Isotope effects measurement

T. Deuterium isotope effects on 13 C chemical shifts In the studies of proton transfer equilibrium of Schiff bases, the most informative are deuterium isotope effects measured for carbons bonded with proton donor groups (C-2 or C-7 for gossypol derivatives). For imines in which... 

TABLE 16. The hydrogen-deuterium kinetic isotope effects measured for the oxidation of mandelic acid" by Pb(OAc)4 in benzene and in benzene-pyridine... 

Ottoboni S, Caldera P, Trevor A, et al. Deuterium isotope effect measurements on the interactions of the neurotoxin l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine with monoamine oxidase B. J Biol Chem 1989 264(23) 13684-13688. 

Reduction of aldonolactones and their derivatives with isotopically modified reducing agents leads to sugars labeled at the anomeric center. Glycosides substituted with deuterium or labeled with tritium are widely employed for kinetic isotope-effect measurements, mechanistic studies, isotope-tracing experiments, and so on. 

Thus the counting rate should be sufficiently high to ensure a precision sufficient for isotope effect measurements a target precision of 0.1% is obtained for total counts of 106. Radiolabeled material used for isotope effect measurements usually has counting rates of 15,000 counts per minute (cpm). To achieve 0.1% precision such samples must be counted for 8 h or more. More often counting time is limited to several cycles of about 10 min each with a commensurate increase in the statistical error. 

It is remarkable that the isotope effects measured for these reactions in the vapor phase are about the same as the solvolysis KIEs measured in aqueous solution. 

Isotope effects on both the carbon and hydrogen of the breaking C-H bond have been measured. However, for this reaction both forward and reverse commitments are sizable so the three equations corresponding to Equation 11.48 have four unknowns the forward and reverse commitments and two intrinsic isotope effects. Measurements of the secondary deuterium kinetic isotope effect (at position 4 of nicotinamide ring of NADP+) and the carbon kinetic isotope effect with the secondary position deuterated introduce two additional equations, but only one more unknown ... 

Secondary isotope effects measure transition-state structure 37 Quantum tunneling in enzyme-catalyzed reactions breakthroughs 42 Experimental phenomenology of quantum tunneling in enzyme-catalyzed... 

Isotope effect measurements have been conducted with labelling at the 6- and 3-positions as well as C-2. These results appear to be no more conclusive than those briefly discussed Scheppele s conclusion concerning y effects at Cg in solvolysis of exo-2-norbomyl brosylate being that they are (a) probably inconsistent with classical theory, but (b) not clearly consistent with non-classical theory. Although not resolving the non-classical issue, the studies indicate many difficulties which obscure the interpretation of any rate data. 

A potential factor affecting the catalytic power of enzyme-catalyzed reactions in which the enzyme forces the attacking moiety into the valence shell of the entity under attack. Compression is thought to be manifested in the transition state complex. The results of kinetic isotope effect measurements have suggested that compression events factor in the catechol methyltransferase re-actionh... 

Table 10.2. These results can be interpreted either as the kinetic effects measured for a single-step proton transfer or as the inverse thermodynamic isotope effects in fast preequilibria [5]. Unfortunately, in contrast to the isotope effects measured for classical hydrogen bonds [24], the effects of denterinm on the thermodynamics of dihydrogen bonding are still nnknown.

Reports in the literature that isolate any of these processes are rare and often require unusual conditions. For example, in addition to the oxazoI-5(4//)-one studies described above, Kemp and Rebek[27l were able to use kinetic isotope effects to distinguish the enolization mechanism from oxazol-5(4//)-one formation in a simple peptide coupling experiment. a-2H-Labeled Bz-L-Leu-OH and Z-Gly-Phe-OH were prepared and coupling reactions to H-Gly-OEt were carried out. In cases where oxazol-5(4//)-one formation is rate-determining, such as with Bz- L-Leu-OH, the isotope effect kHlkD is equal to 1 because the a-proton is not removed until after this rate-determining step. In contrast, enolization requires the direct removal of the a-proton, and the isotope effect measured for this mechanism was as high as 2.9 with Z-Gly-Phe-OH. Therefore, a measurement of the isotope... 

In 1981 Martin and Martin [47] showed that the distribution in organic molecules does not follow a statistical pattern, but it is discriminated by isotopic effects, measurable by NMR and IRMS, respectively. Meanwhile, the system-atics of patterns in natural plant products are being better and better... 

Kinetic isotope effect measurements support this interpretation. A small kinetic isotope effect (kR/kD = 0.71) is observed for p-methoxyphenol in agreement with a ratedetermining nucleophilic attack, while a large kinetic isotope effect (kR/kR = 5.27), observed for p-trifluoromethylphenol, strongly supports a mechanism in which a phenolic H (or D) is transferred to 39 in the rate-determining step. Unfortunately, 39 is a symmetric disilene so that diastereoselectivity could not be determined. It will be interesting to examine whether the diastereoselectivity will be effected by the change in the addition mechanism from electrophilic to nucleophilic. 

Al)-Acetyl-(A6)-aroyl-2,5-dithio-3,4,7,8-tetramethylglycolurils (92) and related compounds undergo a t-butoxide-catalysed Claisen-type condensation.268 Rate constants have been measured for the template-directed process, Hammett plots have been constructed, and deuterium isotope effects measured at the acetyl group. The... 

FIGURE 9.1 Schematic of an apparatus used to determine competitive isotope effects. The reaction chamber (1) is closed off from the atmosphere in the configuration used for kinetic isotope effect measurements and left open in the configuration used for equilibrium isotope effect measurements. 

The organic compounds formed (e.g. thiolanes) may be unstable to pyrolysis and will release the isotopically heavy H20 which will mix with H2S released from the kerogen. The total sulfur isotope effects measure were very small, resulting in a total measured ... 

HO radicals [26]. These results were also consistent with intramolecular kinetic isotope effect measurements on typical Gif oxygenations of l,3-d2-adamantane to... 

Rationalizations of hydroboration stereoselectivity using models with dimeric boranes are thus not viable. Recent secondary isotope-effect measurements may suggest a model (Mann et al., 1986). Addition of a chiral dialkyl borane to 3-deuterio-cw-3-pentene, followed by alkaline oxidation, yields equal amounts of 3- and 4-deuterio-3-hexanols showing that the secondary isotope effects at these alkene sites are vanishingly small. Deuterium substitution at the allylic sites has a much larger effect. Thus similar treatment of 4,4-dideuterio-cw-5-decene yields a 2.86 1 mixture of 4,4-dideuterio- and 6,6-dideuterio-5-decanols. 

Kinetic Isotope Effects Measured at Natural Abundance 153... 

Theory and practice of kinetic isotope effect measurements 153... 

KINETIC ISOTOPE EFFECTS MEASURED AT NATURAL ABUNDANCE... 

SECONDARY ISOTOPE EFFECTS MEASURE TRANSITION-STATE STRUCTURE... 

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