15N-enrichment

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

Powers R, Gronenborn AM, Clore GM, Bax A. Three-dimensional tripleresonance NMR of 13C/15N-enriched proteins using constant-time evolution. J Magn Reson 1991 94 209-213. 

SN]-Labeled 4,6-dimethyl-47/-[l,2,5]oxadiazolo[3,4-rf pyrimidine-5,7-dione 1-oxide 290 is conveniently prepared by nitration of commercially available 6-amino-l,3-dimethyl-l//-pyrimidine-2,4-dione using 15N-enriched nitric acid followed by an intramolecular oxidative cyclization with iodosylbenzene diacetate under mild conditions <2000JOC6670>. 

Vibrational spectra have been used to identify matrix-isolated chalcogen-nitrogen species, which are unstable under ambient conditions.35,36 In this way the species NS, SNS (and the less stable isomer NSS), NNS, NSe, SeNSe and [SeNSe]+ have been identified. The technique of 15N-enrichment has been used in several cases to distinguish S-N from S-S vibrations. 

Solid state 15N NMR has also been used103 to study lyophilized powders of 15N-enriched histidine and imidazole, prepared from aqueous solutions with pH ranging... 

Fig. 7.15 (continued) (c) Superposed experimental and calculated 30.41 MHz (7 T) 15N NMR spectra of 15N enriched DPP at different temperatures and deuterium fractions xd in the mobile sites. The four sharp lines with temperature dependent positions stem from a small quantity of 15N labeled tetramethyltertraaza-[14]annulene reference in a separate capsule (Klein, O., Limbach, H. H., et al. J. Am. Chem. Soc. 126, 11718 (2004))... 

All of the heteroatoms possess at least one naturally occurring isotope with a magnetic moment (Table 15). The nuclei 14N, 170 and 33S also possess an electric quadrupole moment which interacts with the electric field gradient at the nucleus, providing a very efficient mechanism for relaxing the nuclear spin. The consequence of this facilitation of relaxation is a broadening of the NMR signals so that line widths may be 50-1000 Hz or even wider. To some extent this problem is offset by the more extensive chemical shifts that are observed. The low natural abundances and/or sensitivities have necessitated the use of accumulation techniques for all of these heteroatoms. The relative availability of 170 and 15N enriched... 

Two 15N-enriched urea-formaldehyde resins with different crosslink density were studied by tfie solid state CP MAS 15N NMR. Despite at least six expected 15N chemical shifts arising from tertiary, secondary and primary amides in the different structural moieties, both resins exhibit only two major peaks. The lower field resonance is more pronounced in the highly cured resin, suggesting its origin in the tertiary amides. A DD experiment, which would confirm this assumption, does not result in clearly separated secondary and tertiary amides. Thus, from the analytical point of view, it seems that 13C NMR spectra are more useful than 15N NMR spectra, although 1SN resonance data provide a useful supplement 252). 

Pig. 3.19. 13C Chemical shift assignments of 1-Methoxycarbonylphenazine (a) in deuteriochloro-form at 30 C based on long-range UC—I5N couplings of the 10-15N-enriched derivative (b). 

The degree of 15N enrichment between particulate organic matter (POM) and consumers can be used to accurately estimate trophic level (47, 49). Thus isotopic studies permit identification of consumer groups that form nutritional guilds via 13C and analyses, while establishing trophic-level interactions within these groups via 15N analyses. To date, very few combined 13C and 15N analyses of trophic structure in freshwater systems have been published (47, 50-52). 

The relatively smaller 15N enrichments of Cyclops compared to phytoplankton and the other zooplankton in the leucine experiment are consistent with the idea that Cyclops feeds on microheterotrophs that are less labeled with 15N than the phytoplankton. As with the 15N, Cyclops was less enriched in 13C than phytoplankton. But the 13C proved less useful as a tracer because phytoplankton took up the label and because the actual isotopic compositions of bacteria and microheterotrophs were unknown. 

This method of establishing retention time is valid only if isotopic fractionation effects during nitrogen transformations are unimportant. For example, nitrification results in a 15N enrichment of the residual NH4 + pool... 

Grady, M. M. Pillinger, C. T. 1992 The continuing search for the location of 15N-enriched nitrogen in Acfer 182. Meteoritics 27, 226-227. 

Knicker, H. (2002). The feasibility of using DCPMAS 15N 13C NMR spectroscopy for a better characterization of immobilized N-15 during incubation of 13C and 15N enriched plant material. Org. Geochem. 33, 237-246. 

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