Magnetic Resonance (NMR)
- Nuclear Magnetic
Resonance (NMR) is
an analytical technique in which magnetic nuclei absorb energy from an
applied electromagnetic pulse and radiate the energy back.
- NMR is
used for identifying functional
groups and imaging.
- Subatomic particles “spin”
along their axis.
- Atoms with paired spins have no
net overall spin.
- Atoms can have a net overall
spin if the number of protons or the number of protons and neutrons is
atoms have multiple orientations
- In the absence of an external
magnetic field, these orientations have the same energy.
- In the presence of an external
magnetic field, the energy level splits
- The lower energy level contains
more nuclei that the higher energy level.
- Lower energy nuclei be excited
into the higher energy level by electromagnetic radition.
- The frequency of the radition
corresponds to the energy difference between the two nuclei
states (transition frequency).
- After absorption, nuclei relax
to lower energy state
Energy Level Diagram.
field, orientations have the same energy. In the presence
of an applied magnetic
field, the energy level splits into
two levels, high energy and low energy.
- When the
magnetic field at the
nucleis is not equal to the applied magnetic field, the nucleus is
sheilded by surrounding electrons.
- Sheilding: If
electrons produce a
field opposing the applied magnetic field, the applied field strength
must increase in order to reach the transition frequency. This
results in upfield shift
a field with the applied magnetic field, the applied field strength
decrease in order to reach the transition frequency. This results
in downfield shift
bonds are highlighted in
- Primary structure is a sequence of amino acids linked by
amino acid has a unique side chain (R); there are 20 total
- Peptide bonds formed by
dehydration synthesis (highlighted in red)
- Chemical interactions of R
groups dictate higher structure
- Secondary structure is
the formation of alpha helicies and beta sheets
- Tertiary structure is the 3D
folding of the protein
- Quaternary structure is the
assembly of multiple protein subunits
- Protein NMR is a series of 2D NMR techniques
- First is usually 15N-HSQC which
results one signal per amino acid residue
Quantum Coherence (HSQC)
- Hydrogen nuclei are excited and the
energy is transferred to a neighboring 15N
- The chemical shift is evolved on the
- Energy is transferred back to the
hydrogen for detection.
- This method mainly shows H-N correlations. Additional
- amino acids
containing nitrogen in their side chains
an excited hydrogen nuclei
to a neighboring 15N.
“Fingerprint”: each protein has a
Identification of possible problems
due to multiple
conformation of sample heterogenity
Can not assign specific peaks to
Need further more expensive analysis
myglobin from E.coli
(Toiman). Ribbon structure
(right) for the same protein (PDB: 1CQ2).
And Grzesiek, S. "Methodological
Advances in Protein NMR."
Accounts of Chemical Research. 36 (1993): 131-138
Tolman, J.R. et
magnetic dipole interactions in field-oriented
protein: Information for structure determination in solution.”
Natl. Acad. Sci. 92 (1995): 9279-9283.