Catalytic rate constant determinants

In the case of nitrite nucleophile, however, the favourable situation was met that the role of this anion in aromatic nucleophilic substitutions had been thoroughly investigated and host-guest complexation happened under the experimental conditions of catalytic rate constant determination. 

The rate constant /ct, determined by means of Eq. (6-47) or (6-48), may describe either general base or nucleophilic catalysis. To distinguish between these possibilities requires additional information. For example, in Section 3.3, we described a kinetic model for the N-methylimidazole-catalyzed acetylation of alcohols and experimental designs for the measurement of catalytic rate constants. These are summarized in Scheme XVIIl of Section 3.3, which we present here in slightly different form. 

These are rate constants for the hydrolysis of cinnamic anhydride in bicarbonate-carbonate buffers. The pK of bicarbonate is 10.22. Find the rate constant for hydrolysis, at each pH, at zero buffer concentration. Analyze the data to determine if the acid or base component of the buffer, or both, are responsible for catalysis, and give the catalytic rate constant(s). 

Hersh et al. found that the cationic complex [CpFe(CO)2(THF)]BF4 (23) can accelerate the [4 + 2] cycloaddition of acrolein and cyclopentadiene [32]. However, the catalytic activity was higher than expected from rate constants determined in stoichiometric experiments, indicating that a Brpnsted or Lewis acid impurity might accelerate this process and generating doubts about the role of 23. 

The first set of experiments was conducted in methanol. The substrate concentration was varied from 15 to 50 mM at a 200 pM concentration of 1 for the determination of kinetic parameters for the transformation of 8 into 9. The catalytic rate constant was determined to be 0.04 min and the Michael constant was determined to be 40 mM at 30°C. The rate constant is comparable to those reported for other dinuclear Cu(ll) complexes with a comparable Cu -Cu distance of 3.5 A, but about one magnitude lower than those observed for complexes with a shorter intermetallic distances (12-14), e.g. 2.9 A (kcat = 0.21 min ) (12) or 3.075 A (kcat = 0.32 min (13). The rate constant Aion for the spontaneous (imcatalyzed) oxidation of 8 into 9 was determined to be 6 x 10" min and corresponds to the oxidation without catalyst under otherwise identical conditions. The rate acceleration (Arca/Aion) deduced from these values is 60,000-fold. 

The transformation of 8 into 9 was then monitored in 80% aqueous MeOH for substrate concentrations between 0.05 to 0.4 mM, and 12 pM of apparent concentration of 7. Unbuffered nanopure water was always used, as the addition of base accelerates the uncatalyzed oxidation of 8 by air significantly. The catalytic rate constant koat in 80% aqueous MeOH was determined to be 0.13 min. The Michaelis-Menten constant Km was determined to be 0.07 mM, which refers to a higher affinity of the substrate to the metal complex in aqueous methanol than in pure methanol. The rate constant for the spontaneous reaction k on was determined to be 1 X 10 min in 80% aqueous MeOH. The transformation of 8 into 9 is 140,000-fold accelerated over background under these conditions, and is thus more than twice as fast as accelerated than the reaction in pure methanol. 

The obtained obs values are reported as a fimction of the complex concentration (Fig. 8), and a good linear correlation between obs and the complex concentration was observed for both oxidation forms of iron complexes. From the slope of the plot of obs vs. catalyst concentration the catalytic rate constants ( cat) i29) were determined to be (3.7 0.5) X 10 M- s and (3.9 0.5) x 10 M s for [Fe (dapsox)(-H20)2]C104 and [Fe (H2dapsox)(H20)2](NOs)2, respectively 49). It is important to note that, it does not matter whether we start from the Fe(III) or Fe(II) form of the complex, identical spectral changes (Fig. 6a and 6b) and kinetic behavior (Fig. 8) for these two complexes is observed upon reaction with, which is consistent with the redox cychng of the complex during O2 decomposition (Scheme 9). 

Reaction conditions permitting a catalyst to pass through many catalytic rounds. Multiple-turnover conditions are usually obtained by maintaining the substrate concentration in excess over the concentration of active catalyst. This technique usually allows one the opportunity to evaluate the catalytic rate constant ka,t, which is the first-order decay rate constant for the rate-determining step for each cycle of catalysis, and one can evaluate the magnitude of other parameters such as the substrate s dissociation constant or Michaehs constant. 

Although it is not known whether the base is an essential feature of a substrate, the nucleoside moiety is necessary since bis- and tris-p-nitrophenyl phosphate esters are not hydrolyzed (61). The nature of the R substitution on the 3 -OH clearly affects the affinity of the substrate or inhibitor for the enzyme, even though it does not affect the maximal catalytic rate constant (Table I). The importance of the 5 - and 3 -phosphate groups in determining the affinity of inhibitors (3, 66) is consistent with the contribution of these groups to substrate affinity (61). These effects result from the phosphoryl groups themselves rather than... 

The peak height of the DDPV curves is highly sensitive to the parameter A (= ij (k 1 I k2)/D) value which can be used for the determination of the catalytic rate constants. In the case of spherical and disc electrodes, the expression of the peak height can be written as [73, 75]... 

As the ionic radii decrease, the acidity of the ion increases and hence the affinity for the malonate ligand. It is also to be noted that the catalytic rate constant increases with increasing acidity of the lanthanide ion, which is consistent with the proposed mechanism. If the metal ion acidity is a crucial factor in determining the reaction rate, Y(III) should give a rate similar to Ho(III) since both Y(III) and Ho(III) have nearly identical acidity properties. This is shown to be true by data in Figs 7.31 and 7.33. 

Binuclear zinc hydroxide complexes of the Htdmbpo and Hbdmbbppo ligands (Fig. 68) promote the transesterification of HPNP (Fig. 40, bottom).252 Binding constants and catalytic rate constants were determined for the respective complexes. The Htdmpo-ligated zinc complex exhibits a slighter higher rate constant (1.10 x 10-3s-1) than the Hbdmbbppo-ligated complex (8.33 x 10-4s-1). However,... 

Forman and Fridovich (1973) using an indirect assay whereby O2 was generated either by the action of xanthine oxidase on xanthine or by the mechanical infusion of potassium superoxide in tetrahydrofuran. The generated OJ was allowed to react with ferricytochrome c or with tetra-nitromethane and the product formation was monitored spectroscopically. Details of the two assays are given in Section 11.3. Addition of superoxide dismutase inhibits the formation of products. A rate constant of 2 X 10 M sec was determined for all three enzymes. This value agreed with the rate constant determined by pulse radiolysis for the copper/zinc enzyme (Klug-Roth et al., 1973 Fielden et al., 1974). The mechanism of action of the superoxide dismutases has been investigated by the technique of pulse radiolysis which is described in Section II.2. The bovine erythrocyte copper/zinc enzyme is the most studied form as far as the molecular and catalytic properties are concerned (Rotilio and Fielden,... 

We recognized the need for methodology to measure SOD activity directly that would be more accessible to the bench-top scientist than is the method of pulse radiolysis, another direct measure. Consequently, we developed methodology to measure the catalytic dismutation of superoxide by stopped-flow kinetic analysis.By this technique, we directly monitor the decay of superoxide spectrophotometrically in the presence or absence of a putative SOD mimic at a given pH. Kinetic analysis of this decay can determine whether the complex is a SOD mimic (decay of superoxide becomes first-order in superoxide and first-order in complex see equations 1 and 2), or is inactive (decay of superoxide remains second-order for its self-dismutation see equation 3). At least a tenfold excess of superoxide over the putative SOD mimic is used in the stopped-flow assay, to eliminate contributions due to a stoichiometric reaction of the complex with superoxide. A catalytic rate constant for the dismutation of superoxide by the complex can be determined from the observed rate constants of superoxide decay as a function of catalyst concentration. ... 

Based on die pH dependency of die catalytic rate constants, die reactivity of die polyhydric alcohols was attributed to die anion derived from ionization of a hydroxy group in die polyhydric alcohol. The second-order rate constants representing die nucleophilic reactivity of die polyhydric alcohol anions were determined. Results diowed diat die nucleophilic reactivity of dextrose, sucrose, sorbitol and mannitol is similar to other alcohols of comparable pl<. .."... 

Prom this type of analysis, an imcatalyzed decay of superoxide (second-order kinetics) can be distinguished from a catalyzed decay of superoxide (first-order kinetics) in the presence of a large excess of superoxide over the complex being screened (Eq. (8)). A second-order catalytic rate constant ( cat) be obtained (Eq. (8)) for an agent with true catalytic SOD activity. This direct determination of a true cat can be used to directly... 

In the same study (34), the reaction of MnSOD with NO imder aerobic (and thus more physiological) conditions was also studied, and the second-order rate constant for the reaction of MnSOD (E. coli) with NO under aerobic conditions was determined to be 650M s. It should be mentioned that this rate constant is not the catalytic rate constant for NO removal by MnSOD, since the experimental conditions used in the study were not catalytic. Further, it is possible that this constant is largely underestimated because of the relatively slow response time of the NO electrode used in the study to follow the reaction. However, the authors demonstrate that dismutation indeed takes place and that even reduction of NO, considered to be thermodynamically impossible (vide infra), is possible. More importantly, they propose that this reaction could present a defense mechanism against overproduction of NO and its subsequent toxic effects due to the reaction with superoxide and peroxynitrite formation. These two reactions of MnSOD, one with NO and the other with ONOOH, stimulated further studies on SOD mimics with RNS and showed that none of them in fact posses strict selectivity toward superoxide. 

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