Isotopic experiments

The mechanism of the oxidation reaction, resulting from treatment of an alcohol with dicyclohexylcarbodiimide and methyl sulfoxide in the presence of a proton source, was elucidated by isotope experiments (24). These confirmed that the reaction proceeded by formation of a... 

Two separate publications (125, 126) described the synthesis of a number of carbonyl complexes of vanadium. The mononuclear species V(CO) , n = 1-6, have all been identified by using CO matrix-dilution experiments and mixed CO- CO isotope experiments while main-... 

In a preliminary report (2), Fe atoms were reacted with O2, leading to formation of FeiO ), a cyclic isosceles (C2v) species, as suggested by mixed isotope experiments. Reaction of Fe atoms with N2O resulted in formation of FeO. A feature at 887 cm, assigned to a Fe/Nj complex, is probably erroneous, and may be an iron nitride species. In the same triad, the MCD spectrum of matrix-entrapped OSO4 was studied (46). The spectrum was found to be similar to that of Mn04 in a solid lattice, and was assigned accordingly. 

There are three basic types of mechanistic information that are derived from isotopic experiments. 

Finally, we note a very clever isotope experiment done by Tatsuo Matsushima (2 4). The results are summarized in Figure 14 and show that when 1JC0 is preadsorbed on Pt and then a mixture of - - C0 and O2 is introduced into the gas phase, the product CO2 molecules initially formed all are labelled with - C. In fact the experiment is not quite so simple. An initial 002/ C02 ratio is measured (unity in... 

A suggested reaction mechanism based on kinetic and isotope experiments, product distributions, and the effect on these of addition of water to the feed or leaving oxygen out is shown in Figure 13. 

On the other hand, the involvement of vinyl cationic species in the reaction cannot be ruled out in some cases, as shown in Scheme 4. In this context, it was found that the reaction of 3-butyn-2-one with mesitylene can occur without Pd(OAc)2, clearly indicating the involvement of vinyl cations generated from alkynes and H+ in this reaction.47 1 The yield difference in the presence and in the absence of Pd(OAc)2 may be explained by the competition between [Pd(n)02CF3]+ and vinyl cationic species in the electrophilic substitution of aromatic G-H bonds. Recent kinetic isotope experiments suggest a mechanism involving alkyne coordination to Pd(n) followed by electrophilic aromatic substitution.476... 

When o-, m- and p-nitroanisole with 14C-labelled at the methoxy group were irradiated under identical conditions in methanol in the presence of sodium methoxide, only m-nitroanisole underwent methoxy exchange, with the limiting quantum yield (

labelled isotope experiments support a a complex intermediate and indicate an Sjv23Ar mechanism (direct substitution in the triplet state) for this reaction (equation 12) and for 4-nitroveratroles (equation 13). Further evidence from quenching and lifetime experiments also support a direct displacement SAr2Ar mechanism for the photosubstitution reaction of nitroaryl ethers with hydroxide ions13. 

This interpretation offered by Ogston was confirmed by further isotope experiments (Potter and Heidelberger, 1949 Martius and Schorre, 1950). Ochoa and his colleagues also demonstrated that in pigeon-liver preparations which had been freed from aconitase, thus preventing isomerization between citrate and isocitrate, citrate was the product of the condensation of oxaloacetate and acetate. 

Sophisticated isotope experiments were also performed using H2180 (Mildred Cohn) and 32P, and various exchange reactions identified between ATP, ADP, and Pr Analysis of the mode of action of two inhibitors was also relevant. Dinitrophenol (DNP) uncoupled the association between oxidation and ATP generation (Lardy and Elvejhem, 1945 Loomis and Lipmann, 1948). Oligomycin inhibited reaction (ii) above, blocking the terminal phosphorylation to give ATP, but not apparently the formation of A C. 

The biological importance of transamination was confirmed using 15N-labeling experiments (Tannenbaum and Shemin, 1950). 15N-leucine incubated with pig heart muscle gave highly labelled 15N-glutamate, evidence that leucine could be transaminated. Isotope experiments were then extended to the whole range of amino acids. 

Isotopic experiments on the cell cycle could also be performed with ordinary (non-autoradiographic) biochemical procedures if synchronized cells were available. The earliest animal preparation providing such a system for use in vivo was the regenerating rat liver. Stowell (1949) and Abercrombie and Harkness (1951) noted that DNA synthesis in livers... 

In a collaboration between the Abelson and Hecht labs [56b], a series of noncoded amino acids were introduced into dihydrofolate reductase (DHFR) to probe substrate binding and the requirement of an aspartic acid residue for catalytic competence. When aspartic acid analogs mono- or disubstituted at the )0-carbon were substituted for the active site aspartic acid residue, the mutant DHFRs were still able to catalyze the NADPH-dependent reduction of dihydrofolate to tetrahydrofolate at 74 - 86 % of the wild-type rate. While hydride transfer from NADPH is not the rate-limiting step for the wild-type enzyme at physiological pH, a kinetic isotope experiment with NADPD indicated that hydride transfer had likely become the rate-limiting step for the mutant containing the )0,)0-dimethylaspartic acid. 

Figure 1. 3-0-4 lignin substructure model compounds and products of their degradation by ligninolytic cultures of Phanerochaete chrysosporium and by lignin peroxidase. D H, and 0 of 3-ether bond and of H2 0, respectively. B and C show results of stable isotope experiments. 

Stable isotope experiments similar to those with the intact cultures shown in Figure IB and 1C were conducted with lignin peroxidase, and the same results as those obtained with the intact cultures were reproduced with the enzyme system (i6f42,45,46). 

An instrument that measures the isotopic mass ratio of a gas by bombarding the sample in an electron beam, such that the molecular ions generated can be deflected in their trajectories through a magnetic field in accordance to their charge/mass ratios. These devices are extremely accurate and reliable, and many stable isotope experiments can be analyzed by converting the isotopi-caUy substituted metabolite into carbon dioxide, water, or molecular nitrogen prior to I RMS measurements. 

Let us consider the classical secondary kinetic isotope experiments on lysozyme-catalyzed hydrolysis of glycosides. Earlier X-ray work indicated that the enzyme... 

Modifying the selectivity for a particular product is a more challenging task. To understand why Ag is the most selective catalyst for ethylene epoxidation, an highly important reaction practiced industrially for decades, Linic et al. performed detailed spectroscopic and kinetic isotope experiments and DFT calculations, and they concluded that the selectivity between the partial and total oxidation of ethylene on Ag(l 11) is controlled by the relative stability of two different transition states (TS s) that are both accessible to a common oxametallacycle intermediate One results in the closure of the epoxide ring and ethylene oxide (EO), while the other leads to acetaldehyde (AC) via intra-molecular H shift and eventually combustion. The authors... 

Vanderpool, R.A., D. Hoff, and PE. Johnson. 1994. Use of inductively coupled plasma-mass spectrometry in boron-10 stable isotope experiments with plants, rats, and animals. Environ. Health Perspec. 102(Suppl. 7) 13-20. 

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