Kinetic isotope effects product dependence

The time-resolved, chemical behavior of FL depends on the solvent. Irradiation of DAF in cyclohexane gives FLH The lifetime of FL in cyclohexane is 1.4 ns, and the ratio of products obtained (26) indicates that both direct insertion and abstraction-recombination mechanisms are operating (Griller et al., 1984b). Replacement of the cyclohexane by its deuteriated counterpart reveals a kinetic isotope effect of ca 2 (Table 5). 

Using the various simplifications above, we have arrived at a model for reaction 11.9 in which only one step, the chemical conversion occurring at the active site of the enzyme characterized by the rate constant k3, exhibits the kinetic isotope effect Hk3. From Equations 11.29 and 11.30, however, it is apparent that the observed isotope effects, HV and H(V/K), are not directly equal to this kinetic isotope effect, Hk3, which is called the intrinsic kinetic isotope effect. The complexity of the reaction may cause part or all of Hk3 to be masked by an amount depending on the ratios k3/ks and k3/k2. The first ratio, k3/k3, compares the intrinsic rate to the rate of product dissociation, and is called the ratio of catalysis, r(=k3/ks). The second, k3/k2, compares the intrinsic rate to the rate of the substrate dissociation and is called forward commitment to catalysis, Cf(=k3/k2), or in short, commitment. The term partitioning factor is sometimes used in the literature for this ratio of rate constants. 

When isotopes are fractionated kinetically during chemical reactions, the isotope ratio shift of the reaction products relative to the reactants often depends on reaction mechanisms and rates. This contrasts with isotopic fractionations between phases in isotopic equilibrium, where the isotopic differences are thermodynamic quantities and thus do not depend on reaction mechanisms or rates. In this section, we briefly review the well-developed theory for kinetic isotope effects that appears in the S isotope literature. This background serves as a guide for interpreting and predicting Se and Cr isotope systematics. 

The pyridinium chlorochromate (PCC) oxidations of pentaamine cobalt(III)-bound and unbound mandelic and lactic acids have been studied and found to proceed at similar rates.Free-energy relationships in the oxidation of aromatic anils by PCC have been studied. Solvent effects in the oxidation of methionine by PCC and pyridinium bromochromate (PBC) have been investigated the reaction leads to the formation of the corresponding sulfoxide and mechanisms have been proposed. The major product of the acid-catalysed oxidation of a range of diols by PBC is the hydroxyaldehyde. The reaction is first order with respect to the diol and exhibits a substantial primary kinetic isotope effect. Proposed acid-dependent and acid-independent mechanisms involve the rapid formation of a chromate ester in a pre-equilibrium step, followed by rate-determining hydride ion transfer via a cyclic intermediate. PBC oxidation of thio acids has been studied. ... 

Deng and co-workers have also applied the cinchona derivatives to the kinetic resolution of protected a-amino acid N-carboxyanhydrides 51 [48]. A variety of alkyl and aryl-substituted amino acids may be prepared with high se-lectivities (krei=23-170, see Scheme 10). Hydrolysis of the starting material, in the presence of the product and catalyst, followed by extractive workup allows for recovery of ester, carboxylic acid, and catalyst. The catalyst may be recycled with little effect on selectivity (run 1, krei=114 run 2, krei=104). The reaction exhibits first-order dependence on methanol and catalyst and a kinetic isotope effect (A MeOH/ MeOD=l-3). The authors postulate that this is most consistent with a mechanism wherein rate-determining attack of alcohol is facilitated by (DHQD)2AQN acting as a general base. 5-Alkyl 1,3-dioxolanes 52 may also... 

The experimental data for oxidation of benzyl alcohol,1 aliphatic primary and secondary alcohols,2 and cholesterol3 with cetyltrimethylammonium (CTA) dichromate indicated that the reactions occur in a reverse micelle system produced by the oxidant. Michaelis-Menten-type kinetics were observed with respect to the reductants. The product of the oxidation of cholesterol depends on the solvent. In dichloromethane, the product is 7-dehydrocholesterol, whereas with dichloromethane containing acetic acid the product is 5-cholesten-3-one. A low kinetic isotope effect, k /ku = 2.81, was observed in the oxidation of methanol- this, combined with the rate data and the reverse solvent isotope effect [ (H20)/fc(D20) = 0.76], suggests that these reactions... 

An important consequence of the for one substrate measured by its inhibition of the hydrolysis of a second being its is that the ratio of rates of two competing substrates depends only on their kcatIKm ratio, at all substrate concentrations. This is shown in eqn. (5.23). It is particularly germane to the measurement of kinetic isotope effects from the changes in isotopic enrichment in substrate or product such measurements always give the effect on... 

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