Tracers oxygen-18-labeled

Oxygen occurs in Nature in three isotopic species 160 (99.759%), nO (0.0374%), and 180 (0.2039%). The rare isotopes, particularly lsO, can be concentrated by fractional distillation of water, and concentrates containing up to 99 at. % lsO or up to 90 at. % nO as well as other labeled compounds are commercially available. Oxygen-18 has been widely used as a tracer in studying reaction mechanisms of oxygen compounds. Oxygen-17 has a nuclear spin 5/2, but because of the low abundance of this isotope and appreciable quadrupole moment, enriched materials and Fourier transform nmr techniques are required. 

Fractional distillation of NO provides another effective route and, as the heavier isotope of oxygen is simultaneously enriched, the product has a high concentration of N 0. Many key nitrogen compounds are now commercially available with N enriched to 5%, 30% or 95%, e.g. N2, no, NO2, NH3, HNO3 and several ammonium salts and nitrates. Fortunately the use of these compounds in tracer experiments is simplified by the absence of exchange with atmospheric N2 under normal conditions, in marked contrast with labelled H, C and O compounds where contract with atmospheric moisture and CO2 must be avoided. 

Tracer studies using show that in these redox reactions H2( 0)2 is converted to ( 0)2, conhrming that no oxygen from the solvent (which is not labelled) is incorporated and the 0—0 bond is not broken. 

The isotopic oxygen exchange rates between these ions and H2O can be represented by a first-order process provided only tracer quantities of labeled reactants are introduced into the chemically equilibrated solutions. Applied to Equations (81) and (82) this rate law results in Equations (83) and (84) as follows... 

During labeling and storage, different factors are responsible for the degradation of radioiodinated tracers, among which are radiolysis and the presence of trace impurities. Iodine loss can occur due to oxygen, light, heat, pH, or solvents. 

Compounds labeled with 20-min half-lived carbon-11 were first intro- duced into biomedical research over 40 years ago (1-6). Before the discovery of carbon-14, compounds such as propionic acid (3) and lactic acid (2) labeled with carbon-11 were synthesized and utilized in biochemical research. After the introduction of carbon-14 as a biomedical tracer, interest in the shorter-lived carbon-11 decreased, and only in the past decade has there been renewed interest in labeling compounds with carbon-11 as well as with other short-lived positron-emitters such as oxygen-15, nitrogen-13, and fluorine-18. 

Covalent protein binding studies require the use of radiolabeled compounds. H-tracers are often used because they are relatively easy to synthesize and are of high specific radioactivity. However, possibilities exist that the H-label may be lost if oxygenation of the test compound occurs directly on the carbons where the H-label is attached. As a consequence, observed binding values could be underestimated. tracers, on the contrary, are less likely to lose the label, and therefore are preferred. However, possibilities of losing a radiolabel should be considered for any given radiolabeled tracer. Therefore, evaluation of the loss of radiolabels needs to be performed before covalent... 

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