Biphasic kinetics

Biexponential kinetics, 72-76 Biphasic kinetics, 72-76 Bloch equations, 261 Branching reactions, 189 Brpnsted-Bjerrum equation, 204... 

As mentioned above, V(0 Pr)3-modiricd silica treated with BuOOH shows no activity towards cyclohexene at room temperature. However, the material obtained by grafting Ti(0 Pr)4 directly on unmodified silica heated at 500°C, eq 4, does convert olefins to their epoxides upon treatment with BuOOH. The catalyst 4 also shows biphasic kinetic behavior towards cyclohexene, Figure lc. 

In the meantime temperature-dependent stopped-flow measurements were conducted on the latter complex in order to determine the activation parameters of the N-N cleavage reaction (24). Plots of the absorption intensity at 418 nm vs. time at T — —35 to +15°C indicate biphasic kinetics with two rate constants 0bs(p and obs(2)> in analogy to our measurements of the tungsten complex. This time, however, both rates depended upon the acid concentration. Interestingly much smaller rate constants 0bs(i) and 0bs(2)> were found for all acid concentrations than given by Henderson et al. for his (single) rate constant kobs (up to 1 order of magnitude). Furthermore plots of 0bs(i) and kohs(2) vs. the acid concentration showed no saturation behavior but linear dependencies with slopes k and k and intercepts k und k, respectively (s — acid dependent and i — acid independent), Eq. (2) ... 

Biphasic kinetics of Fe2+ transport in a wild-type strain of El. pylori suggested the presence of high- and low-affinity uptake systems. The high-affinity system (apparent Ks = 0.54 jimoldm ) is absent in a mutant lacking the feoB gene. Transport via FeoB is highly specific for Fe2+, and was inhibited by FCCP,... 

The kinetics of complex formation with Zn2+ can be followed by monitoring the change in the fluorescence intensity [ 17g]. In the case of 1, the change in the fluorescence intensity with time indicates a biphasic kinetics with the incorporation rate constants k1 = 4.9 x 105 M 1 s 1 followed by a first order process... 

The fact that diffusion models describe a number of chemical processes in solid particles is not surprising since in most cases, mass transfer and chemical kinetics phenomena occur simultaneously and it is difficult to separate them [133-135]. Therefore, the overall kinetics of many chemical reactions in soils may often be better described by mass transfer and diffusion-based models than with simple models such as first-order kinetics. This is particularly true for slower chemical reactions in soils where a fast reaction is followed by a much slower reaction (biphasic kinetics), and is often observed in soils for many reactions involving organic and inorganic compounds. 

Biphasic kinetics will in principle be observed with (1.123). The associated first-order rate constants k and are related to the rate constants of (1.123) by the relationships (Sec. 1.8.2)... 

The NO dissociation rate constants are summarized in Table HI (50) and are smaller than those seen with NO-metmyoglobin complexes. NP2 and NP3 (A ofr 0.1 s ) release NO approximately 10 times more slowly than NPl and NP4 kofs 2-3 s ) at pH 8.0, and the NO release rate for all nitrophorins decreases as the pH is lowered to 5.0. The NO release curves cannot be fit with a single exponential, indicating two off rates at each pH, as previously noted for both recombinant and insect-derived NPl (46), and which has also been recently reported for recombinant NP2 (145). The biphasic kinetics suggest the presence of slowly interconverting conformations. The values obtained are pH and protein dependent, ranging from 2.6 to 0.05 (Table HI) (50), values that are considerably slower than found for sperm whale metmyoglobin... 

Biphasic kinetics were observed for the removal of iron from ferritin by licams, A, A, A "-tris (5-sulfo-2,3-dihydroxybenzoyl)-l,5,10-triazadecane (272) - Fe + is removed from the N-terminal site about twice as fast as from the C-terminal site E SOkJmol for the latter process). It is... 

A multiple-path mechanism has been elaborated for dissociation of the mono- and binuclear tris(hydroxamato)-iron(III) complexes with dihydroxamate ligands in aqueous solution. " Iron removal by edta from mono-, bi-, and trinuclear complexes with model desferrioxamine-related siderophores containing one, two, or three tris-hydroxamate units generally follows first-order kinetics though biphasic kinetics were reported for iron removal from one of the binuclear complexes. The kinetic results were interpreted in terms of discrete intrastrand ferrioxamine-type structures for the di-iron and tri-iron complexes of (288). " Reactivities for dissociation, by dissociative activation mechanisms, of a selection of bidentate and hexadentate hydroxamates have been compared with those of oxinates and salicylates. ... 

Selected entries from Methods in Enzymology [vol, page(s)] Applications, 249, 263-283 basic kinetics, 249, 243-245 biphasic kinetics, 249, 253 competitive inhibitor protection, 249, 247,... 

The interpretation of the biphasic kinetics of formation of PhjC- is provided in Scheme 56. The critical feature of Scheme 56 is sequential H-transfer to 131h with the intermediacy of the radical cation 135h. Long-wavelength absorption, consistent with this species, is observed during the PhsCH trapping experiments. ... 

The elimination of [i C]triethanolamine from the blood of mice administered 1.0 mg/kg bw intravenously showed first-order biphasic kinetics with a rapid (0.58-h half-life) and a slow phase (10.2-h half-life). The slow phase half-lives for elimination of triethanolamine in mice after dermal exposure to 1000 and 2000 mg/kg bw in acetone were 9.7 h and 18.6 h. Skin absorption rates (as blood concentration-time curves) after dermal application of aqueous and neat [I CJtriethanolamine to mouse skin (2000 mg/kg bw, enclosed by a glass ring) showed no significant change with the use of water as the vehicle (Waechter Rick, 1988, cited in Knaak et al, 1997). 

In most of the reactions studied, the appearance of biphasic kinetics showed that the full oxidation process is achieved through the stepwise oxidation of one of the metal centers, rendering a mixed-valence species which is further oxidized to the final product. The question arises in Eq. (35) as to the alternative first oxidation of iron or ruthenium. The stoichiometry of the first oxidation step with bridging 4- and 3-cyanopyridines (with the N-atom of py binding to... 

Figure 5.1 shows the spectrophoto metrically determined liberation of p-nitro phenol (pNPOH) from pNPOAc in the absence (curve a) and presence (curve b) of metal catalyst. The catalyzed reaction exhibits biphasic kinetics, characterized by an initial burst of pNPOH release, followed by a slower linear phase. 

In spite of some claims to the contrary, venom exonuclease is capable of attacking double-stranded high molecular DNA. In fact, double-stranded DNA is a better substrate than denatured DNA. Bjork (1 ) studied the rates of degradation of native and heat-denatured DNA using a pH stat. Denatured DNA was degraded at a steady rate, which was dependent on the ionic strength of the medium. An increase in NaCl concentration from 1 to 100 mM decreased the rate of hydrolysis by a factor of two. With native DNA a two-phase reaction was observed. The initial, very rapid, rate was independent of NaCl concentration. After about one-third of the linkages had been hydrolyzed, the rate slowed down to that of denatured DNA and became salt dependent. Similar biphasic kinetics was observed previously with DNA that was denatured by an exhaustive dialysis (40). 

Chiba et al. [163] studied the oxidative metabolism of omeprazole in 14 human liver microsomes in relation to the 4 -hydroxylation capacity of S-mephenytoin. The formation of 5-hydroxyomeprazole and omeprazole sulfone from omeprazole exhibited a biphasic kinetic behavior, indicating that at least two distinct enzymes are involved in either of the metabolic pathways of omeprazole. These findings suggest that S-mephenytoin 4 -hydroxylase is an enzyme primarily responsible for the 5-hydroxylation of omeprazole and further metabolism of omeprazole sulfone, but not for the sulfoxidation of omeprazole in human liver microsomes. 

The biphasic kinetics curve for the reactions of polymers is very typical and is found frequently in the polymer literature. Daglen and Tyler showed62 that equation 27 gave excellent fit to these systems as well, which suggests the presence of reaction spheres is common in the mechanism of solid-state polymer photodegradation. 

There has been a study of the influence of ion-pairing reagents and solvents on the stability of the hydroxide adduct (11) of the diethylamide of 3,5-dinitrobenzoic acid e.g. (11) is stabilized by quaternary ammonium salts in benzene. The resulting methodology has been used to perform highly enantioselective biphasic kinetic resolutions of several racemic electron-deficient amides.50... 

A second type of nonhyperbolic saturation kinetics became apparent during studies on the metabolism of naproxen to desmethylnaproxen (32). Studies with human liver microsomes showed that naproxen metabolism has biphasic kinetics and is activated by dapsone (T. Tracy, unpublished results). The unactivated data shows what appears to be a typical concentration profile for metabolism by at least two different enzymes. However, a similar biphasic profile was obtained with expressed enzyme (25). This biphasic kinetic profile is observed with the two-substrate model when V/rn2 > Eml and Kml Km2. The appropriate equation for the two-site model when [S] < Kml is... 

Serpone et al. have examined colloidal titanium dioxide sols (prepared by hydrolysis of TiCl4) with mean particle diameters of 2.1, 13.3, and 26.7 nm by picosecond transient absorption and emission spectroscopy [5]. Absorption decay for the 2.1 nm sols was found to be a simple first-order process, and electron/hole recombination was 100% complete by 10 ns. For the 13.3 and 26.7 nm sols absorption decay follows distinct second-order biphasic kinetics the decay times of the fast components decrease with increase in particle size. 10 ns after the excitation pulse, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (x > 20 ns) reflect recombination of deep-trapped electrons and holes. 

The inactivation of bovine Fi -ATPase by FSBA exhibited biphasic kinetics. A double reciprocal plot of the inactivation rate for the fast phase against FSBA concentration gave a curved line, whereas the same type of plot for the slow phase yielded a straight line, giving a Ka value of 0.23 mM. The slow phase was diminished when the enzyme was inactivated in the presence of 0.2 M phosphate. On complete inactivation, about three moles of FSBA bound to one mole of bovine Fi -ATPase. Regardless of the presence of phosphate, His-427 of the (8-subunit was dominantly modified at pH 6, whereas Tyr-368 of the /8-subunit was dominantly modified at pH 8. At pH 7, the two residues were modified in a similar ratio. 

By means of stopped-flow spectrophotometry, the reaction of flavopapain IV withN-benzyl-l,4-dihydronicotinamide (NBzNH) has been studied anaerobically. Using conditions of excess substrate and following the decrease in absorbance of Eox (at 427 nm) with time, we observed biphasic kinetic behavior. The experimental data could be fitted to a scheme using two consecutive first-order processes, and rate constants could be determined for the two phases. A reasonable interpretation of our results is that a labile intermediate is formed in the reaction, as illustrated in Equation 4, where ES represents the intermediate. In this interpretation, the second, substrate-independent, slower phase of the reaction corresponds to the breakdown of the intermediate ES (fc3 step in Equation 4). The initial, faster phase of the reaction corresponds to the formation of the intermediate from Eox and N-benzyl-l,4-dihydronicotinamide. Using Equation 5, the calculated rate constants for this phase, fcf, can be related to Ks and k2- Values of kCat and Km were calculated from the relationships of Equations 6 and 7, using the measured values of the kinetic parameters k2, 3, and Ks, and the numbers obtained were in reasonable agreement with the values obtained aerobically. 

From Equation (5.139), called the Michaelis-Menten equation, one sees that the rate of formation of P is proportional to the concentration of A at low concentrations (i.e., first-order) and independent of the concentration of A at high concentrations (i.e., zero-order). The general biphasic kinetic profile is shown in Figure 5.21. Usually, experimental runs are carried out in such a way that the concentration of A is very high compared to that of [E]0. All of the enzyme is bound to the substrate and the reaction takes place at a maximum velocity. Then Equation (5.139) becomes ... 

SA in Table II - nmoles/hr/mg protein and N.D - not detectable. Tobacco, soy cultures and barley seedlings were the best source of ALS, both in terms of specific activity and total units. The enzyme preparations from all sources were unstable in buffer solutions in spite of protective thiol agents. The inactivation of ALS in the crude extract of tobacco showed a distinct biphasic kinetics, implying the presence of at least two isozymes (unpublished observations). The presence of two ALS genes in tobacco (29) and at least three in microorganisms (18) has also been noted by other workers. ALS from barley was most amenable to purification. Table III gives a profile for the rapid purification of this enzyme with high recovery. 

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