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11.
Nitrogen dioxide ((*)NO(2)) participates in a variety of biological reactions. Of great interest are the reactions of (*)NO(2) with oxymyoglobin and oxyhemoglobin, which are the predominant hemeproteins in biological systems. Although these reactions occur rapidly during the nitrite-catalyzed autoxidation of hemeproteins, their roles in systems producing (*)NO(2) in the presence of these hemeproteins have been greatly underestimated. In the present study, we employed pulse radiolysis to study directly the kinetics and mechanism of the reaction of oxymyoglobin (MbFe(II)O(2)) with (*)NO(2). The rate constant of this reaction was determined to be (4.5 +/- 0.3) x 10(7) M(-1)s(-1), and is among the highest rate constants measured for (*)NO(2) with any biomolecule at pH 7.4. The interconversion among the various oxidation states of myoglobin that is prompted by nitrogen oxide species is remarkable. The reaction of MbFe(II)O(2) with (*)NO(2) forms MbFe(III)OONO(2), which undergoes rapid heterolysis along the O-O bond to yield MbFe(V)=O and NO(3-). The perferryl-myoglobin (MbFe(V)=O) transforms rapidly into the ferryl species that has a radical site on the globin ((*)MbFe(IV)=O). The latter oxidizes another oxymyoglobin (10(4) M(-1)s(-1) < k(17) < 10(7) M(-1)s(-1)) and generates equal amounts of ferrylmyoglobin and metmyoglobin. At much longer times, the ferrylmyoglobin disappears through a relatively slow comproportionation with oxymyoglobin (k(18) = 21.3 +/- 5.3 M(-1)s(-1)). Eventually, each (*)NO(2) radical converts three oxymyoglobin molecules into metmyoglobin. The same intermediate, namely MbFe(III)OONO(2), is also formed via the reaction peroxynitrate (O(2)NOO(-)/O(2)NOOH) with metmyoglobin (k(19) = (4.6 +/- 0.3) x 10(4) M(-1)s(-1)). The reaction of (*)NO(2) with ferrylmyoglobin (k(20) = (1.2 +/- 0.2) x 10(7) M(-1)s(-1)) yields MbFe(III)ONO(2), which in turn dissociates (k(21) = 190 +/- 20 s(-1)) into metmyoglobin and NO(3-). This rate constant was found to be the same as that measured for the decay of the intermediate formed in the reaction of MbFe(II)O(2) with (*)NO, which suggests that MbFe(III)ONO(2) is the intermediate observed in both processes. This conclusion is supported by thermokinetic arguments. The present results suggest that hemeproteins may detoxify (*)NO(2) and thus preempt deleterious processes, such as nitration of proteins. Such a possibility is substantiated by the observation that the reactions of (*)NO(2) with the various oxidation states of myoglobin lead to the formation of metmyoglobin, which, though not functional in the gas transport, is nevertheless nontoxic at physiological pH. 相似文献
12.
Judit E. Puskas Sam W. P. Chan Kimberley B. McAuley Sohel Shaikh Gabor Kaszas 《Journal of polymer science. Part A, Polymer chemistry》2005,43(22):5394-5413
This article is a critical analysis of kinetic dataavailable on carbocationic polymerizations. A survey of published propagation rate constant (kp) data revealed several orders of magnitude differences. In this article, an explanation of this apparent discrepancy is offered with a case study involving the carbocationic polymerization of 2,4,6‐trimethylstyrene (TMS). With the polymerization mechanism originally proposed for this system, kp = 1.35 × 104 L mol?1 s?1 was extracted from experimental data with the Predici polyreaction package. The alternative mechanism yielded kp = 1.01 × 107 L mol?1 s?1, close to that predicted by Mayr's Linear Free Energy Relationship (LFER). We propose that true rate constants can only be obtained from direct competition experiments or from kinetic interpretation based on independently proven mechanisms. The second part of this review discusses critical analysis of the temperature and concentration dependence of various living IB systems. Comparison of the temperature dependence in systems initiated with 2‐ chloro‐2,4, 4‐ trimethylpentane (TMPCl)/TiCl4 from various laboratories yielded of ΔH ~?25 and ?34.5 kJ/mol for high and low TMPCl/TiCl4 ratios, respectively. Aromatic (cumyl‐type) initiators show ΔH ~ ?40 kJ/mol, whereas H2O/TiCl4 in the presence of the strong electron‐ pair donor dimethylacetamide gave ΔH = ?12 kJ/mol. The significant differences indicate different underlying mechanisms with complex elementary reactions. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5394–5413, 2005 相似文献
13.
Klaus Burger Gabor Radics Lothar Hennig Christoph B?ttcher Jan Spengler Fernando Albericio 《Monatshefte für Chemie / Chemical Monthly》2005,5(3):763-776
Starting from L-malic, L-citramalic, and rac. thiomalic acids routes to L-homoisoserine, 2-methyl-L-homoisoserine and rac. homoisocysteine have been developed. The new orthogonally protected and carboxy-activated building blocks are GABA as well as -hydroxy and -mercapto acid derivatives, suitable for the construction of peptide and depsipeptide surrogates. 相似文献
14.
Margaret E. Carrington Gabor Kunstatter Christopher D. Phillips Marcelo E. Rubio 《Entropy (Basel, Switzerland)》2022,24(11)
We study numerically the evolution of an expanding system of scalar fields. The initial configuration is non-isotropic and rotating. We calculate the energy–momentum tensor and angular momentum vector of the system. We compare the time scales associated with the isotropization of the transverse and longitudinal pressures, and the decay of the initial angular momentum. We show that even a fairly large initial angular momentum decays significantly faster than the pressure anisotropy. 相似文献
15.
Isothiocyanato-functionalized cyanine dyes 7 and 11 for labelling of proteins at amino groups have been synthesized. The dyes and their adducts with amines show strong absorbance and fluorescence in the near-infrared region of 750-850 nm. 相似文献
16.
Dr. Azusa Kondoh Dipl.‐Chem. Alexander Arlt Barbara Gabor Prof. Alois Fürstner 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(24):7731-7738
The lichen‐derived glycoconjugate gobienine A is structurally more complex than most glycolipids isolated from higher plants by virtue of the all‐cis substituted γ‐lactone substructure embedded into its macrocyclic frame. A concise entry into this very epimerization‐prone and hence challenging structural motif is presented, which relies on an enantioselective cyanohydrin formation, an intramolecular Blaise reaction, a palladium‐catalyzed alkoxycarbonylation, and a diastereoselective hydrogenation of the tetrasubstituted alkene in the resulting butenolide. This strategy, in combination with ring‐closing olefin metathesis for the formation of the macrocyclic perimeter, allowed the proposed structure of gobienine A ( 1 ) to be formed in high overall yield. The recorded spectral data show that the structure originally attributed to gobienine A is incorrect and that it is not the epimerization‐prone ester site on the butanolide ring that is the locus of misassignment; rather, the discrepancy must be more profound. 相似文献
17.
Dr. Tran Quang Hung Dr. Ferial Terki Dr. Souleymane Kamara Mourad Dehbaoui Prof. Salam Charar Brajalal Sinha Prof. CheolGi Kim Dr. Philippe Gandit Il'ya A. Gural'skiy Dr. Gabor Molnar Dr. Lionel Salmon Dr. Helena J. Shepherd Dr. Azzedine Bousseksou 《Angewandte Chemie (International ed. in English)》2013,52(4):1058-1058
18.
Christophe Bliard Pal Herczegh Alain Olesker Gabor Lukacs 《Journal of carbohydrate chemistry》2013,32(1):103-113
Abstract L-Oleandrose is the carbohydrate constituent of the potent anthelmintic agents the avermectins. Diethylaminosulfur tri-fluoride treatment of appropriate uloses did not give gem-difluoro sugars. Trifluorofluoroxymethane or xenon difluoride addition to the double bond of 4-O-benzoyl-6-deoxy-2-fluoro-3-O-methyl-L-glucal produced protected 2,2-difluorooleandrose derivatives activated at their anomeric center and ready for glycosidation. 相似文献
19.
Ing. Günter Seidel Barbara Gabor Dr. Richard Goddard Dipl.‐Ing. Berit Heggen Prof. Walter Thiel Prof. Alois Fürstner 《Angewandte Chemie (International ed. in English)》2014,53(3):879-882
Carbophilic catalysts that are based on AuI allow a host of different nucleophiles to be added across various π systems. 1 – 3 Although many of these reactions are thought to proceed via gold carbenoids, the challenge to observe and characterize these putative intermediates has basically been unmet. 4 The current mechanistic interpretation therefore largely relies on indirect evidence and computational data, some of which are subject to debate. 5 In an attempt to fill this gap, we pursued a potential route to gold carbenoids by formal transmetalation of chromium or tungsten Fischer carbene complexes with [LAu]+. Whereas this transformation proceeds with exceptional ease as long as a stabilizing heteroelement is present on the carbene center, it stops half‐way in its absence. Rather unusual bimetallic arrays are formed, which allow the charge density to delocalize over several positions. The obvious difficulty of releasing an “unstabilized” gold carbenoid has potential mechanistic implications for the understanding of π‐acid catalysis in general. 相似文献
20.
Mapping Platinum Species in Polymer Electrolyte Fuel Cells by Spatially Resolved XAFS Techniques 下载免费PDF全文
Dr. Shinobu Takao Dr. Oki Sekizawa Dr. Shin‐ichi Nagamatsu Dr. Takuma Kaneko Dr. Takashi Yamamoto Dr. Gabor Samjeské Dr. Kotaro Higashi Dr. Kensaku Nagasawa Dr. Takuya Tsuji Dr. Motohiro Suzuki Dr. Naomi Kawamura Dr. Masaichiro Mizumaki Prof. Dr. Tomoya Uruga Prof. Dr. Yasuhiro Iwasawa 《Angewandte Chemie (International ed. in English)》2014,53(51):14110-14114
There is limited information on the mechanism for platinum oxidation and dissolution in Pt/C cathode catalyst layers of polymer electrolyte fuel cells (PEFCs) under the operating conditions though these issues should be uncovered for the development of next‐generation PEFCs. Pt species in Pt/C cathode catalyst layers are mapped by a XAFS (X‐ray absorption fine structure) method and by a quick‐XAFS(QXAFS) method. Information on the site‐preferential oxidation and leaching of Pt cathode nanoparticles around the cathode boundary and the micro‐crack in degraded PEFCs is provided, which is relevant to the origin and mechanism of PEFC degradation. 相似文献