Isotope exchange studies

In our laboratory we have been investigating the mechanism of action of mandelic acid racemase from Pseudomonas putida (101), which catalyzes the racemization of either D or L-mandelic acid, 47. Evidence from kinetic and isotopic exchange studies indicates that the racemization proceeds via an... 

The acid-catalysed hydrolysis of the acylal, 1-phenoxyethyl propionate (13), has been studied using the PM3 method in the gas phase. The kinetics and mechanism of the hydrolysis of tetrahydro-2-furyl and tetrahydropyran-2-yl alkanoates (14) in water and water-20% ethanol have been reported. In acidic and neutral media, kinetics, activation parameters, isotope-exchange studies, substituent effects, solvent effects and the lack of buffer catalysis pointed clearly to an Aai-1 mechanism with formation of the tetrahydro-2-furyl or tetrahydropyran-2-yl carbocation as the rate-limiting step (Scheme 1). There is no evidence of a base-promoted Bac2 mechanism up to pH 12. ... 

The aqueous chemistry of chromium(in) above 100 °C has been investigated with particular reference to corrosion phenomena and the possibility of hydro-thermal synthesis of chromite in serpentine rocks. Isotopic exchange studies have indicated that the CrO unit exchanges intact between [Cr(H20)6] and [CrMo6024H6] . This appears to be the first established exchange of such a unit. ° [CrlHjOljlEtOH)] has a stability constant of 6.5 x 10" and is only stable in perchlorate solutions with >80% EtOH. ... 

Selected entries from Methods in Enzymology [vol, page(s)] Equilibrium isotope exchange study of kinetic mechanism, 249, 466 site-directed mutagenesis of Escherichia coli enzyme, 249, 93 positional isotope exchange studies, 249, 423 product inhibition studies of three substrates three products reactions, 249, 207-208. 

Control of enzymic activity arising from the modulated access of substrates to a channel leading to the active site. Such a scheme was suggested for aspartate carbamo-yltransferase which has its complement of active sites located on the interior surface of the complex comprised of catalytic and regulatory subunits. Nonetheless, isotope exchange studies of this enzyme suggest that this form of enzyme regulation does not apply in the case of aspartate transcarbamoylase . ... 

In ping pong reactions, Haldane relations can also be written for the individual half-reactions. In such cases, Haldane expressions assist in analyzing isotope exchange studies involving these partial reactions. 

Ye, R. W., Toro-Suarez, 1., Tiedje, J. M., and Averill, B. A. (1991). H2 0 isotope exchange studies on the mechanism of reduction of nitric oxide and nitrite to nitrous oxide by denitrifying bacteria. J. Biol. Chem. 266, 12848-12851. 

Isotope exchange studies with perdeuterated Cg-Cis alkanes led to the surprising observation that H/D exchange did not occur in alkanes under Fischer-Tropsch synthesis conditions (Fe catalyst, 270°C, 11.9 atm).510 In contrast, under the... 

The hydrogenation of benzene and its alkyl-substituted derivatives takes place stepwise (Scheme 11.5). On the basis of instrumental and isotope exchange studies, the involvement of tt-adsorbed (8, 9) and cr-adsorbed (10) species was suggested 96-98... 

How is ATP made No covalent intermediates have been identified, and isotopic exchange studies indicate a direct dehydration of ADP and P to form bound ATP.245 For the nucleophilic terminal phospho group of ADP to generate a high-energy linkage directly by attack on the phosphorus atom of P an OH ion must be eliminated (Eq. 18-13). This is not a probable reaction at pH 7, but it would be reasonable at low pH. Thus, one function of the oriented ATP... 

T. H. Kowalczyk, P. J. Horn, W. H. Pan and N. F. B. Phillips (1996). Initial rate and equilibrium isotope exchange studies on the ATP-dependent activity of polyphosphate glucokinase from... 

Hydrogen-deuterium exchange experiments have been used extensively to study both structural parameters and dynamic and mechanistic aspects. They are utilized in conjunction with both NMR and IR studies. Mass spectrometry, with its high sensitivity and resolution, is ideally suited for isotopic exchange studies. 

The mechanisms of these reactions are discussed by Palmer and Harris, and after allowing for the inevitable uncertainty caused by ion pairing, they conclude that the evidence tends to favor a clear dissociative (five-coordinate intermediate) mechanism .1202 Additional support for a dissociative mechanism in the substitutions of aquachlororhodium(III) complexes comes from Cl- isotopic exchange studies of [RhCls]3- and [RhCl5(H20)]2, which suggested that the reactions proceed via... 

Both antipodes racemize rapidly in solution, although less rapidly than the corresponding trioxalatochromates racemization is complete within 24 hours. Racemization is reported to occur even in the solid state. Isotopic exchange studies have shown only limited parallelism between the mechanisms of oxalate exchange and racemization. Optical rotatory dispersion measurements - have been used to determine the absolute configuration of the trioxalatocobaltate(III) ion. 

Heats of adsorption were scarce, limiting any thermodynamic analysis of catalytic reactions, while the significance of surface structure had not been addressed. Where heats of adsorption were available there was a conflict between data obtained by those advocating the clean surface (Beeck) and technical catalyst (Eucken) approaches. Taylor drew attention to Muller s (then recent) field emission microscope and the potential of deuterium in isotopic exchange studies an aspect that Kemball, who had worked in Princeton used to much advantage in unravelling the mechanism of hydrocarbon... 

Hexokinase does not yield parallel reciprocal plots, so the Ping Pong mechanism can be discarded. However, initial velocity studies alone will noi discriminate between the rapid equilibrium random and steady-state ordered mechanisms. Both yield ihe same velocity equation and families of intersecting reciprocal plots. Other diagnostic procedures must be used (e.g., product inhibition, dead-end inhibition, equilibrium substrate binding, and isotope exchange studies). These procedures are described in detail in the author s Enzyme Kinetics behavior and Analysis of Rapid Equilibrium and Steady-State Enzyme Systems, Wiley-Interscience (1975),... 

E.Kh. Kurumchin and M.V. Perfiliev, An isotope exchange study of the behaviour of... 

---