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Biomedical subjects

HW Lee

Publications and source records attributed to HW Lee.

At least 19 recordsLinked to original sources
PubMed2000

Kinetics and mechanism of addition of benzylamines to benzylidene-1,3-indandiones in acetonitrile

Nucleophilic addition reactions of benzylamines (BA; XC6H4CH2NH2) to benzylidene-1,3-indandiones (BID; YC6H4CH=C(C=O)2C6H4) have been studied in acetonitrile at 25.0 degrees C. The rate is first-order with respect to BA and BID, and no base catalysis is observed. The structure-reactivity behaviors (k2, rhoX, betaX, and betaY) are intermediate between the two series of addition reactions of BA to beta-nitrostyrene (NS) and benzylidenemalononitrile (BMN) in acetonitrile. The normal kinetic isotope effects, kH/kD > 1.0, involving deuterated BAs (XC6H4CH2ND2) are smaller than those for the reactions of NS and BMN suggesting a somewhat looser bond formation in the transition state. The reaction is predicted to proceed in a single step with concurrent C(alpha)-N bond formation and proton transfer to C(beta). A hydrogen-bonded, four-center type cyclic transition state is proposed.

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PubMed2000

Kinetics and mechanism of the pyridinolysis of phenacyl bromides in acetonitrile

Kinetic studies of the reactions of substituted phenacyl bromides (YC6H4COCH2Br) with pyridines (XC5H4N) are carried out in acetonitrile at 45.0 degrees C. A biphasic Bronsted plot is obtained with a change in slope from a large (betaX approximately equals 0.65-0.80) to a small (betaX approximately 0.36-0.40) value at pKa = 3.2-3.6, which can be attributed to a change in the rate-determining step from breakdown to formation of a tetrahedral intermediate in the reaction path as the basicity of the pyridine nucleophile increases. This mechanism is supported by the faster rates with pyridines than with anilines and the change of cross-interaction constant rhoXY from a large positive (rhoXY = +1.4) to a small positive (rhoXY approximately +0.1) value. The large magnitude of Hammett rhoX (= -5.5 to -6.9) values for the pyridines with electron-withdrawing substituents and positive deviations of the pi-acceptors, p-CH3CO and p-CN, are quite similar to those for the pyridinium ion formation equilibria. The activation parameters are also in line with the proposed mechanism.

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PubMed2000

Kinetics and mechanism of the addition of benzylamines to benzylidenemalononitriles in acetonitrile

Nucleophilic addition reactions of benzylamines (BA) to benzylidenemalononitrile (BMN) have been studied in acetonitrile at 15.0 degrees C. The rate is first-order with respect to both BA and BMN and no base catalysis is observed. The rate decreases as the electron-withdrawing power of the substituent (Y) in the substrate increases (rho Y < 0). This is in contrast to the similar reactions of beta-nitrostyrenes (NS) with BAs in acetonitrile and the addition reactions of NS and BMN in aqueous solution (rho Y > 0). This sign change of rho Y is considered to result from the strong electron-withdrawing power of the (CN)2 group in BMN, which leads to polarization of C alpha delta+C delta- (CN)2 in the transition state. The mechanism of amine addition to BMN in acetonitrile is radically different from that in water. The reaction is predicted to proceed concertedly in a single step with a hydrogen-bonded, four-center cyclic transition state.

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PubMed2000

Broad Halpha Wing Formation in the Planetary Nebula IC 4997.

The young and compact planetary nebula IC 4997 is known to exhibit very broad wings with a width exceeding 5000 km s-1 around Halpha. We propose that the broad wings are formed through Rayleigh-Raman scattering that involves atomic hydrogen, by which Lybeta photons with a velocity width of a few 102 km s-1 are converted to optical photons and fill the Halpha broad wing region. The conversion efficiency reaches 0.6 near the line center, where the scattering optical depth is much larger than 1, and rapidly decreases in the far wings. Assuming that close to the central star there exists an unresolved inner compact core of high density, nH approximately 109-1010 cm-3, we use the photoionization code "CLOUDY" to show that sufficient Lybeta photons for scattering are produced. Using a top-hat-incident profile for the Lybeta flux and a scattering region with a H i column density NHi=2x1020 cm-2 and a substantial covering factor, we perform a profile-fitting analysis in order to obtain a satisfactory fit to the observed flux. We briefly discuss the astrophysical implications of the Rayleigh-Raman processes in planetary nebulae and other emission objects.

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PubMed2000

Pyridinolysis of phenyl-substituted phenyl chlorophosphates in acetonitrile

The kinetics and mechanism of the reactions of phenyl-substituted phenyl chlorophosphates, I, with pyridines in acetonitrile are investigated at 25.0 degrees C. The rates are much faster and the transition state is much earlier on the reaction coordinate with smaller beta(X) (0.16-0.18) and smaller negative beta(XY) (-0.011) values than those for the corresponding reactions with anilines. The vertical approach of the pyridine ring to an apical position of the trigonal bipyramidal transition state enables the p(pi)-d(pi) overlap between the pi orbital of pyridine and an empty d-orbital of phosphorus with less steric hindrance in a concerted process. The activation parameters are in line with the proposed mechanism.

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PubMed2000

Allozyme Variation and Genetic Relationships among Species of Cimicifuga (Ranunculaceae) from Korea.

Allozyme investigation of the five Cimicifuga taxa in Korea was conducted to assess genetic and clonal diversity within populations and genetic divergence among populations and taxa. Levels of allozyme variation maintained in Korean Cimicifuga taxa were comparable to those for most herbaceous perennials. In general, samples excluding copies of the same multilocus genotype maintained higher levels of genetic diversity than the total samples within populations. Copies of homozygous genotypes at several loci resulting from clonal spread lead to decreased levels of genetic diversity within populations, indicating that clonal reproduction found in Cimicifuga affects population genetic structure. In general, more widely distributed species such as C. dahurica and C. japonica harbored higher levels of allozyme diversity than the other taxa examined. Although two varieties of C. heracleifolia are geographically and reproductively isolated, the genetic and clonal structure of var. bifida seems to resemble var. heracleifolia, indicating that the two varieties may have had a similar evolutionary history. However, the allozyme data strongly indicate that the two morphological types (Groups I and II) of C. simplex should be treated as separate species.

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PubMed2000

Dynamical localization and partial-barrier localization in the Paul trap

We investigate the nature of quantum localization exhibited by the center-of-mass motion of an ion in a Paul trap interacting with a standing laser field. Depending upon system parameters and the initial location of the ion, quantum suppression of chaotic diffusion is dominated by dynamical localization or localization due to partial barriers formed of broken separatrices and cantori.

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PubMed2000

Phase diagram of congested traffic flow: An empirical study

We analyze traffic data from a highway section containing one effective on-ramp. Based on two criteria, local velocity variation patterns and expansion (or nonexpansion) of congested regions, three distinct congested traffic states are identified. These states appear at different levels of the upstream flux and the on-ramp flux, thereby generating a phase digram of the congested traffic flow. Observed traffic states are compared with recent theoretical analyses and both agreeing and disagreeing features are found.

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PubMed1999

Screening for L-sorbose and L-sorbosone dehydrogenase producing microbes for 2-keto-L-gulonic acid production.

Acetic acid bacteria incompletely oxidize L-sorbose to 2-keto-L-gulonic acid (2KLG) by L-sorbose- and L-sorbosone dehydrogenases. In order to isolate novel microorganisms with these enzyme activities, a new screening method has been studied with a presumption that microorganisms reuse their metabolic products when principal carbon sources are exhausted. When various keto-aldonic acid-producing microorganisms were tested for the ability to grow in minimal media containing such products as 2,5-diketo-gluconic acid, 2-keto-D-gluconic acid, 5-keto-D-gluconic acid or 2-keto-L-gulonic acid, they grew with these keto-aldonic acids as the sole carbon source. By enriching the isolates collected from screening samples for their growth in minimal medium containing 2KLG as the sole carbon source, as much as 50% of selected strains showed L-sorbose- and L-sorbosone dehydrogenase activities. In spite of the presence of these enzymes, no significant amount of 2KLG was detected in the culture broth, possibly due to 2KLG reductase activity, indicating that the direct screening for 2KLG producer microorganisms would be less successful. These results suggest that the screening strategy using 2KLG as a carbon source is a useful method for the selective screening of microorganisms with L-sorbose- and L-sorbosone dehydrogenases, and that a similar strategy may be applied to other cases.

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