15N isotope

Nitrogen is an A + 1 element, while the natural abundance of 15N isotope constitutes about 0.4% of 14N. Since besides nitrogen only carbon contributes notably to the intensity of the A + 1 peak, it is possible to estimate the number of nitrogen atoms in the molecule on the basis of the nitrogen mle and the differences in the abundances of 13C ii 15N isotopes. 

Most of the early methods for assigning isotopically labeled nucleic acids were developed using RNA samples, since 13C and 15N isotopes can be incorporated in RNA more easily than in DNA. While most of the magnetization transfer pathways in RNA and DNA are the same, the latter contains thymine rather than uracil. To remove the ambiguity of intra- and inter-residue H6-CH3 peak assignment in thymine, the HCCCH through-bond method was proposed [51] as a more sensitive alternative to NOE or ROE. 

Preston et al. [698,699] have described novel approaches to 15N-isotope dilution determination of ammonium and of free amino acids in natural waters, incorporating chemical derivatization and conventional GC-MS analysis. 

The predictive capabilities of results of theoretical calculations of isotope effects have again been questioned,94 following an experimental and theoretical study of the decarboxylation of 3-carboxybenzisoxazole at room temperature (Kemp s reaction). The experimentally determined 15N isotope effect in acetone is 1.0312 0.0006 and the 13C isotope effect (1.0448, 1.0445, 1.0472, and 1.0418 in 1,4-dioxane, acetonitrile, DMF, and water, respectively) is independent of solvent polarity even though the reaction rate is markedly solvent dependent. Theoretical models at die semiempirical (AMI, PM3, SAMI) and ab initio (up to B3LYP/6-31+ + G ) levels were all unable to predict die experimental results quantitatively. 

As an example, Figure 12.2 show the concentration of two DNA species in a CsCl solution. The initial solution had a density of 1.739 g cm3. After spinning at 44,776 revolutions per minute at 298.15 K, the DNA species form two sharp bands. Species 1 is a bacterial virus DNA with a molecular weight of 2 x 104 kg mol-1. Species 2 is the same DNA except that it contains the 15N isotope rather than 14 N. 

Proteins that are to be studied by NMR must be specially prepared. Natural proteins almost exclusively contain 12C and 14N, neither of which is an NMR-active isotope. For analysis, the protein must be enriched with 13C and 15N. Isotopically-enriched proteins can be isolated from bacteria that are grown in the presence of 13C-enriched glucose and 15NH4C1. The resulting proteins are then handed off to the NMR experts. 

Activation with 14 MeV neutrons has been used to determine the oxygen content of various metals such as beryllium 20>, Cl, F, O, Na, Si, and various rare earths in complex molten salt electrolytes 45>, the protein content of food products by means of the nitrogen content 46>, i60/180 and 14N/15N isotopic ratios in stable isotope tracer experiments 47,48), and in a wide variety of other applications. One application we... 

Mutchler, T., Sullivan, M.J., and Fry, B. (2004) Potential of 15N isotope enrichment to resolve ambiguities in coastal trophic relationships. Mar. Ecol. Prog. Ser. 266, 27-33. 

So far, three computational studies of isotope effects related to the ODCase mechanism have been published Singleton, Beak and Lee used 13C isotope effects to elucidate the mechanism by which the uncatalyzed decarboxylation of orotic acid takes place.46 Phillips and Lee calculated 15N isotope effects and compared them to known experimental values to show that oxygen-protonation mechanisms are viable for the enzyme-catalyzed process.47 Kollman and coworkers focused on the 15N isotope effect associated with C5-protonation.27 Each study is described further below. 

Computational approach. Phillips and Lee calculated 15N isotope effects47 using the quiver program.63 The relevant geometries were optimized at B3LYP/ 6-311+ + G 49 A scaling factor of 0.96 was used for the frequencies. 

Results. The experimental 15N isotope effect at N1 for the decarboxylation of OMP in ODCase (Scheme 1) was measured by Cleland et al. to be 1.0068.66 Comparison of this normal isotope effect with IEs measured for the model compounds picolinic acid (17) and A-methyl picolinic acid (18) led Cleland and coworkers to conclude that the normal IE observed for OMP decarboxylation is indicative of the lack of a bond order change at Nl. This conclusion was based on the following reasoning. The IE for the decarboxylation of picolinic acid (17) is 0.9955 this inverse value is due to the change in bond order incurred when the proton shifts from the carboxylate group to the N in order to effect decarboxylation (equation 2) the N is ternary in the reactant, but becomes quaternary in the intermediate, which results in the inverse IE. The decarboxylation of A-methyl picolinic acid (18) involves no such bond order change (equation 3), and the observed normal IE of 1.0053 reflects this. 

Phillips and Lee calculated the 15N isotope effect for the decarboxylation of 1-methyl orotate (lb) via 2-protonation (4b) and via 4-protonation (6b). They found that in both cases, the calculated isotope effect is normal 1.0043 for 2-protonation, and 1.0054 for 4-protonation. An examination of the optimized structures showed clearly that very little bond order change occurs at Nl, regardless of which oxygen is protonated. Phillips and Lee also benchmarked their calculations by computing the IEs for protonation of pyridine and for decarboxylation of picolinic acid (17) and A-methyl picolinic acid (18) the results of these calculations are in agreement with the experimental values mentioned above. Therefore, Philips and Lee asserted that... 

Recent general spectral studies of such nitriles include the vibration spectra of 2-pyrazinecarbonitrile and a (> 99%)15N-isotopic version 1172 the mass spectra of 2,3-pyrazinedicarbonitrile, its 5,6-diphenyl derivative, and 2,3,5,6-pyrazinetetracarboni-trile for comparison with those of analogous heterocyclic nitriles 1406 and the 15C NMR spectra of 2-pyrazinecarbonitrile and the like for correlation with their reactivities toward acetone enolate anions.251 The structure-activity relationship of pyrazinecarbonitriles as herbicides has been reported.1048... 

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