Direct analysis of stable isotopes with a quadrupole mass spectrometer

Caprioli, Richard M.; Fies, W. F.; Story, M. S.

doi:10.1021/ac60340a033

Cited by 20 publications

(9 citation statements)

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“…We were able to determine the 180 content of the benzoylglycine methyl ester derived from each of these substrates with high precision on a quadrupole mass spectrometer adapted so that we could do repeated injections, multiple analyses within a single injection, and computer averaging of the results, in a fashion similar to that reported by Kwart (25) and others (26,27). With care, we find that we can determine the 180 content of such samples with 0.2% reproducibility.…”

mentioning

confidence: 57%

Evidence for the general base mechanism in carboxypeptidase A-catalyzed reactions: partitioning studies on nucleophiles and H2(18)O kinetic isotope effects.

Breslow¹,

Chin²,

Hilvert³

et al. 1983

Proc. Natl. Acad. Sci. U.S.A.

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Methanol does not detectably compete with water in carboxypeptidase-catalyzed cleavage of any substrate, although it is preferentially reactive in a model for the proposed nucleophilic mechanism for the enzyme that involves an anhydride intermediate. To test for such a common intermediate in the cleavage of related peptide and ester substrates, a method has been developed to examine H2160-H2'80 kinetic isotope-partitioning effects. The finding that benzoylglycylphenylalanine has an isotope effect of 1.019 ± 0.002 while benzoylglycyl-,B-L-phenyllactate shows a small inverse isotope effect excludes most versions of a nucleophilic mechanism having a common anhydride intermediate. The bulk of the available evidence strongly favors the previously proposed general base mechanism.Much attention has been devoted to studies of the mechanism by which carboxypeptidase catalyzes hydrolysis (1-9). In spite of this the overall mechanistic path is still not clear, to say nothing of the details of the path.Three functional groups of the enzyme seem to play catalytic roles: a bound Zn2+, the COO group of glutamate-270, and the phenolic OH of tyrosine-248. The tyrosine is apparently required for the cleavage of peptides but not for the cleavage of ester substrates of the enzyme (8). The Zn2+ can be replaced by other metal ions (1), with different effects on the catalytic ability of the enzyme to hydrolyze peptides and esters. In most mechanistic suggestions (9), the Zn2+ acts as a Lewis acid by coordinating with the carbonyl oxygen of the scissile group in the substrate, and the phenol assists protonation of the leaving group either by simple proton transfer or by acting as both donor and acceptor to facilitate an internal switch of protons in the tetrahedral intermediate (6). However, there is no universal agreement on this; Mock (10) has suggested a mechanism in which the tyrosine phenol group acts, in its ionized form, as a base rather than as an acid. (13), it was shown that this mechanism can be assisted with a phenolic group, in imitation of the combined role normally suggested for glutamate-270 and tyrosine-248 in enzyme-catalyzed amide cleavage. In contrast, a general base mechanism for assistance by a COO-is found in the hydrolysis of aspirin (14), even though the nucleophilic mechanism would also have been available to this substance.In chemical systems, the choice of one or the other mechanism seems to be dictated by whether the intermediate can be formed by a fragmentation or not. The maleamic acids or the corresponding maleate half-esters can form anhydride intermediates, converting stable amide or ester groups to enthalpically less-stable anhydride groups, because of the driving force associated with fragmentation of the substrate and the corresponding entropy increase associated with departure of the leaving group (15). In the case of aspirin, the anhydride intermediate would be formed by a rearrangement, rather than a fragmentation; without the entropy driving force of a fragmentation, this is apparently...

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mentioning

confidence: 57%

Evidence for the general base mechanism in carboxypeptidase A-catalyzed reactions: partitioning studies on nucleophiles and H2(18)O kinetic isotope effects.

Breslow¹,

Chin²,

Hilvert³

et al. 1983

Proc. Natl. Acad. Sci. U.S.A.

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“…In this multiple ion mode, the mass spectrometer continuously cycles, collecting ions from up to four separate masses. Details of this method of analysis, which provides accuracies of 0.2-0.4%, have been published elsewhere (Caprioli et al, 1974). Each analysis reported in this paper represents an average of at least three independent isotope measurements for a given sample.…”

Section: Methodsmentioning

confidence: 99%

Oxygen-18 studies of the mechanisms of yeast and muscle aldolases

Heron¹,

Caprioli²

1974

Biochemistry

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“…Thus, a sensitivity in the range of 0-20% will be considered necessary for a successful analysis. By use of this criteria, only the Caprioli et al (15) mass spectral measurement errors permit a successful analysis (Table II). Furthermore, since extremely small mass spectral measurement errors are required, small perchlorination errors are also a requirement.…”

Section: Isomeride Composition Calculation Sensitivitymentioning

confidence: 99%

Labeled perchlorination reagent for determination of polychlorinated biphenyls

Burkhard¹,

Armstrong²

1981

Anal. Chem.

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Perchlorinating polychlorinated biphenyls with an Isotopically labeled perchlorination reagent results In an altered isotopic abundance distribution In the decachlorobiphenyl formed. Analysis of this abundance distribution allows determination of the Isomerides and Aroclor mixtures In the original sample. The theory and the mathematics for using this technique are described. A computer sensitivity analysis Including both perchlorination and mass spectral measurement errors was performed. The computer sensitivity analysis Indicated that extremely pure chlorine-37 reagents, extremely accurate Ion abundance measurements, and low perchlorination errors are required for a successful analysis. These requirements make this technique appear to be unsuitable for routine analyses of polychlorinated biphenyls.

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Direct analysis of stable isotopes with a quadrupole mass spectrometer

Cited by 20 publications

References 4 publications

Evidence for the general base mechanism in carboxypeptidase A-catalyzed reactions: partitioning studies on nucleophiles and H2(18)O kinetic isotope effects.

Evidence for the general base mechanism in carboxypeptidase A-catalyzed reactions: partitioning studies on nucleophiles and H2(18)O kinetic isotope effects.

Oxygen-18 studies of the mechanisms of yeast and muscle aldolases

Labeled perchlorination reagent for determination of polychlorinated biphenyls

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