Labeled atoms reactions isotope transfer

Atomic force microscopy, polysilanes, 3, 599 Atomic layer epitaxy, for semiconductor growth, 12, 11 Atom transfer radical polymerization type reactions, isotope labeling studies, 1, 567... 

Oxiranes (or epoxides) are conveniently formed by delivery of electrophilic oxygen to one of the faces of an alkene. This is achieved with a peroxy acid (peracid) in a single-step reaction (Scheme 4.2) that necessarily delivers oxygen syn to the double bond. Isotope labelling experiments have shown that the oxygen atom that is transferred to the alkene is the one that is further from the carbonyl group of the peroxy acid. 

The general principles of catalytic reactions can be applied for the description of the kinetics of the transfer of labelled atoms by reaction. Usually identification of molecules by labelling is done by substitution of an atom with an isotope. It is assumed that the content of labelled atoms is determined only in the participants of the reaction, i.e. in the reactants and the products - but not in the intermediates and that only the total content of isotopes in equivalent atoms of a given substance is determined. The significance of the last restriction is that no distinction is made between H2+D2 and 2HD. It is also assumed that a complex reaction proceeds in a stationary manner. 

For the sake of clarity we will consider a one-stage reversible reaction between the molecules A and A with the formation of molecules B and B It is supposed that the atoms are substituted by their isotopes. The rate of transfer of an isotope is defined as the number of labelled atoms transfered from the molecules of one species to the molecules of another species per unit time per unit reaction space. The fraction of labelled atoms in the molecules A and B will be denoted by C,a and ,b, the number of equivalent atoms that could be labelled and are transfered from the substance A to the substance B in one elementary act of the reaction is denoted by p. Thus the rate of transfer of the label by the forward reaction is... 

Here, 0/ and a are respectively the steady-state concentrations of surface species and the relative label concentrations (isotope fractions) in them rf is the rate of a chemical reaction step y is the number of label atoms transferring from one substance to another in elementary reaction step (an analog of stoichiometric coefficient), P is a dimensionless coefficient equal to LW/VNa, where V is the gas-phase volume (crn ), L is the number of active sites per gram of catalyst (mol/g), W is the catalyst weight (g), Na is the number of gas molecules per unit volume (mol/cm ), and 0 Cf(x.j) is the operator depending on the mass transfer regime in the reactor ... 

It may be concluded that for reactions where the proton is less or more than one-half transferred in the transition state, i.e. the A—H and H—B force constants are unequal, the primary hydrogen-deuterium kinetic isotope effect will be less than the maximum of seven. The maximum isotope effect will be observed only when the proton is exactly half-way between A and B in the transition state. This relationship is also found for carbon kinetic isotope effects where the isotopically labelled carbon is transferred between two atoms in the reaction10,11. This makes interpreting carbon isotope effects difficult. 

The precise nature of the reaction path is ambiguous, because the same products would be obtained regardless of whether an oxygen atom or a chlorine atom were transferred. As they pointed out in their paper, this ambiguity can be resolved by using nitryl chloride with isotopically labeled nitrogen. Pertinent to this question are the calculations of Herschbach, Johnston, Pitzer, and Powell,199 who computed pre-exponential factors based on two alternative activated complexes... 

Jhe development of chemistry in the 20th century has been dominated and motivated by the electronic theory of the chemical bond and the role of electrons in chemical reactivity. The electronic structure of the chemical bond could be deduced by more or less direct methods, such as electronic excitation spectra, dipole moments, or paramagnetism but there was no direct indication for the transfer of electrons in chemical reactions. Using isotopic techniques it has been possible to demonstrate bond cleavage and atom transfer reactions, but it is impossible to label an electron and trace its transfer from one molecule to another. It was not until the discovery of the radiolytically produced solvated electron that electron transfer processes could be examined directly and unambiguously. 

When the hydrogen transferred as hydride to the cofactor is retransferred to the same carbon atom in the product, the movement is far more difficult to detect. The conversion of D-glucose 6-phosphate (58) into lL-mt/o-inositol 1-phosphate (61) occurs by cyclization of the carbon skeleton, with formation of a new bond between C-l and C-6. When each carbon atom in turn was specifically labeled with tritium, there was complete retention of tritium, even in the presence of added NADH, although there was an apparent, small isotope-effect with D-glucose-5-t 6-phosphate.19 The mechanism proposed for the cyclization19 was an initial oxidation at C-5 to give NADH and xylo-hexos-5-ulose 6-phosphate (59), followed by an aldol reaction causing cyclization to lL-myo-inosose-2 1-phosphate (60), which is then... 

An ingenious method for detecting the transfer of the terminal phosphate of ATP in ATP-coupled enzyme reactions has been devised, If an enzyme becomes reversibly phosphorylated, and the /9-phosphate of the bound ADP is free to rotate, an isotopic label in the terminal oxygen bridge will become scrambled between three oxygen atoms (Scheme 2). In order to demonstrate the formation of a complex... 

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