Isotopic labels oxygen

A plot of the potential-dependence of the Raman intensities of a Ru binuclear p-oxo water oxidation catalyst showed evidence for one-, two-, and four-electron oxidized species. Oxygen isotopic labeling was used to characterize the catalytically active fully oxidized state.91... 

The tracing of the oxygen species is usually performed by oxidation first in 16C>2 and then 18C>2 enriched gas in the gas/solid interface. Thin ( 20 A) oxide film growths on single crystal metals at moderate temperatures can be studied by oxygen isotope labeling combined with high-resolution microscopy. In the case of liquid/solid and electrochemical interfaces there is another complication that arises from the existence of an electric field and the presence of an electrolyte solution with adsorbable anions and solvents. 

To further study the reaction between methane and the surface oxygen, we used oxygen isotope labeling to preferentially populate the surface state with Determining the isotope label of oxygen in carbon dioxide provides us with information on the reactions of methane with the lattice and the surface oxygen. The results at 973 K are shown in Fig. 7. 

By using oxygen isotope labels, the barium hydroxide catalyzed hydrolysis of the nucleotide cyclophosphate (40) to yield a 4 1 mixture of 3 - and 5 -monophosphates was shown by P-n.m.r. to occur with inversion at phosphorus. 

Important evidence in support of this mechanism was provided by carrying out hydrolysis with oxygen-isotope-labeled water (H2 0). The isotope test showed that the alkoxysilane cleavage is carried out by breaking the Si-OC bond rather than by cleavage of the SiO-C bond, resulting in the formation of the heavy silanol =Si OH and not the light =Si OH. 

The stereochemical relationship between the reactant and the product revealed by the isotopic labeling shows that oxygen becomes bonded to carbon on the same side from which H IS lost As you will see m this and the chapters to come determining the three dimensional aspects of a chemical or biochemical transformation can be a subtle yet powerful tool for increasing our understanding of how these reactions occur... 

In an extension of the work described m the preceding section Bender showed that basic ester hydrolysis was not concerted and like acid hydrolysis took place by way of a tetrahedral intermediate The nature of the experiment was the same and the results were similar to those observed m the acid catalyzed reaction Ethyl benzoate enriched m 0 at the carbonyl oxygen was subjected to hydrolysis m base and samples were isolated before saponification was complete The recovered ethyl benzoate was found to have lost a por tion of Its isotopic label consistent with the formation of a tetrahedral intermediate... 

Isotopic labeling experiments have established that C—O bond rupture occurs between the carbonyl carbon and oxygen substitution at the alcohol C—O bond is not involved. 

In a similar experiment, unlabeled 4-butanolide was allowed to stand in an acidic solution in which the water had been labeled with 0. When the lactone was extracted from the solution after four days, it was found to contain 0. Which oxygen of the lactone do you think became isotopically labeled ... 

The oxygen in water is primarily (99.8%) l60, but water enriched with the heavy isotope, 80 is also available. When an aldehyde or ketone is dissolved in 180-enriched water, the isotopic label becomes incorporated into the carbonyl group. Explain. 

The mechanism shown in Figure 21.7 is supported by isotope-labeling studies. When ethyl propanoate labeled with lsO in the ether-like oxygen is hydrolyzed in aqueous NaOH, the l80 label shows up exclusively in the ethanol product. None of the label remains with the propanoic acid, indicating that saponification occurs by cleavage of the C-OR bond rather than the CO—R bond. 

When a carboxylic acid is dissolved in isotopically labeled water, the label rapidly becomes incorporated into both oxygen atoms of the carboxylic acid. Explain. 

A 1,2-shift has been observed in radicals bearing an OCOR group at the p-carbon where the oxygen group migrates as shown in the interconversion of 36 and 37. This has been proven by isotopic labeling experiments and other mechanistic explorations. A similar rearrangement was observed with phosphatoxy alkyl radicals such as 38. ... 

After approximately 3 min on stream in the isotopic mixture of CH4/02/C 02/He, a steady-state value in the rate of formation is obtained (Fig. 3). This value is used to estimate the relative contribution of the CO2-reforming route to the overall production of CO (direct + indirect routes). Proper analysis of this result, taking into consideration scrambling of isotopic oxygen Figure 3. Transient isotopic labelling experiment atoms between the CO2 and O2 molecules... 

Characterization of various types of damage to DNA by oxygen-derived species can be achieved by the technique of gas chromatography-mass spectrometry (GC-MS), which may be applied to DNA itself or to DNA-protein complexes such as chromatin (Dizdaroglu, 1991). For GC-MS, the DNA or chromatin is hydrolysed (usually by heating with formic acid) and the products are converted to volatile derivatives, which are separated by gas chromatography and conclusively identified by the structural evidence provided by a mass spectrometer. Stable isotope-labelled bases may be used as internal standards... 

The oxidation reactions were performed in a 25 mL ronnd bottom flask. In a typical reaction the catalyst (0.5 % Fe mol) was added to 0.125 M olefin solntion in acetone then dry TBHP (3.5 M in CH2CI2 or in PhCl) was added in one step, and the reaction mixture was stirred and heated in an oil bath at 40°C for 7 h. For the allyhc oxidation of cyclohexene with isotopically labelled oxygen ( 02) the following procedure was carried out the suspension of the catalyst (0.5% Fe mol) in cyclohexene (4 mL, 0.125 M) was frozen and the air in the reactor was evacuated and replaced by an oxygen (21% mol) - argon (79% mol) mixture. Then, the suspension was allowed to warm at room temperatnre and 1.3 mmol of degasified TBHP was added to the solution and the reaction mixtnre was stirred at 40°C for 3 h. 

When a sample of the same Kevlar-29 fabric was identically treated in l 02-atmosphere except in the absence of photolysis (in the dark) for 24 hours, the results of C02 evolved at 25° and 196°C from the GC-analysis were the same as the samples under photolysis (e.g., Table I). However, the gross difference was in the analyses of the GC-mass spectroscopic data, where no oxygen-18-labelled C02 isotopes evolved (i.e., there was no C02 or 48co2 above the natural background levels). ... 

A kinetic isotope effect 160/180 of 2% in the spontaneous hydrolysis of the 2,4-dinitrophenyl phosphate dianion, whose ester oxygen is labeled, suggests a P/O bond cleavage in the transition state of the reaction, and thus also constitutes compelling evidence for formation of the metaphosphate 66,67). The hydrolysis behavior of some phosphoro-thioates (110) is entirely analogous 68). 

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