetic field strength is assume to be in equipartition with the postshock thermal energy behind the accretion shock. We also assume that the injected energy spectrum is a power-law with the exponential cutoff at the maximum energy which is calculated from the condition that the energy gain rate for diffusive shock acceleration is balanced by the loss rate due to the interactions with the cosmic background radiation. In contrast to the earlier paper we have adopted here the description of the cosmic ray diffusion by Jokipii (1987) which leads to considerably higher particle energies. Finally the injected particle spectrum at the clusters is integrated over the cosmological distance to earth by considering the energy loss due to the interactions with the cosmic background radiation. Our calculations show that the expected spectrum of high-energy protons from the cosmological ensemble of the cluster accretion shocks could match well the observed cosmic ray spectrum near $10^{19}$eV with reasonable parameters and models if about $10^{-4}$ of the infalling kinetic energy can be injected into the intergalactic space as the high energy particles.

Intracellular Ca2+ release mediates ursolic acid-induced apoptosis in human leukemic HL-60 cells.

Abstract: The effect of ursolic acid (UA) on tumor cell apoptosis was investigated using HL-60 human promyelocytic leukemia cells as a model cellular system. Treatment with UA resulted in a concentration-dependent decreased cell viability assessed by MTT assay. UA also induced genomic DNA fragmentation, a hallmark of apoptosis, indicating that the mechanism by which UA induced cell death was through apoptosis. The intracellular Ca2+ level was increased by treatment with UA. Intracellular Ca2+ inhibitors, such as intracellular Ca2+-release blockers (dantrolene, TMB-8 and ruthenium red) and an intracellular Ca2+ chelator (BAPTA/AM), significantly blocked the UA-induced increased intracellular Ca2+ concentration. These inhibitors also blocked the effects of UA on cell viability and apoptosis. These results suggest that enhanced intracellular Ca2+ signals may be involved in UA-induced apoptosis in HL-60 cells. Int. J. Cancer 73:725–728, 1997. © 1997 Wiley-Liss, Inc.

Three crystal structures of vacuum-dehydrated zeolite x, m46si100al92o384,m = mg2+, ca2+ and ba2+

Abstract: The crystal structures of vacuum-dehydrated, fully Mg2+-, Ca2+-, and Ba2+-exchanged zeolite X (Mg46(H2O)4−X, Ca46−X, and Ba46−X; X = Si100Al92O384) have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21 °C (a = 24.671(5), 25.024(4), and 25.266(5) A, respectively). Their structures were refined to the final error indices Rw = 0.046 with 439 reflections, 0.037 with 434 reflections, and 0.049 with 485 reflections, respectively, for which I > 3σ(I). In Mg46(H2O)4−X, Mg2+ ions are found at four crystallographic sites: 14 per unit cell are located at site I at the center of the hexagonal prism (Mg−O = 2.262(6) A), only four are at site I‘ in the sodalite cavity near the hexagonal prism (Mg−O = 2.221(5) A), four, each coordinated to an H2O molecule, are located at site II‘ in the sodalite cavity (Mg−O = 2.223(12) A), and the remaining 24 are at site II near single six-oxygen rings in the supercage (Mg−O = 2.184(5) A). In Ca46−X, Ca2+ ions are found at only two si...

Preparation and Properties of Water-borne Polyurethanes

Abstract: Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea(Received 22 July 1996; accepted 6 August 1996)Abstract: Following a prepolymer mixing process, polyurethane (PU) aniono-mer dispersions were prepared from polyethylene adipate glycol (PEA), iso-phorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI) and dimethylolpropionic acid (DMPA) as a potential ionic centre. E†ects of prepolymer molec-ular weight, PEA molecular weight, hard segment content, DMPA content,degree of neutralization and mixed diisocyanates have been studied in terms ofparticle size and viscosity of emulsion, and surface, mechanical and dynamicmechanical properties of the emulsion-cast Ðlms. Particle size decreased andemulsion viscosity increased with increase in prepolymer molecular weight andPEA molecular weight, DMPA content, the degree of neutralization and IPDIcontent in mixed isocyanate systems. The decrease in particle size was due toincreased chain Nexibility and/or hydrophilicity of the PU. The mechanical anddynamic mechanical properties of the PU ionomer dispersions were interpretedin terms of soft segmentEhard segment phase separations, hard segment content,chain Nexibility and coulombic forces.Key words: polyurethanes, anionomer dispersions, structureEproperty corre-lation, mechanical properties, dynamic mechanical properties.

A spatial data mining method by Delaunay triangulation

Abstract: It becomes an important task to discover significant pattern or characteristics which may implicitly exist in huge spatial dntabases, such as geographical or medical databases. In this paper, we present a spatial data mining method named SMiYN (Spatial data Mining by Triangulated Irregular Network), which is based on Delaunay Triangulation. Sh47ZN demonstrates important advantages over the previous works. First, it discovers even sophisticated pattern like nested doughnuts, and hierarchical structure of cluster distribution. Second, in order to execute SMTIN, we do not need to know a priori the nature of distribution, for example the number of clusters, which is indispensable to other methods. Third, experiments show that SMTIN requires less CPU processing time than other methods such as BIRCH and CLARANS. Finally it is not ordering sensitive and handles efticiently outliers.

Synthesis of High Strength Bulk Amorphous Zr–Al–Ni–Cu–Ag Alloys with a Nanoscale Secondary Phase

Abstract: Bulk amorphous alloys in a cylindrical form were prepared in the diameter range up to about 5 mm for Zr 65 Al 7.5 Ni 10 Cu 17.5 - x Ag x (x=0 to 5 at%) by copper mold casting. These bulk amorphous alloys exhibit a wide supercooled liquid region before crystallization and the temperature interval defined by the difference between glass transition temperature (T g ) and crystallization temperature (T x ), ΔT x (= T x - T g ) is 111 K for the 0%Ag alloy and 63 K for the 5%Ag alloy. The replacement of Cu by 5 at%Ag causes the change in the crystallization process from a single stage of Am → Zr 2 (Cu, Ni) + Zr 2 (Al, Ni) to two stages of Am → Am' + Zr 3 (Al, Ag) 2 → Zr 3 (Al, Ag) 2 + Zr 2 (Cu, Ni). The Zr 3 (Al, Ag) 2 phase has a spherical shape with a small size of about 20 nm and is dispersed homogeneously at an interparticle spacing of about 30 nm. The change in the crystallization process by the addition of Ag is presumably due to the generation of the distinction of bonding nature among the constituent elements because of the positive heats of mixing for Ag-Ni and Ag-Cu pairs. The dispersion of the nanoscale Zr 3 (Al, Ag) 2 phase causes a significant increase in tensile fracture strength (σ f ) from 1150MPa for the amorphous single phase to 1520MPa for the amorphous phase containing about 14% volume fraction of Zr 3 (Al, Ag) 2 phase. The shear-type fracture mode along the maximum shear plane remains unchanged. The effectiveness of the Zr 3 (Al, Ag) 2 phase for the increase in σ f is presumably because the size of the phase is too small to contain dislocations. The success of synthesizing the high-strength bulk amorphous alloy consisting of nanoscale Zr 3 (Al, Ag) 2 phase embedded in an amorphous matrix is encouraging for future development of bulk amorphous alloys as a high-strength material.

Production of inulo-oligosaccharides using endo-inulinase from a pseudomonas sp.

Abstract: Inulo-oligosaccharides were produced from inulin by using high activities of an endo-acting inulinase. The total yields of oligosaccharide were slightly decreased as the concentration of inulin increased from 50 to 200g/l. Under the optimal reaction conditions, the products consist of inulo-oligosaccharides ranging from DP (degrees of polymerization)2 to DP7, where the major oligosaccharides are 29.8% DP2, 21.4% DP3, and 8.1% DP4 oligomer, respectively. The maximum yield was 75.6% when 50g inulin/l and 15 units/g substrate were used.

Oleanolic acid and ursolic acid stabilize liposomal membranes

Abstract: The effects of oleanolic acid (OA) and ursolic acid (UA) on the fluidity and stability of dipalmitoyl phosphatidylcholine (DPPC) liposomal membrane were monitored by measuring the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene labeled in the liposomal membrane and the leakage of calcein from the probe-encapsulated liposomes. The experiments with the liposomes made of DPPC and OA or UA showed that OA and UA exhibited a moderate fluidity-modulating effect for the liquid-crystalline liposomal membrane, and a strong condensing effect for both crystalline and liquid-crystalline liposomal membranes. Their effects were comparable to those of cholesterol. These results suggest that their fluidity-modulating and condensing effects might have some implications in their biological functions.

Crystal Structures of Dehydrated Fully Mn2+-Exchanged Zeolite X and of Its Ethylene Sorption Complex

Abstract: The crystal structures of dehydrated Mn46Si100Al92O384 (a = 24.721(5) A) and of its ethylene sorption complex Mn46Si100Al92O384·30C2H4 (a = 24.690(5) A) have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21(1) °C. Each crystal was prepared by dehydration at 380 °C and 2 × 10-6 Torr for 2 days; the complex was then prepared by exposure to 300 Torr of ethylene gas at 24(1) °C, and its structure was determined in this atmosphere. Their structures were refined to the final error indices, R1 = 0.050 and R2 = 0.044 with 316 reflections and Rl = 0.064 and R2 = 0.061 with 313 reflections, respectively, for which I > 3σ(I). In each structure, 16 Mn2+ ions, each octahedrally coordinated by six oxygens of the zeolite framework, fill site I at the centers of the double six-rings. The remaining 30 Mn2+ ions in each structure are at site II in the supercage. Each site-II Mn2+ ion in the empty structure is three-coordinate and is quite close to the plane of the three oxy...