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21.
Ermakova S Sokolova R Kim SM Um BH Isakov V Zvyagintseva T 《Applied biochemistry and biotechnology》2011,164(6):841-850
Fucoidans were isolated by water extraction and ion-exchange chromatography from brown algae Eclonia cava, Sargassum hornery, and Costaria costata collected near of Korean coasts. The structures of fucoidans were investigated. Fucoidan from E. cava was mixture of sulfated rhamnogalactofucan and galactofucan. Fucoidan from C. costata was a sulfated galactofucan. Fucoidan isolated from S. hornery was separated into three fractions: a homofucan sulfate, a homofucan but without sulfate groups, and a sulfated rhamnofucan.
The results clearly showed that fucoidans play an inhibitory role in colony formation in human melanoma and colon cancer cells
and may be effective antitumor agents. 相似文献
22.
A kinetic study is reported for the reactions of 4-nitrophenyl X-substituted benzoates (1a-1) and Y-substituted phenyl benzoates (2a-1) with two anionic nucleophiles (OH(-) and CN(-)) and three amines (piperidine, hydrazine, and glycylglycine) in 80 mol% H(2)O-20 mol% dimethyl sulfoxide (DMSO) at 25.0 +/- 0.1 degrees C. Each Hammett plot exhibits two intersecting straight lines for the reactions of 1a-1 with the anionic nucleophiles and piperidine, while the Yukawa-Tsuno plots for the same reactions are linear. The Hammett plots for the reactions of 2a-1 with hydrazine and glycylglycine demonstrate much better linear correlations with sigma(-) constants than with sigma degrees or sigma constants, indicating that the leaving group departure occurs at the rate determining step (RDS). On the contrary, sigma(-) constants result in poorer Hammett correlation than sigma degrees constants for the corresponding reactions with OH(-) and CN(-), indicating that the leaving group departure occurs after the RDS for the reactions with the anionic nucleophiles. The large rho(X) value (1.7 +/- 0.1) obtained for the reactions of 1a-1 with the anionic nucleophiles supports the proposal that the reactions proceed through an addition intermediate with its formation being the RDS. 相似文献
23.
Synthesis of Spirocyclic Ethers by Enantioselective Copper‐Catalyzed Carboetherification of Alkenols
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Shuklendu D. Karyakarte Chanchamnan Um Ilyas A. Berhane Prof. Sherry R. Chemler 《Angewandte Chemie (International ed. in English)》2018,57(39):12921-12924
Spirocyclic ethers can be found in bioactive compounds. This copper‐catalyzed enantioselective alkene carboetherification provides 5,5‐, 5,6‐ and 6,6‐spirocyclic products containing fully substituted chiral carbon centers with up to 99 % enantiomeric excess. This reaction features the formation of two rings from acyclic substrates, 1,1‐disubstituted alkenols functionalized with either arenes, alkenes, or alkynes, and clearly constitutes a powerful way to synthesize chiral spirocyclic ethers. 相似文献
24.
A kinetic study is reported for alkaline hydrolysis of X‐substituted phenyl diphenylphosphinates ( 1 a – i ). The Brønsted‐type plot for the reactions of 1 a – i is linear over 4.5 pKa units with βlg=?0.49, a typical βlg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σo and σ? constants are linear but exhibit many scattered points, while the corresponding Yukawa–Tsuno plot results in excellent linear correlation with ρ=1.42 and r=0.35. The r value of 0.35 implies that leaving‐group departure is partially advanced at the rate‐determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO? ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (DN + AN) mechanism is also ruled out on the basis of the small βlg value. As the substituent X in the leaving group changes from H to 4‐NO2 and 3,4‐(NO2)2, ΔH ≠ decreases from 11.3 kcal mol?1 to 9.7 and 8.7 kcal mol?1, respectively, while ΔS ≠ varies from ?22.6 cal mol?1 K?1 to ?21.4 and ?20.2 cal mol?1 K?1, respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1 a – i . 相似文献
25.
A kinetic study is reported for reactions of Y-substituted phenyl X-substituted cinnamates (1a-e and 3a-g) and benzoates (2a-e and 4a-g) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. Reactions of 2,4-dinitrophenyl X-substituted cinnamates (1a-e) and benzoates (2a-e) with amines result in linear Yukawa-Tsuno plots. The rho(X) values are much smaller for the reactions of 1a-e than for those of 2a-e. A distance effect and the nature of the reaction mechanism (i.e., a concerted mechanism for 1a-e) have been suggested to be responsible for the small rho(X) values. The Br?nsted-type plots for the reactions of 2,4-dinitrophenyl X-substituted cinnamates (1a, 1c, and 1e) with amines are curved with a decreasing betanuc value from 0.65 to 0.3-0.4. The reactions of Y-substituted phenyl cinnamates (3a-g) with morpholine also result in a curved Br?nsted plot, while the corresponding reactions of Y-substituted phenyl benzoates (4a-e) exhibit a linear Br?nsted plot. It has been concluded that the curved Br?nsted plots found for the reactions of the cinnamates (1a, 1c, 1e, and 3a-g) are not due to a change in the rate-determining step (RDS) but due to a normal Hammond effect for a concerted mechanism, that is, an earlier transition state (TS) for a more reactive amine or substrate. 相似文献
26.
Misook Kang Myeong-Heon Um Jong-Yul Park 《Journal of molecular catalysis. A, Chemical》1999,150(1-2):195-203
In order to synthesize the NiAPSO-34 crystal with a pure CHA structure having a uniform size and stable catalytic activity, factors having influence on the crystallization at gel preparation were investigated. The results showed that the mixing order of starting materials influences on the crystallization rate. The concentration of the template was related to the morphology of crystals and the crystallinity, while the particle size obtained was controlled by the silica source and the amount of nickel. Moreover, it was elucidated that the mixing orders of starting materials influences on the crystal growth rate. Seed crystal addition (SAPO-34; 0.8 μm) and ultrasonic-wave treatment after the gel formation were effective in decreasing smaller particle size and in narrowing the size distribution. 相似文献
27.
The retinal chromophore (11Z)‐3‐diazo‐4‐oxoretinal ( 1 ) with two photo‐labile moieties has been synthesized by semi‐hydrogenation of an 11‐yne precursor with activated Zn in aqueous media. Incorporation of 1 into opsin yielded diazoketo rhodopsin (DK‐Rh), which, upon bleaching, gave rise to intermediates batho‐Rh, lumi‐Rh, meta‐Rh, and meta‐II‐Rh corresponding to those of native Rh but at lower temperatures. Photoaffinity labeling of DK‐Rh and these bleaching intermediates showed that the ionone ring cross‐linked to Trp265 of helix F in DK‐Rh and batho intermediate, and to Ala169 of helix D in lumi, meta‐I, and meta‐II intermediates. These results demonstrate the occurrence of large conformational changes along the visual transduction path, which, in turn, is responsible for activation of the G‐protein. 相似文献
28.
Multiparameter kinetic analysis of alkaline hydrolysis of a series of aryl diphenylphosphinothioates: models for P=S neurotoxins
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Ik‐Hwan Um Jeong‐Yoon Han Young‐Hee Shin Julian M. Dust 《Journal of Physical Organic Chemistry》2017,30(7)
Alkaline hydrolysis of a series of X‐substituted‐phenyl diphenylphosphinothioates ( 2a‐i ) in 80 mol%/20 mol% DMSO at 25.0 ± 0.1°C has been studied kinetically and assessed through a multiparameter approach. Substrates 2a to 2i are approximately 12 to 22 times less reactive than their P=O analogues 1a to 1i (ie, the thio effect). The Brønsted‐type plot for the reactions of 2a to 2i is linear with βlg = ?0.43, consistent with a concerted mechanism. Hammett plots correlated with σo and σ? constants also support a concerted mechanism; the Yukawa‐Tsuno plot results in an excellent linear correlation with ρX = 1.26 and r = 0.30, indicating that expulsion of the leaving group occurs in the rate‐determining step (RDS). The ΔH? value increases from 10.5 to 11.7 and 13.9 kcal/mol as substituent X in the leaving group changes from 3,4‐(NO2)2 to 4‐NO2 and H, in turn, while TΔS? remains constant at ?6.0 kcal/mol. The strong dependence of ΔH? on the electronic nature of substituent X also indicates that the leaving group departs in the RDS. The reaction mechanism and origin of the thio effect are discussed by comparison of the current kinetic results with those reported for the reactions of 1a to 1i . The results suggest that for useful OP neurotoxins the mechanism of abiotic hydrolysis is concerted (with varying degrees of asynchronicity) when the substrate bears good leaving groups. 相似文献
29.
A kinetic study is reported for reactions of 4-nitrophenyl benzoate (1c) and O-4-nitrophenyl X-substituted thionobenzoates (2a-e) with a series of pyridines in 80 mol % H2O/20 mol % dimethyl sulfoxide (DMSO) at 25.0 +/- 0.1 degrees C. O-4-Nitrophenyl thionobenzoate (2c) is more reactive than its oxygen analogue 1c toward all the pyridines studied. The Br?nsted-type plot is linear with beta(nuc)=1.06 for reactions of 1c but curved for the corresponding reactions of 2c with beta(nu)c decreasing from 1.38 to 0.38 as the pyridine basicity increases, indicating that the reaction mechanism is also influenced on changing the electrophilic center from C=O to C=S. The curvature center of the curved Br?nsted-type plots (defined as pK(a)(o)) occurs at pKa = 9.3 regardless of the electronic nature of the substituent X in the nonleaving group. The Hammett plot for reactions of 2a-e with 4-aminopyridine is nonlinear, i.e., the substrates having an electron-donating substituent exhibit negative deviations from the Hammett plot. However, the Yukawa-Tsuno plot for the same reactions exhibits good linear correlation, indicating that the negative deviations shown by these substrates arise from stabilization of the ground state through resonance interaction between the electron-donating substituent X and the C=S bond. 相似文献
30.
Soojin Lee Borim Kim Kyungmoon Park Youngsoon Um Jinwon Lee 《Applied biochemistry and biotechnology》2012,166(7):1801-1813
meso-2,3-Butanediol (meso-2,3-BDO) is essential for the synthesis of various economically valuable biosynthetic products; however, the production of
meso-2,3-BDO from expensive carbon sources is an obstacle for industrial applications. In this study, genes involved in the synthesis
of 2,3-BDO in Klebsiella pneumoniae were identified and used to genetically modify Escherichia coli for meso-2,3-BDO production. Two 2,3-BDO biosynthesis genes—budA, encoding acetolactate, and meso-budC, encoding meso-SADH—from K. pneumoniae were cloned into the pUC18 plasmid and introduced into E. coli. In 2 l batch culture, the SGSB03 E. coli strain yielded meso-2,3-BDO at 0.31 g/gglucose (with a maximum of 15.7 g/lculture after 48 h) and 0.21 g/gcrude glycerol (with a maximum of 6.9 g/lculture after 48 h). Batch cultures were grown under optimized conditions (aerobic, 6% carbon source, 37 °C, and initial pH 7). To
find the optimal culture conditions for meso-2,3-BDO production, we evaluated the enzyme activity of meso-SADH and the whole cell conversion yield (meso-2,3-BDO/acetoin) of the E. coli SGSB02, which contains pSB02. meso-SADH showed high enzyme activity at 30–37 °C and pH 7 (30.5–41.5 U/mg of protein), and the conversion yield of SGSB02 E. coli was highest at 37–42 °C and a pH of 7 (0.25–0.28 g
meso-2,3-BDO/gacetoin). 相似文献