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In this paper, we develop a new technique to study the treewidth of graphs with bounded degree. We show that the treewidth of a graph G = (V, E) with maximum vertex degree d is at most for sufficiently large d, where C is a constant. 相似文献
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We measured the following variables to investigate the effects of fasting and temperature on swimming performance in juvenile qingbo (Spinibarbus sinensis): the critical swimming speed (Ucrit), resting metabolic rate and active metabolic rate of fish fasting for 0 (control), 1, 2 and 4 weeks at low and high acclimation temperatures (15 and 25 °C). Both fasting treatment and temperature acclimation had significant effects on all parameters measured (P<0.05). Fasting at the higher temperature had a negative effect on all measured parameters after 1 week (P<0.05). However, when acclimated to the lower temperature, fasting had a negative effect on Ucrit until week 2 and on , and metabolic scope (MS, –) until week 4 (P<0.05). The values of all parameters at the lower temperature were significantly lower than those at the higher temperature in the identical fasting period groups except for of the fish that fasted for 2 weeks. The relationship between fasting time (T) and Ucrit was described as Ucrit(15)=−0.302T2−0.800T+35.877 (r=0.781, n=32, P<0.001) and Ucrit(25)=0.471T2−3.781T+50.097 (r=0.766, n=32, P<0.001) at 15 and 25 °C, respectively. The swimming performance showed less decrease in the early stage of fasting but more decrease in the later stage at the low temperature compared to the high temperature, which might be related to thermal acclimation time, resting metabolism, respiratory capacity, energy stores, enzyme activity in muscle tissue and energy substrate utilization changes with fasting between low and high temperatures. The divergent response of the swimming performance to fasting in qingbo at different temperatures might be an adaptive strategy to seasonal temperature and food resource variation in their habitat. 相似文献
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Andrew Wojcicki 《Inorganica chimica acta》1985,100(1):125-133
Studies are reported on the chemical reduction of the homobinuclear bis(μ-phosphido) metal complexes (R = Ph or Me), and (M = Mo or W). Two reduction pathways have been observed which result in different two-electron transformations: (1) with Na or LiAlH4, electron transfer to yield the corresponding symmetric dianions of the type LnM(μ-PR2)2MLn2? without metalmetal bond and (2) with M′BR′3H(M′ = Li, Na, or K; R′ = Et or sec-Bu), hydride transfer to give monoanionic complexes of the type or (M = Fe, Mo, or W; L = CO or NO; R = Ph or Me). The monoanionic complexes can be deprotonated with n-BuLi at ?78 °C to the corresponding unsymmetric dianions (M = Fe; L = CO or NO; R = Ph) or symmetric dianions LnM(μ-PR2)2MLn2? (M = Mo or W; L = CO; R = Ph). The unsymmetric dianions isomerize on slight warming to the symmetric dianions, which undergo protonation by CF3COOH to yield the aforementioned monoanions. Reactions of several members of these three classes of binuclear anions with CF3COOH, alkylating reagents, 1,1-diiodohydrocarbons and metal diiodo complexes have resulted in the synthesis of new binuclear and trinuclear compounds. Examples include , (R = Me, Et, n-Pr, or i-Pr), (M = Mo or W; R = Me or Ph), , , , and Fe2Co(η5-C5H5)(CO)(NO)4(μ-PPh2)2. Synthetic and mechanistic studies on these reactions are presented. 相似文献
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Owing to their ability to break glycosidic bonds in recalcitrant crystalline polysaccharides such as cellulose, the catalysis effected by lytic polysaccharide monooxygenases (LPMOs) is of major interest. Kinetics of these reductant-dependent, monocopper enzymes is complicated by the insoluble nature of the cellulose substrate and parallel, enzyme-dependent, and enzyme-independent side reactions between the reductant and oxygen-containing cosubstrates. Here, we provide kinetic characterization of cellulose peroxygenase (oxidative cleavage of glycosidic bonds in cellulose) and reductant peroxidase (oxidation of the reductant) activities of the LPMO TrAA9A of the cellulose-degrading model fungus Trichoderma reesei. The catalytic efficiency of the cellulose peroxygenase reaction (kcat = 8.5 s−1, and ) was an order of magnitude higher than that of the reductant (ascorbic acid) peroxidase reaction. The turnover of H2O2 in the ascorbic acid peroxidase reaction followed the ping-pong mechanism and led to irreversible inactivation of the enzyme with a probability of 0.0072. Using theoretical analysis, we suggest a relationship between the half-life of LPMO, the values of kinetic parameters, and the concentrations of the reactants. 相似文献
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The objective of this work was to explore the hypothesis that nitric oxide (NO) affects Fe bioavailability in sorghum (Sorghum bicolor (L.) Moench) embryonic axes. NO content was assessed in embryonic axes isolated from seeds control or exposed to NO-donors, employing spin trapping electron paramagnetic resonance (EPR) methodology. NO donors such as sodium nitroprusside (SNP) and diethylenetriamine NONOate (DETA NONOate), released NO that permeated inside the axes increasing NO content. Under these conditions low temperature EPR was employed to study the labile iron pool. A 2.5 fold increase was observed in NO steady state concentration after 24 h of exposure to NO donors that was correlated to a 2 fold increase in the Fe labile pool, as compared to control axes. This observation provides experimental evidence for a potential role of NO in Fe homeostasis.Key words: iron, labile iron pool, nitric oxide, sorghumNitric oxide (NO) has a wide range of functions, among them promotion of growth and seed germination were described in several plant species.1 Evidences for its participation in Fe homeostasis in planta arise from the fact that Fe deficiency can be reverted enhancing NO level.2 Moreover, it is expected that NO acts as intercellular messenger3 being transported from the site of its synthesis. Nitrosylated Fe complexes, formed by reaction of NO with Fe2+ and biological thiols, have been proposed as NO carriers, since they are relative stable molecules.4The ability of Fe of changing its oxidation state and redox potential in response to changes in the nature of the ligand makes this metal essential for almost all living organisms.5 Fe-containing enzymes are the key components of many essential biological reactions. However, the same biochemical properties that make Fe beneficial might be a drawback in some particular conditions, when improperly shielded Fe can catalyze one-electron reductions of O2 species that lead to the production of reactive free radicals. The toxicity of Fe depends on the Fenton reaction, which produces the hydroxyl radical (·OH) or an oxoiron compound (LFeO2+) and on its reactions with lipid hydroperoxides.6Most of the current information about NO functions in plants comes from pharmacological studies using NO donors, which generate NO either spontaneously, or after metabolic activation. Moreover, NO production from numerous compounds strongly depends on pH, temperature, light and the presence of reductants.7 SNP and DETA NONOate have different kinetics and mechanisms of NO release. However, both are suitable compounds for long-term treatments, since their stability is higher than other NO donors.In this work we evaluated NO steady state concentration in sorghum embryonic axes 24 h after imbibition, in control seeds (distilled water) and in seeds placed either in 1 mM SNP or DETA NONOate. SNP contains Fe in its chemical structure, thus a control was carried out employing photodegraded SNP, which consist of 1 mM SNP solution which had been left under light until all NO was released from the molecule. As it is shown in FW (mg axis−1) Electrolyte leakage (%) NO (nmol g−1 FW) LIP(nmol g−1 FW) Control 6.8 ± 0.3 29 ± 2 2.4 ± 0.2 8 ± 1 SNP 10.8 ± 0.6* 20 ± 1* 6.0 ± 0.9* 19 ± 2* Photodegraded SNP 6.6 ± 0.3 27 ± 2 2.5 ± 0.6 9 ± 1 DETA NONOate 9.7 ± 0.9* 18 ± 1* 6.2 ± 0.6* 15.2 ± 0.5*