Nitrogen-15 NMR
Nitrogen-15 nuclear magnetic resonance spectroscopy (nitrogen-15 NMR spectroscopy, or just simply 15N NMR) is a version of nuclear magnetic resonance spectroscopy that examines samples containing the 15N nucleus.15N NMR differs in several ways from the more common 13C and 1H NMR. To lift the restraint of spin 1 found in 14N, 15N NMR is employed in samples for detection since it has a ground-state spin of ½. Since14N is 99.64% abundant, incorporation of 15N into samples often requires novel synthetic techniques. Two sources of nitrogen-15 are the positron emission of oxygen-15 and the beta decay of carbon-15.
Nitrogen-15 is frequently used in nuclear magnetic resonance spectroscopy (NMR), because unlike the more abundant nitrogen-14, that has an integer nuclear spin and thus a quadrupole moment, 15N has a fractional nuclear spin of one-half, which offers advantages for NMR like narrower line width. Proteins can be isotopically labeled by cultivating them in a medium containing nitrogen-15 as the only source of nitrogen. In addition, nitrogen-15 is used to label proteins in quantitative proteomics (e.g. SILAC).
15N NMR has complications not encountered in 1H and 13C NMR spectroscopy. The 0.36% natural abundance of 15N results in a major sensitivity penalty. Sensitivity is made worse by its low gyromagnetic ratio (γ = -27.126 × 106 T−1s−1), which is 10.14% that of 1H. The signal to noise ratio for 1H is about 300 fold greater than 15N at the same magnetic field.
The physical properties of 15N are quite different from other nuclei. Its properties along with several common nuclei are summarized in the below table.
The International Union of Pure and Applied Chemistry (IUPAC) recommends using CH3NO2 as the experimental standard; however in practice many spectroscopists utilize pressurized NH3(l) instead. For 15N, chemical shifts referenced with NH3(l) are 380.5 ppm upfield from CH3NO2 (δNH3 = δCH3NO2 + 380.5 ppm). Chemical shifts for 15N are somewhat erratic but typically they span a range of -400 ppm to 1100 ppm with respect to CH3NO2. Below is a summary of 15N chemical shifts for common organic groups referenced with respect to NH3, whose chemical shift is assigned 0 ppm.
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