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51.
Nucleophilic addition of the peroxynitrite anion, ONOO(-), to the two prototypical carbonyl compounds, acetaldehyde and acetone, was investigated in the pH interval 7.4-14. The process is initiated by fast equilibration between the reactants and the corresponding tetrahedral adduct anion, the equilibrium being strongly shifted to the reactant side. The adduct anion also undergoes fast protonation by water and added buffers. Consequently, the rate of the bimolecular reaction between ONOO(-) and the carbonyl is strongly dependent on the pH and on the concentration of the buffer. The pK(a) of the carbonyl-ONOO adduct was estimated to be approximately 11.8 and approximately 12.3 for acetone and acetaldehyde, respectively. It is shown that both the anionic and the neutral adducts suffer fast homolysis along the weak O-O bond to yield free alkoxyl and nitrogen dioxide radicals. The yield of free radicals was determined to be about 15% with both carbonyl compounds at low and high pH, while the remainder collapses to molecular products in the solvent cage. The rate constants for the homolysis of the adducts vary from ca. 3 x 10(5) to ca. 5 x 10(6) s(-1), suggesting that they cannot act as oxidants in biological systems. This small variation around a mean value of about 10(6) s(-1) suggests that the O-O bond in the adduct is rather insensitive to its protonation state and to the nature of its carbonyl precursor. An overall reaction scheme was proposed, and all the corresponding rate constants were evaluated. Finally, thermokinetic considerations were employed to argue that the formation of dioxirane as an intermediate in the reaction of ONOO(-) with acetone is an unlikely process. 相似文献
52.
Bekers O op den Buijsch RA de Vries JE Wijnen PA van Dieijen-Visser MP 《Electrophoresis》2002,23(12):1878-1881
We describe the application of capillary electrophoresis to detect DNA fragments, obtained after amplifying a part of the apolipoprotein E (apoE) gene with polymerase chain reaction (PCR). Compared to conventional agarose slab gel electrophoresis (AGE), CE appears the method of choice with regard to resolution and sensitivity, to detect DNA fragments in the range of 20-100 base pairs. Especially discrimination between apoE2/E2 and apoE2/E3 genotypes is more reliable with CE than with AGE, this being of great clinical value in the diagnosis of familiary dysbetalipoproteinemia. 相似文献
53.
Ben L. M. Van Baar Peter C. Burgers John L. Holmes Johan K. Terlouw 《Journal of mass spectrometry : JMS》1988,23(5):355-363
By combining results from a variety of mass spectrometric techniques (metastatle ion, collisional activation, collision-induced dissociative ionization, neutralization–reionization spectrometry and appearance energy measurements) and the classical method of isotopic labelling, a unified mechanism is proposed for the complex unimolecular chemistry of ionized 1,2-propanediol. The key intermediates involved are the stable hydrogen-bridged radical cations [CH2?C(H)? H…?O…?O(H)CH3]+˙, which were generated independently from [4-methoxy, 1-butanol]+˙ (loss of C2H4) and [1-methoxyglycerol]+˙ (loss of CH2O), [CH3? C?O…?H…?O(H)CH3]+˙ and the related ion-dipole complex [CH2?C(OH)CH3/H2O]+˙. The latter species serves as the precursor for the loss of CH3˙ and in this reaction the same non-ergodic behaviour is observed as in the loss of CH3˙ from the ionized enol of acetone. 相似文献
54.
β-Glucosidase hydrolyzes cellobiose to glucose and is an important enzyme in the consortium used for hydrolysis of cellulosic and lignocellulosic feedstocks. In the present work, β-glucosidase was covalently immobilized on non-porous magnetic particles to enable re-use of the enzyme. It was found that particles activated with cyanuric chloride and polyglutaraldehyde gave the highest bead-related immobilized enzyme activity when tested with p-nitrophenyl-β-D-glucopyranoside (104.7 and 82.2 U/g particles, respectively). Furthermore, the purified β-glucosidase preparation from Megazyme gave higher bead-related enzyme activities compared to Novozym 188 (79.0 and 9.8 U/g particles, respectively). A significant improvement in thermal stability was observed for immobilized enzyme compared to free enzyme; after 5 h (at 65 °C), 36 % of activity remained for the former, while there was no activity in the latter. The performance and recyclability of immobilized β-glucosidase on more complex substrate (pretreated spruce) was also studied. It was shown that adding immobilized β-glucosidase (16 U/g dry matter) to free cellulases (8 FPU/g dry matter) increased the hydrolysis yield of pretreated spruce from ca. 44 % to ca. 65 %. In addition, it was possible to re-use the immobilized β-glucosidase in the spruce and retain activity for at least four cycles. The immobilized enzyme thus shows promise for lignocellulose hydrolysis. 相似文献
55.
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57.
Ultraviolet B radiation induces upregulation of calcitonin gene-related peptide levels in human Finn chamber skin samples 总被引:1,自引:0,他引:1
Sleijffers A Herreilers M van Loveren H Garssen J 《Journal of photochemistry and photobiology. B, Biology》2003,69(3):149-152
One of the neuropeptides that plays a role in UVB-induced immunomodulation is calcitonin gene-related peptide (CGRP), as demonstrated in several animal studies. CGRP can be detected in human skin, but effects of UVB exposure on CGRP levels in human skin are not known. We determined CGRP levels in human Finn chamber skin samples of 15 UVB-irradiated and 10 control volunteers. Filter samples were collected prior to and immediately after a UVB exposure protocol (5 consecutive days, with one personally determined minimal erythema dose (MED(jp)) per day). CGRP levels in filter samples were determined using a commercially available radioimmunoassay kit. CGRP could be detected in the filter samples and volunteers showed statistically significantly increased levels after UVB exposure. In addition, the CGRP levels of UVB-exposed volunteers were positively correlated with the dose of UVB in J/m(2) that they received on 5 consecutive days. In other words, higher UVB doses resulted in higher CGRP levels. In summary, CGRP, a mediator in UVB-induced immunomodulation, could be detected in human Finn chamber skin samples, and was significantly increased after UVB exposure. The CGRP level appeared to depend on the amount of UVB the volunteers received. 相似文献
58.
p-Carborane has been vinylated on the 2-B-atom in high yields using the Heck reaction. Thus, the reaction between 2-iodo-p-carborane and various styrenes [4-H-, 4-C(6)H(4)-, 4-Cl-, 4-Br-, 4-NO(2)-, 4-CH(3)O-, and 4-CH(3)-] resulted in the production of the corresponding trans-beta-(2-B-p-carboranyl)styrene in DMF solution when reacted in the presence of silver phosphate and the palladacycle Herrmann's catalyst. 相似文献
59.
Veerman JJ Bon RS Hue BT Girones D Rutjes FP van Maarseveen JH Hiemstra H 《The Journal of organic chemistry》2003,68(11):4486-4494
Several 2-substituted and 2,5-disubstituted piperazine-3,6-diones were synthesized starting from readily available alpha-amino acids. After activation of a lactam carbonyl via introduction of a methoxycarbonyl group onto nitrogen, this carbonyl was selectively reduced. Treatment of the resulting urethane with protic acid generated the corresponding N-acyliminium ion, which was trapped by a nucleophilic C2-side chain to provide 2,6-bridged piperazine-3-ones. Several aromatic, heteroaromatic, and nonaromatic side chains were used as pi-nucleophiles. In addition, the effect of the presence of a C5-methyl group on the stereochemical outcome of the cyclization was examined. 相似文献
60.