• 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.

Spin 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 odd. These atoms have multiple orientations Examples: 13C, 15N 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.  Without an external magnetic 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.  source: http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm

• 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 (diamagnetic shift)

• Desheilding: If the electrons produce a field with the applied magnetic field, the applied field strength must decrease in order to reach the transition frequency.  This results in downfield shift (paramagnetic shift)

Primary Structure.  Peptide bonds are highlighted in red.

Primary structure is a sequence of amino acids linked by peptide bonds Each 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 Heteronuclear Single Quantum Coherence  (HSQC) Hydrogen nuclei are excited and the energy is transferred to a neighboring 15N The chemical shift is evolved on the nitrogen Energy is transferred back to the hydrogen for detection. This method mainly shows H-N correlations.  Additional peaks are seen from amino acids containing nitrogen in their side chains

Transfer of Energy between an excited hydrogen nuclei to a neighboring 15N.

 
HSQC spectrum (left) for myglobin from E.coli (Toiman).  Ribbon structure (right) for the same protein (PDB: 1CQ2). 

http://www.protein-nmr.org.uk/spectra.html

http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/nmr1.htm