Magnetic resonance is widely used for spectroscopic measurements of
biological systems. The concept of magnetic resonance can be applied to
two probes: with the nuclei, as is done in nuclear magnetic resonance
(NMR), or with the electrons, as done in electron paramagnetic resonance
(EPR). The basic concepts of NMR are discussed, including the use of
different pulse techniques for improvements in the signal quality. The use
of NMR to determine protein structures is presented with the example of
a protein associated with neurological disease. Also discussed is the rela-
tionship between NMR and the technique of magnetic resonance imaging
(MRI) that is commonly used in hospitals. The presentation switches to
the basic concepts of EPR followed by the use of this technique to under-
stand the function of proteins, including heme proteins and ribonucleotide
reductase, a critical enzyme that catalyzes the conversion of nucleotides
into deoxynucleotides.
NMR
Every particle that has a spin also has a magnetic dipole moment, with
protons, neutrons, and electrons having a spin of 1/2 (Chapter 17). The
protons and neutrons forming a nucleus will tend to pair up their spins.
In general, nuclei with even numbers of protons and neutrons will have
zero spin, those with an odd number of either protons or neutrons will
have a half-integer spin, and those with both protons and neutrons being
odd will have an integer spin (Table 16.1). Since the isotopes of any given
atom have different numbers of neutrons, the nuclear spin of isotopes
will necessarily be different. For proteins, most work is done on the NMR
signals arising from the protons that have a spin of 1/2. The NMR signals
arising from protons can be eliminated by substitution with deuterium.
The non-zero spins of the isotopes^13 C and^17 O provide the opportunity to
perform specific isotopic substitutions and to measure the NMR signals from
those isotopes.