Nitrogen-15 nuclear magnetic resonance spectroscopy (nitrogen-15 NMR spectroscopy, or just simply
15N NMR) is a version of
nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...
that examines samples containing the
15N nucleus.
[Witanowski, M (1974). “Nitrogen N.M.R. Spectroscopy”. Pure and Applied Chemistry. 37, pp. 225-233. ] 15N NMR differs in several ways from the more common
13C and
1H NMR. To circumvent the difficulties associated with measurement of the quadrupolar, spin-1
14N nuclide,
15N NMR is employed in samples for detection since it has a ground-state spin of ½. Since
14N is 99.64% abundant, incorporation of
15N into samples often requires novel synthetic techniques.
Nitrogen-15 is frequently used in
nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...
(NMR), because unlike the more abundant nitrogen-14, that has an integer
nuclear spin
In atomic physics, the spin quantum number is a quantum number (designated ) which describes the intrinsic angular momentum (or spin angular momentum, or simply spin) of an electron or other particle. The phrase was originally used to describe th ...
and thus a quadrupole moment,
15N has a fractional nuclear spin of one-half, which offers advantages for NMR like narrower line width.
Proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
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
Proteomics is the large-scale study of proteins. Proteins are vital parts of living organisms, with many functions such as the formation of structural fibers of muscle tissue, enzymatic digestion of food, or synthesis and replication of DNA. In ...
(e.g.
SILAC Stable Isotope Labeling by/with Amino acids in Cell culture (SILAC) is a technique based on mass spectrometry that detects differences in protein abundance among samples using non-radioactive isotopic labeling. It is a popular method for quantitativ ...
).
Implementation
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 × 10
6 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 strength.
Physical properties
The physical properties of
15N are quite different from other nuclei. Its properties along with several common nuclei are summarized in the below table.
Chemical shift trends
The
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
(IUPAC) recommends using CH
3NO
2 as the experimental standard; however in practice many spectroscopists utilize pressurized NH
3(l) instead. For
15N, chemical shifts referenced with NH
3(l) are 380.5 ppm upfield from CH
3NO
2 (δ
NH3 = δ
CH3NO2 + 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 CH
3NO
2. Below is a summary of
15N chemical shifts for common organic groups referenced with respect to NH
3, whose chemical shift is assigned 0 ppm.
Gyromagnetic ratio
Unlike most nuclei, the gyromagnetic ratio for
15N is negative. With the spin precession phenomenon, the sign of γ determines the sense (clockwise vs counterclockwise) of precession. Most common nuclei have positive gyromagnetic ratios such as
1H and
13C.
Applications
Tautomerization
15N NMR is used in a wide array of areas from biological to inorganic techniques. A famous application in organic synthesis is to utilize
15N to monitor tautomerization equilibria in heteroaromatics because of the dramatic change in
15N shifts between tautomers.
Protein NMR
15N NMR is also extremely valuable in protein NMR investigations. Most notably, the introduction of three-dimensional experiments with
15N lifts the ambiguity in
13C–
13C two-dimensional experiments. In
solid-state nuclear magnetic resonance
Solid-state NMR (ssNMR) spectroscopy is a technique for characterizing atomic level structure in solid materials e.g. powders, single crystals and amorphous samples and tissues using nuclear magnetic resonance (NMR) spectroscopy. The anisotropic pa ...
(ssNMR), for example,
15N is most commonly utilized in NCACX, NCOCX, and CANcoCX pulse sequences.
Investigation of nitrogen-containing heterocycles
15N NMR is the most effective method for investigation of structure of heterocycles with a high content of nitrogen atoms (tetrazoles, triazines and their annelated analogs).
15N labeling followed by analysis of
13C–
15N and
1H–
15N couplings may be used for establishing structures and chemical transformations of nitrogen heterocycles.
INEPT
Insensitive nuclei enhanced by polarization transfer Insensitive nuclei enhancement by polarization transfer (INEPT) is a signal enhancement method used in NMR spectroscopy. It involves the transfer of nuclear spin polarization from spins with large Boltzmann population differences to nuclear spins ...
(INEPT) is a signal resolution enhancement method. Because
15N has a gyromagnetic ratio that is small in magnitude, the resolution is quite poor. A common pulse sequence which dramatically improves the resolution for
15N is INEPT. The INEPT is an elegant solution in most cases because it increases the Boltzmann polarization and lowers T
1 values (thus scans are shorter). Additionally, INEPT can accommodate negative gyromagnetic ratios, whereas the common
nuclear Overhauser effect The nuclear Overhauser effect (NOE) is the transfer of nuclear spin polarization from one population of spin-active nuclei (e.g. 1H, 13C, 15N etc.) to another via cross-relaxation. A phenomenological definition of the NOE in nuclear magnetic res ...
(NOE) cannot.
See also
*
Heteronuclear single quantum coherence spectroscopy (HSQC)
*
Two-dimensional nuclear magnetic resonance spectroscopy Two-dimensional nuclear magnetic resonance spectroscopy (2D NMR) is a set of nuclear magnetic resonance spectroscopy (NMR) methods which give data plotted in a space defined by two frequency axes rather than one. Types of 2D NMR include correlation ...
*
Triple-resonance nuclear magnetic resonance spectroscopy Triple resonance experiments are a set of multi-dimensional nuclear magnetic resonance spectroscopy (NMR) experiments that link three types of atomic nuclei, most typically consisting of 1H, 15N and 13C. These experiments are often used to assign ...
References
{{NMR by isotope
Nuclear magnetic resonance