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1.
Scherer P. Sanders Stephen J. Harrison Periannan Kuppusamy J. T. Sylvester Jay L. Zweier 《Free radical biology & medicine》1994,16(6):753-761
Superoxide anions (O2.−) generated by the reaction of xanthine with xanthine oxidase were measured by the reduction of cytochrome c and by electron paramagnetic resonance (EPR) spectroscopy using the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Studies were performed to determine the relative sensitivities of these two techniques for the measurement of O2.−. Mixtures of xanthine, xanthine oxidase, DMPO generated two adducts, a transient DMPO-OOH and a smaller but longer-lived DMPO-OH. Both adducts were inhibited by superoxide dismutase (SOD), demonstrating they originated from O2.−, and were also significantly decreased when the experiments were performed using unchelated buffers, suggesting that metal ion impurities in unchelated buffers alter the formation or degradation of DMPO-adducts. O2.−, generated by concentrations of xanthine as low as 0.05 μM, were detectable using EPR spin trapping. In contrast, mixtures of xanthine, xanthine oxidase, and cytochrome c measured spectrophotometrically at 550 nm demonstrated that concentrations of xanthine above 1 μM were required to produce measurable levels of reduced cytochrome c. These studies demonstrate that spin trapping using DMPO was at least 20-fold more sensitive than the reduction of cytochrome c for the measurement of superoxide anions. However, at levels of superoxide generation where cytochrome c provides a linear measurement of production, EPR spin trapping may underestimate radical production, probably due to degradation of DMPO radical adducts. 相似文献
2.
Kinetics of the reactions of nitrogen dioxide with glutathione,cysteine, and uric acid at physiological pH 总被引:2,自引:0,他引:2
Nitrogen dioxide (NO2•) is a key biological oxidant. It can be derived from peroxynitrite via the interaction of nitric oxide with superoxide, from nitrite with peroxidases, or from autoxidation of nitric oxide. In this study, submicromolar concentrations of NO2• were generated in < 1 μs using pulse radiolysis, and the kinetics of scavenging NO2• by glutathione, cysteine, or uric acid were monitored by spectrophotometry. The formation of the urate radical was observed directly, while the production of the oxidizing radical obtained on reaction of NO2• with the thiols (the thiyl radical) was monitored via oxidation of 2,2′-azino-bis-(3-ethylthiazoline-6-sulfonic acid). At pH 7.4, rate constants for reaction of NO2• with glutathione, cysteine, and urate were estimated as 2 × 107, 5 × 107, and 2 × 107 M−1 s−1, respectively. The variation of these rate constants with pH indicated that thiolate reacted much faster than undissociated thiol. The dissociation of urate also accelerated reaction with NO2• at pH > 8. The thiyl radical from GSH reacted with urate with a rate constant of 3 × 107 M−1 s−1. The implications of these values are: (i) the lifetime of NO2• in cytosol is < 10 μs; (ii) thiols are the dominant ‘sink’ for NO2• in cells/tissue, whereas urate is also a major scavenger in plasma; (iii) the diffusion distance of NO2• is 0.2 μm in the cytoplasm and < 0.8 μm in plasma; (iv) urate protects GSH against depletion on oxidative challenge from NO2•; and (v) reactions between NO2• and thiols/urate severely limit the likelihood of reaction of NO2• with NO• to form N2O3 in the cytoplasm. 相似文献
3.
Degradation of hyaluronic acid by oxidants such as HO· and HOCl/ClO− is believed to be important in the progression of rheumatoid arthritis. While reaction of hyaluronic acid with HO· has been investigated extensively, reaction with HOCl/ClO− is less well defined. Thus, little is known about the site(s) of HOCl/ClO− attack, the intermediates formed, or the mechanism(s) of polymer degradation. In this study reaction of HOCl/ClO− with amides, sugars, polysaccharides, and hyaluronic acid has been monitored by UV-visible (220–340 nm) and EPR spectroscopy. UV-visible experiments have shown that HOCl/ClO− reacts preferentially with N-acetyl groups. This reaction is believed to give rise to transient chloramide (R—NCl—C(O)—R′) species, which decompose rapidly to give radicals via either homolysis (to produce N· and Cl·) or heterolysis (one-electron reduction, to give N· and Cl−) of the N—Cl bond. The nature of the radicals formed has been investigated by EPR spin trapping. Reaction of HOCl/ClO− with hyaluronic acid, chondroitin sulphates A and C, N-acetyl sugars, and amides gave novel, carbon-centered, spin adducts, the formation of which is consistent with selective initial attack at the N-acetyl group. Thus, reaction with hyaluronic acid and chondroitin sulphate A, appears to be localized at the N-acetylglucosamine sugar rings. These carbon-centered radicals are suggested to arise from rapid rearrangement of initial nitrogen-centered radicals, formed from the N-acetyl chloramide, by reactions analogous to those observed with alkoxyl radicals. The detection of increasing yields of low-molecular-weight radical adducts from hyaluronic acid and chondroitin sulphate A with increasing HOCl/ClO− concentrations suggests that formation of the initial nitrogen-centered species on the N-acetylglucosamine rings, and the carbon-centered radicals derived from them, brings about polymer fragmentation. 相似文献
4.
Bruno Eto Michel Boisset Peter Eden Ambikaipakan Balasubramaniam Jehan-Franois Desjeux 《Peptides》1995,16(8):1403-1409
The aim of our study was to determine whether a meal modifies the antisecretory response induced by PYY and the structural requirements to elicit antisecretory effects of analogue PYY(22–36) for potential antidiarrhea therapy. The variations in short-circuit current (Isc) due to the modification of ionic transport across the rat intestine were assessed in vitro, using Ussing chambers. In fasted rats, PYY induced a dose- and time-dependent reduction in Isc, with a sensitivity threshold at 5 × 10−11 M (ΔIsc −2 ± 0.5 μA/cm2). The reduction was maximal at 10−7 M (Isc −23 ± 2 μA/cm2), and the concentration producing half-maximal inhibition was 10−9 M. At 10−7 M, reduction of Isc by PYY reached 90% of response to 5 × 10−5 M bumetanide. The PYY effect was partly reversed by 10−5 M forskolin (Isc +13.43 ± 2.91 μA/h·cm2, p < 0.05) or 10−3 M dibutyryl adenosine 3′,5′ cyclic monophosphate (Isc +12 ± 1.69 μA/cm2, p < 0.05). Naloxone and tetrodotoxin did not alter the effect of PYY. In addition, PYY and its analogue P915 reduced net chloride ion secretion to 2.85 and 2.29 μEq/cm2 (p < 0.05), respectively. The antisecretory effect of PYY was accompanied by dose- and time-dependent desensitization when jejunum was prestimulated by a lower dose of peptide. The antisecretory potencies exhibited by PYY analogues required both a C-terminal fragment (22–36) and an aromatic amino acid residue (Trp or Phe) at position 27. At 10−7 M the biological activity of PYY was lower in fed than fasted rats (p < 0.001). Our results confirm the antisecretory effect of PYY, but show that the fed period is accompanied by desensitization, similar to the transient desensitization observed in the fasted period with cumulative doses. This suggests that PYY may act as a physiological mediator that reduces intestinal secretion. 相似文献
5.
The unique capabilities of EPR spin trapping of nitric oxide based on a ferrous-dithiocarbamate spin trap have been demonstrated in a study verifying the source of the nitrogen and oxygen atoms in nitric oxide produced from activated macrophages. Spin trapping experiments were performed during nitric oxide generation from activated mouse peritoneal macrophages using the ferrous complex of N-methyl D-glucam-ine dithiocarbamate as a spin trap. When 15N-substituted arginine was given to the activated macrophages in the presence of the spin trap, a characteristic EPR spectrum of the nitric oxide spin adduct was obtained, which indicates the presence of the l5N atom in the nitric oxide molecule. The hyperfine splitting (hfs) constant of the l5N nucleus was 17.6 gauss. When l7O-containing dioxygen (55%) was supplied to the medium, an EPR spectrum consistent with the “O-substituted nitric oxide spin adduct was observed in the composite spectrum. The hfs of “O was estimated to be 2.5 gauss. The l4NO spin adduct observed after prolonged incubation in the medium which contains [l5N]L-arginine as the only extracellular source of arginine demonstrates that arginine is recycled through its metabolite in activated macrophages. 相似文献
6.
Victor Ciornea Liudmila Mingalieva Jean-Pierre Costes Ghenadie Novitchi Irina Filippova Ravil T. Galeev Sergiu Shova Violeta K. Voronkova Aurelian Gulea 《Inorganica chimica acta》2008,361(7):1947-1957
Five heterometallic compounds with formulae [Ba(H2O)4Cr2(μ-OH)2(nta)2] · 3H2O (I), [M(bpy)2(H2O)2] [Cr2(OH)2(nta)2] · 7H2O, where M2+ = Zn, (II); Ni, (III); Co, (IV) and [Mn(H2O)3(bpy)Cr2(OH)2(nta)2] · (bpy) · 5H2O (V); bpy = 2,2′-bipyridine, (nta = nitrilotriacetate ion) have been prepared by reaction of I with the corresponding MII-sulfates in the presence of 2,2′-bipyridine. Substances I–V have been characterized by magnetic susceptibility measurements, EPR and X-ray determinations. I represents a 2D coordination polymer formed by coordination of centrosymmetrical dimeric chromium(III) units and Barium cations. The 10-coordinate Ba polyhedron is completed by four water molecules. Compounds II–IV are isostructural and consist of non-centrosymmetric dimeric anions [Cr2(μ-OH)2(nta)2]2−, complex cations [MII(bpy)2(H2O)2]2+ and solvate water molecules. The octahedral coordination of chromium atoms implies four donor atoms of the nta3− ligands and two bridging OH groups. Multiple hydrogen bonds of coordinated and solvate water molecules link anions and cations in a 3D network. A similar [Cr2(μ-OH)2(nta)2]2− unit is found in V. The bridging function is performed by a carboxylate oxygen atom of the nta ligand that leads to the formation of a trinuclear complex [Mn(bpy)(H2O)2Cr2(μ-OH)2(nta)2]. Experimental and calculated frequency and temperature dependences of EPR spectra of these compounds are presented. The fine structure appearing on the EPR spectra of compound V is analyzed in detail at different temperatures. It is established that the main part of the EPR signals is due to the transitions in the spin states of a spin multiplet with S = 2. Analyses of experimental and calculated spectra confirm the absence of interaction between metal ions (MII) and Cr-dimers in complexes III and IV and the presence of weak Mn–Cr interactions in V. The temperature dependence of magnetic susceptibilities for I–V was fitted on the basis of the expression derived from isotropic Hamiltonian including a bi-quadratic exchange term. 相似文献
7.
Various sulfidic anions and the oxidizing cations [Ru(NH3)6]3+ and N,N′-dimethyl-4,4′-bipyridinium2+ (paraquat2+) form ion pairs in aqueous solutions which display outer-sphere charge-transfer (CT) absorptions. The CT energies are used to establish a series of sulfidic anions with increasing CT donor strength: SCN−2O3 2−4 3−3S3−2 −
2S2 −
8.
Ademir Neves Cludio N. Verani Marcos A. de Brito Ivo Vencato Antnio Mangrich Glaucius Oliva Dulce D.H.F. Souza Alzir A. Batista 《Inorganica chimica acta》1999,290(2):2221-212
Copper(II) complexes were synthesized and characterized by means of elemental analysis, IR and visible spectroscopies, EPR and electrochemistry, as well as X-ray structure crystallography. The group consists of discrete mononuclear units with the general formula [Cu(II)(Hbpa)2](A)2·nH2O, where Hbpa=(2-hydroxybenzyl-2-pyridylmethyl)amine and A=ClO4 −, n=2 (1), CH3COO−, n=3 (2), NO3 −, n=2 (3) and SO4 2−, n=3 (4). The structures of the ligand Hbpa and complex 1 have been determined by X-ray crystallography. Complexes 1–4 have had their UV–Vis spectra measured in both MeCN and DMF. It was observed that the compounds interact with basic solvents, such that molecules coordinate to the metal in axial positions in which phenol oxygen atoms are coordinated in the protonated forms. The values were all less than 1000 M−1 cm−1. EPR measurements on powdered samples of 1–3 gave g/A values between 105 and 135 cm−1, typical for square planar coordination environments. Complex 4·3H2O exhibits a behaviour typical for tetrahedral coordination. The electrochemical behaviour for complexes 1 and 2 was studied showing irreversible redox waves for both compounds. 相似文献
9.
A. Wozniak J.B. Hutchison 《The Journal of steroid biochemistry and molecular biology》1993,44(4-6):641-645
To study mechanisms of aromatase inhibition in brain cells, a highly effective non-steroidal aromatase inhibitor (Fadrozole; 4-[5,6,7,8-tetra-hydroimidazo-(1,5-a)-pyridin-5-yl] benzonitrile HCl; CGS 16949A) was compared with endogenous C-19 steroids, known to be formed in the preoptic area, which inhibit oestrogen formation. Using a sensitive in vitro tritiated water assay for aromatase activity in avian (dove) preoptic tissue, the order of potency, with testosterone as substrate was: Fadrozole (Ki < 1 × 10−9 M) > 4-androstenedione 5-androstanedione > 5-dihydrotestosterone (Ki = 6 × 10−8 M) > 5β-androstanedione > 5β-dihydrotestosterone (Ki = 3.5 × 10−7 M) > 5-androstane-3, 17β-diol (Ki = 5 × 10−6 M) > 5β-androstane-3β,17β-diol. Five other steroids, 5β-androstane-3,17β-diol, 5-androstane-3β,17β-diol, progesterone, oestradiol and oestrone, showed no inhibition at 10−4 M. The kinetics indicate that endogenous C-19 steroids show similar competitive inhibition of the aromatase as Fadrozole. Mouse (BALB/c) preoptic aromatase was also inhibited by Fadrozole. We conclude that endogenous C-19 metabolites of testosterone are effective inhibitors of the brain aromatase, and suggest that they bind competitively at the same active site as Fadrozole. 相似文献
10.
Magnetic field-dependent recombination measurements together with magnetic field-dependent triplet lifetimes (Chidsey, E.D., Takiff, L., Goldstein, R.A. and Boxer, S.G. (1985) Proc. Natl. Acad. Sci USA 82, 6850–6854) yield a free energy change ΔG(P+H− − 3P*) = 0.165 eV ±0.008 at 290 K. This does not depend on whether nuclear spin relaxation in the state 3P* is assumed to be fast or slow compared to the lifetime of this state. This value, being (almost) temperature independent, indicates ΔG(P+H− − 3P*) ΔH(P+H− − 3P*) and is consistent with ΔG(1P* − P+H−) and ΔH(1P* − 3P*) from previous delayed fluorescence and phosphorescence data, implying ΔG ΔH for all combinations of these states. 相似文献
11.
Goltzman D 《The Journal of steroid biochemistry and molecular biology》2007,103(3-5):587-591
We employed genetically modified mice to examine the role of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] on skeletal and calcium homeostasis. In mice expressing the null mutation for 25-hydroxyvitamin D 1 hydroxylase (1OHase−/−), or the vitamin D receptor (VDR−/−), 1,25(OH)2D3 and calcium were both required for optimal epiphyseal growth plate development, serum calcium and phosphorus alone were sufficient to mineralize skeletal tissue independent of 1,25(OH)2D3 and the VDR, and endogenous 1,25(OH)2D3 and the VDR were essential for baseline bone formation. In 2-week-old 1OHase−/− mice and in 2-week-old mice homozygous for the PTH null mutation(PTH−/−), PTH and 1,25(OH)2D3 were each found to exert independent and complementary effects on skeletal anabolism, with PTH predominantly affecting appositional trabecular bone growth and 1,25(OH)2D3 influencing both endochondral bone formation and appositional bone growth. Endogenous 1,25(OH)2D3 maintained serum calcium homeostasis predominantly by modifying intestinal and renal calcium transporters but not by producing net bone resorption. Administration of exogenous 1,25(OH)2D3 to double mutant PTH−/−1OHase−/− mice produced skeletal effects consistent with the actions of endogenous 1,25(OH)2D3. These studies reveal an important skeletal anabolic role for both endogenous and exogenous 1,25(OH)2D3 and point to a potential role for 1,25(OH)2D3 analogs in the treatment of disorders of bone loss. 相似文献
12.
Van Remmen H Qi W Sabia M Freeman G Estlack L Yang H Mao Guo Z Huang TT Strong R Lee S Epstein CJ Richardson A 《Free radical biology & medicine》2004,36(12):1625-1634
To examine the effect of compound deficiencies in antioxidant defense, we have generated mice (Sod2+/−/Gpx1−/−) that are deficient in Mn superoxide dismutase (MnSOD) and glutathione peroxidase 1 (Gpx1) by breeding Sod2+/− and Gpx1−/− mice together. Although Sod2+/−/Gpx1−/− mice showed a 50% reduction in MnSOD and no detectable Gpx1 activity in either mitochondria or cytosol in all tissues, they were viable and appeared normal. Fibroblasts isolated from Sod2+/−/Gpx1−/− mice were more sensitive (4- to 6-fold) to oxidative stress (t-butyl hydroperoxide or γ irradiation) than fibroblasts from wild-type mice, and were twice as sensitive as cells from Sod2+/− or Gpx1−/− mice. Whole-animal studies demonstrated that survival of the Sod2+/−/Gpx1−/− mice in response to whole body γ irradiation or paraquat administration was also reduced compared with that of wild-type, Sod2+/−, or Gpx1−/− mice. Similarly, endogenous oxidative stress induced by cardiac ischemia/reperfusion injury led to greater apoptosis in heart tissue from the Sod2+/−/Gpx1−/− mice than in that from mice deficient in either MnSOD or Gpx1 alone. These data show that Sod2+/−/Gpx1−/− mice, deficient in two mitochondrial antioxidant enzymes, have significantly enhanced sensitivity to oxidative stress induced by exogenous insults and to endogenous oxidative stress compared with either wild-type mice or mice deficient in either MnSOD or Gpx1 alone. 相似文献
13.
Kozlov AV Szalay L Umar F Kropik K Staniek K Niedermüller H Bahrami S Nohl H 《Biochimica et biophysica acta》2005,1740(3):382-389
The aim of this study was to compare rat tissues with respect to their reactive oxygen and nitrogen species (RONS) generating activities as a function of age. We quantified the RONS generation in vivo in young (6 months) and in old (30 months) male Sprague-Dawley rats using the recently developed spin trap 1-hydroxy-3-carboxy-pyrrolidine, applied intravenously. This spin trap reacts with superoxide radical and peroxynitrite yielding a stable spin adduct which is detectable by means of electron paramagnetic resonance (EPR) spectroscopy in frozen tissues. In old rats RONS generation was significantly increased compared to their young counterparts in the following order: blood相似文献
14.
Comparative measurements of bacterial total counts and volumes of flow cytometry (FCM), transmission electron (TEM), and epifluorescence microscopy (EFM), were undertaken during a four week mesocosm experiment. Total counts of bacteria measured by TEM, EFM, and FCM were in the range of 1 · 106−6 cells ml−1, 1 · 106−3 · 1016 cells ml−1, and 5 · 105 cells ml−1 respectively. The mean volume of the bacterial community, measured by means of EFM and TEM, increased from 0.12–0.15 μm3 at the start of the experiment to 0.39–0.53 μm3 at the end. Generally, there was good agreement between the two methods and regression analyses gave r = 0.87 (p < < 0.01) for cell volume and r = 0.97 (p < < 0.01) for cell number. DAPI stained bacteria with volumes less than 0.2 μm3 were not detected by flow cytometry and these were generally an order of magnitude lower than counts made by TEM and EFM. For samples where the mean bacterial cell volume was longer than 0.3 μm3, all three methods were in agreement both with respect to counts and volume estimates. 相似文献
15.
Crystal and microparticle effects on MDCK cell superoxide production: oxalate-specific mitochondrial membrane potential changes 总被引:1,自引:0,他引:1
Meimaridou E Jacobson J Seddon AM Noronha-Dutra AA Robertson WG Hothersall JS 《Free radical biology & medicine》2005,38(12):1553-1564
We have previously shown that crystals of calcium oxalate (COM) elicit a superoxide (O2−) response from mitochondria. We have now investigated: (i) if other microparticles can elicit the same response, (ii) if processing of crystals is involved, and (iii) at what level of mitochondrial function oxalate acts. O2− was measured in digitonin-permeabilized MDCK cells by lucigenin (10 μM) chemiluminescence. [14C]-COM dissociation was examined with or without EDTA and employing alternative chelators. Whereas mitochondrial O2− in COM-treated cells was three- to fourfold enhanced compared to controls, other particulates (uric acid, zymosan, and latex beads) either did not increase O2− or were much less effective (hydroxyapatite +50%, p < 0.01), with all at 28 μg/cm2. Free oxalate (750 μM), at the level released from COM with EDTA (1 mM), increased O2− (+50%, p < 0.01). Omitting EDTA abrogated this signal, which was restored completely by EGTA and partially by ascorbate, but not by desferrioxamine or citrate. Omission of phosphate abrogated O2−, implicating phosphate-dependent mitochondrial dicarboxylate transport. COM caused a time-related increase in the mitochondrial membrane potential (Δψm) measured using TMRM fluorescence and confocal microscopy. Application of COM to Fura 2-loaded cells induced rapid, large-amplitude cytosolic Ca2+ transients, which were inhibited by thapsigargin, indicating that COM induces release of Ca2+ from internal stores. Thus, COM-induced mitochondrial O2− requires the release of free oxalate and contributes to a synergistic response. Intracellular dissociation of COM and the mitochondrial dicarboxylate transporter are important in O2− production, which is probably regulated by Δψm. 相似文献
16.
The selective picosecond excitation of Rhodopseudomonas sphaeroides (R-26) reaction centers (RCs) at 870 nm induces the formation of the transient state within <1 ps followed by the conversion into the state PF (P± Bph±− during 7 ± 2 ps at both 293 K and 110 K. The transient state including the intense bleaching at 800 nm has been shown not to be due: (a) to photon excitation at 870 nm; (b) the excitation of P+; (c) photoselection effects. The transient state is interpreted as the state 1[P+B−] in agreement with earlier works. The primary formation of the state 1P+B−] and the big effective singlet-triplet splitting in this state correspond to the spectral splitting of the P band at 900 nm in R-26 RCs and at 1000 nm in Rhodopseudomonas viridis RCs found at 4.2 K and attributed to the optical transition to both 1P and 1[P+B−] states. 相似文献
17.
Vladimír Miík I. Tong Mak Richard E. Stafford William B. Weglicki 《Free radical biology & medicine》1993,15(6):611-619
In the present study, using the technique of EPR spin trapping with DMPO a spin trap, we demonstrated formation of thiyl radicals from thiol-containing angiotensin converting enzyme (ACE) inhibitor captopril (CAP) and from its stereoisomer epicaptopril (EPICAP), a non-ACE inhibitor, in the process of .OH radical scavenging. Splitting constants of DMPO/thiyl radical adducts were identical for both thiols and were aN = 15.3 G, and aH = 16.2 G. Bimolecular rate constants for the reaction of CAP and EPICAP with .OH radicals were close to a diffusion-controlled rate (≈ 2 × 1010 M−1s−1). Our data also show that both CAP and EPICAP reduce Fe(III) ions and that their respective thiyl radicals are formed in this reaction. In the presence of Fe(III), H2O2, and CAP, or EPICAP, .OH radicals were produced by a thiol-driven Fenton mechanism. Copper(II) ions were also reduced by these thiols, but no thiyl radicals could be detected in these reactions, and no .OH or other Fenton oxidants were observed in the presence of H2O2. Our data show direct evidence that thiol groups of CAP and EPICAP are involved in scavenging of .OH radicals. The direct .OH radical scavenging, together with the reductive “repair” of other sites of .OH radical attack, may contribute to the known protective effect of CAP against ischemia/reperfusion-induced arrhythmias. The formation of reactive thiyl radicals in the reactions of the studied compounds with .OH radicals and with Fe(III) ions may play a role in some of the known adverse effects of CAP. 相似文献
18.
High-pressure liquid-chromatography and microcalorimetry have been used to determine equilibrium constants and enthalpies of reaction for the disproportionation reaction of adenosine 5′-diphosphate (ADP) to adenosine 5′-triphosphate (ATP) andadenosine 5′-monophosphate (AMP). Adenylate kinase was used to catalyze this reaction. The measurements were carried out over the temperature range 286 to 311 K, at ionic strengths varying from 0.06 to 0.33 mol kg−1, over the pH range 6.04 to 8.87, and over the pMg range 2.22 to 7.16, where pMg = -log a(Mg2+). The equilibrium model developed by Goldberg and Tewari (see the previous paper in this issue) was used for the analysis of the measurements. Thus, for the reference reaction: 2 ADp3− (ao) AMp2− (ao)+ ATp− (ao), K° = 0.225 ± 0.010, ΔG° = 3.70 +- 0.11 kJ mol −1, ΔH° = −1.5 ± 1. 5 kJ mol −1, °S ° = −17 ± 5 J mol−1 K−1, and ACPp°≈ = −46 J mo1l−1 K−1 at 298.15 K and 0.1 MPa. These results and the thermodynamic parameters for the auxiliary equilibria in solution have been used to model the thermodynamics of the disproportionation reaction over a wide range of temperature, pH, ionic strength, and magnesium ion morality. Under approximately physiological conditions (311.15 K, pH 6.94, [Mg2+] = 1.35 × 10−3 mol kg−1, and I = 0.23 mol kg−1) the apparent equilibrium constant (KA′ = m(ΣAMP)m(ΣATP)/[ m(ΣADP)]2) for the overall disproportionation reaction is equal to 0.93 ± 0.02. Thermodynamic data on the disproportionation reaction and literature values for this apparent equilibrium constant in human red blood cells are used to calculate a morality of 1.94 × 10−4 mol kg−1 for free magnesium ion in human red blood cells. The results are also discussed in relation to thermochemical cycles and compared with data on the hydrolysis of the guanosine phosphates. 相似文献
19.
O. I. Trifonova E. A. Ochertyanova N. G. Akhmedov V. A. Roznyatovsky D. N. Laikov N. A. Ustynyuk Yu. A. Ustynyuk 《Inorganica chimica acta》1998,280(1-2):328-338
σ-Methyl-(η5-indenyl) chromium tricarbonyl (III) rearranges quantitatively into η6-1-endo-methylindene) chromium tricarbonyl (IV) in C6D6 solution at 30–60°C. Methyl group attachment to the positions 2 or 3 of indenyl ligand in (III) has no influence on the activation parameters of this ricochet inter-ring haptotropic rearrangement (ΔG#=23.6 kcal mol−1; ΔH#=18.9±0.2 kcal mol−1; ΔS#=−18.6±0.2 cal K−1 mol−1). (IV) undergoes further irreversible isomerization at 60–120° into (ν6-3-methylindene) chromium tricarbonyl (V) with a higher activation barrier (ΔG#=28.5±0.1 kcal mol−1) via two consecutive [1,5]-sigmatropic hydrogen shifts. The mechanisms of both rearrangements have been studied in detail using density functional theory (DFT) calculations with extended basis sets. Calculations show that the rearrangement (III) → (IV) proceeds in two steps. Methyl group migration from chromium into position 1 of the indenyl ligand is the rate-determining step leading to the formation of the 16-electron intermediate (VII). The calculated activation barrier (Ea=19.6 kcal mol−1) is in good agreement with the experimental one. Further rearrangement (VII) → (V) proceeds via a trimethylenemethane-type transition state (XVIII) with an activation barrier 11.8 kcal mol−1. The coordination of the chromium tricarbonyl group at the six-membered ring has only minor influence on the kinetic parameters of the hydrogen [1,5]-sigmatropic shift in indene. 相似文献
20.
Kurz CR Kissner R Nauser T Perrin D Koppenol WH 《Free radical biology & medicine》2003,35(12):1529-1537
The reaction of peroxynitrous acid with monohydroascorbate, over the concentration range of 250 μM to 50 mM of monohydroascorbate at pH 5.8 and at 25°C, was reinvestigated and the rate constant of the reaction found to be much higher than reported earlier (Bartlett, D.; Church, D. F.; Bounds, P. L.; Koppenol, W. H. The kinetics of oxidation of L-ascorbic acid by peroxynitrite. Free Radic. Biol. Med. 18:85–92; 1995; Squadrito, G. L.; Jin, X.; Pryor, W. A. Stopped-flow kinetics of the reaction of ascorbic acid with peroxynitrite. Arch. Biochem. Biophys. 322:53–59; 1995). The new rate constants at pH 5.8 are k1 = 1 × 106 M−1 s−1 and k−1 = 500 s−1 for 25°C and k1 = 1.5 × 106 M−1 s−1 and k−1 = 1 × 103 s−1 for 37°C. These values indicate that even at low monohydroascorbate concentrations most of peroxynitrous acid forms an adduct with this antioxidant. The mechanism of the reaction involves formation of an intermediate, which decays to a second intermediate with an absorption maximum at 345 nm. At low monohydroascorbate concentrations, the second intermediate decays to nitrate and monohydroascorbate, while at monohydroascorbate concentrations greater than 4 mM, this second intermediate reacts with a second monohydroascorbate to form nitrite, dehydroascorbate, and monohydroascorbate. EPR experiments indicate that the yield of the ascorbyl radical is 0.24% relative to the initial peroxynitrous acid concentration, and that this small amount of ascorbyl radicals is formed concomitantly with the decrease of the absorption at 345 nm. Thus, the ascorbyl radical is not a primary reaction product. Under the conditions of these experiments, no homolysis of peroxynitrous acid to nitrogen dioxide and hydroxyl radical was observed. Aside from monohydroascorbate's ability to “repair” oxidatively modified biomolecules, it may play a role as scavenger of peroxynitrous acid. 相似文献