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91.
Jian‐Hua Zhao Hsuan‐Liang Liu Hsin‐Yi Lin Chih‐Hung Huang Hsu‐Wei Fang Shau‐Wei Tsai 《中国化学会会志》2007,54(4):835-842
In this study, we present the results from the semiempirical molecular orbital calculations for the acylation step in the lipase‐catalyzed ester hydrolysis. The results reveal that the lowest energy path for the formation of the tetrahedral intermediate is for the serine residue of the catalytic triad to attack the substrate, followed by coupling heavy atom movement and proton transfer. The calculations of four active site models show that the cooperation of the aspartate group and the oxyanion hole is capable of lowering the activation energy by about 16 kcalmol?1. Our results further suggest that the lipase‐catalyzed ester hydrolysis adopts the single proton transfer mechanism. 相似文献
92.
The oxidation of cysteine by [Mo(CN)(8)](3-) in deoxygenated aqueous solution at a moderate pH is strongly catalyzed by Cu(2+), to the degree that impurity levels of Cu(2+) are sufficient to dominate the reaction. Dipicolinic acid (dipic) is a very effective inhibitor of this catalysis, such that with 1 mM dipic, the direct oxidation can be studied. UV-vis spectra and electrochemistry show that [Mo(CN)(8)](4-) is the Mo-containing product. Cystine and cysteinesulfinate are the predominant cysteine oxidation products. The stoichiometric ratio (Deltan(Mo(V))/Deltan(cysteine)) of 1.4 at pH 10.8 is consistent with this product distribution. At pH 1.5, the reaction is quite slow and yields intractable kinetics. At pH 4.5, the rates are much faster and deviate only slightly from pseudo-first-order behavior. With 2 mM PBN (N-phenyl-tert-butyl nitrone) present at pH 4.5, the reaction rate is about 20% less and shows excellent pseudo-first-order behavior, but the stoichiometric ratio is not significantly changed. The rates also display a significant specific cation effect. In the presence of spin-trap PBN, the kinetics were studied over the pH range 3.48-12.28, with [Na(+)] maintained at 0.09-0.10 M. The rate law is -d[Mo(V)]/dt = k[cysteine](tot)[Mo(V)], with k = {2(k(b)K(a1)K(a2)[H(+)] + k(c)K(a1)K(a2)K(a3))}/([H(+)](3) + K(a1)[H(+)](2) + K(a1)K(a2)[H(+)] + K(a1)K(a2)K(a3)), where K(a1), K(a2), and K(a3) are the successive acid dissociation constants of HSCH(2)CH(NH(3)(+))CO(2)H. Least-squares fitting yields k(b) = (7.1 +/- 0.4) x 10(4) M(-1) s(-1) and k(c) = (2.3 +/-0.2) x 10(4) M(-1) s(-1) at mu = 0.1 M (NaCF(3)SO(3)) and 25 degrees C. A mechanism is inferred in which k(b) and k(c) correspond to electron transfer to Mo(V) from the thiolate forms of anionic and dianionic cysteine. 相似文献
93.
High-performance liquid chromatography (HPLC) was combined with electrospray ionization tandem mass spectrometry (ESI-MS/MS) to develop a sensitive and selective method for the quantitative measurement of N(7)-(2-hydroxyethyl)guanine (N(7)-HEG) adducts in DNA obtained from ethylene oxide-exposed biological samples. Selected reaction monitoring (SRM) was used as the detection mode while the fragmentation product ion at m/z 152 generated from the precursor protonated N(7)-HEG (m/z 196) was monitored. The detection limits for N(7)-HEG were estimated by twofold serial dilution and determined to be 4 fmol in neat standard solution and 16 fmol when a matrix effect is considered. When the mass spectrometer was operated in the selected ion monitoring mode using only the first quadrupole (without MS/MS function), the detection limits increased to 128 fmol and 1 pmol (when matrix effect is considered), respectively. A good linear correlation (R(2) = 0.999) was observed for signal intensities obtained by injecting 16 fmol--33 pmol of N(7)-HEG into the HPLC/ESI-MS/MS system. Hep G2 cells were incubated for 8 h with medium containing various concentrations of ethylene oxide (ranging from 0.05 to 5.0 mM). A dose-response relationship was established, indicating that the adduct formation increases with the exposure level. The method shows potential, although the detection limit needs to be lowered for practical applications, for use in monitoring N(7)-HEG formation in other biological systems. 相似文献
94.
Christian Pichot Marie-France Llauro Quang-Tho Pham 《Journal of polymer science. Part A, Polymer chemistry》1981,19(10):2619-2633
The compositions and sequence distributions of vinyl acetate–butyl acrylate copolymers obtained with batch and semicontinuous emulsion polymerizations have been studied by 1H and 13C NMR. The batch process gives heterogeneous copolymers while with the semicontinuous one the sequence distribution is statistical. These differences in sequence distributions have been related to the physical properties of the copolymers. 相似文献
95.
The kinetics of the reaction NH2 + NO → N2 + H2O were studied, using a conventional flash photolysis system. A value of k1 = (1.1 ± 0.2) × 1010 & mole?1 s?1 was obtained at room temperature and in the pressure range 2–700 torr in the presence of nitrogen. A slight negative temperature coefficient was observed between 300 and 500 K, equivalent to a negative activation energy of 1.05 ± 0.2 kcal mole?1. 相似文献
96.
Campos LM Dang H Ng D Yang Z Martinez HL Garcia-Garibay MA 《The Journal of organic chemistry》2002,67(11):3749-3754
A detailed thermochemical analysis of the alpha-cleavage and decarbonylation reactions of acetone and several ketodiesters was carried out with the B3LYP/6-31G* density functional method. The heats of formation of several ground-state ketones and radicals were calculated at 298 K to determine bond dissociation energies (BDE) and radical stabilization energies (RSE) as a function of substituents. Results show that the radical-stabilizing abilities of the ketone substituents play a very important role on the thermodynamics of the alpha-cleavage and decarbonylation steps. An excellent correlation between calculated values and previous experimental observations suggests that photochemical alpha-cleavage and decarbonylation in crystals should be predictable from knowledge of excitation energies and the RSE of the substituent. 相似文献
97.
Institute of Chemistry, Moldavian SSR Academy of Sciences, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 759–760, September–October, 1988. 相似文献
98.
Hon Man Lee Chi Ying Lu Chih Yuan Chen Wen Ling Chen Hung Ching Lin Pei Ling Chiu Pi Yun Cheng 《Tetrahedron》2004,60(27):5807-5825
A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)2 produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs. 相似文献
99.
Pham TN 《Physical review D: Particles and fields》1992,46(7):2976-2981
100.
R A Wind N Zumbulyadis R H Young Y Hung L Li R H Nuttall G Maciel 《Solid state nuclear magnetic resonance》1992,1(2):55-65
We have investigated the EPR and DNP behavior of a molecularly doped polymer modeling those used to transport electronic charge in electrophotography. The EPR spectra show no evidence of the superexchange reported for a closely related system based on tri-p-tolylamine. The difference may be due to larger charge-transfer matrix elements in the latter system. An unambiguous interpretation of the observed 1H DNP was rendered difficult by the unanticipated asymmetry of the EPR spectra. We report extensive data on the "three-spin" effect evident in the DNP-enhanced 13C NMR spectra, and comment on its potential for characterizing polymer interfaces. 相似文献