Introduction to NMR and Its Application in Metabolite Structure Determination

XiAOHUA Huang, Robert Powers, Adrienne Tymiak, Robert Espina, and Vikram Roongta 

In this chapter we provide a brief introduction to NMR theory and describe the most common experiments used for metabolite structure determination by NMR. Selected examples of metabolite structure elucidation are presented to illustrate relevant hardware, key parameters, and NMR methodology. Detailed explanations of NMR fundamentals have been previously reviewed in a series of books that are listed at the end of this chapter (Berger and Brunn, 2004 Breitmaier, 2002 Freeman, 1997 Friebolin, 2005 Homans, 2005 Keeler, 2005 Martin and Zektzer, 1988). 

An NMR signal arises from an interaction between the nuclear spin (I) of an atom with an external magnetic field (Bq). Elements with even atomic mass and number have a zero nuclear spin, where only nuclei with 1 0 produce an observable NMR signal. Nuclei with I A generate broad resonance lines in an NMR spectrum and are not generally useful for structure elucidation. Fortunately, most of the elements that comprise metabolites correspond to nuclei that are observable by NMR. They are listed in Table 12.1 along with their relevant NMR properties. 

After the rf pulse, the net magnetization will continue to precess in the X,Y plane about the external magnetic field (Bg). The system will also slowly relax back to the Z-axis equilibrium position. The frequency of the X,Y precession, also known as the Larmor frequency, is related to Bg, y, and more importantly, to the local chemical environment of the nucleus. Small differences in the chemical environment of a nucleus will result in parts per million (ppm) differences in the Larmor frequency, which is known as the NMR Chemical 

FIGURE 12.1 Relationship between the magnetic field strength and NMR sensitivity. 

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