The 15N HSQC experiment is probably the most frequently recorded experiment in protein NMR. The HSQC experiment can be performed either using the natural abundance of the 15N isotope, or using isotopically labeled protein. The latter can be recorded on much lower concentrations of protein, but requires recombinant expression of the protein.
Each residue of the protein (except proline) has an amide proton attached to a nitrogen in the peptide bond. If the protein is folded, the peaks are usually well dispersed, and most of the individual peaks can be distinguished. Being a relatively cheap and quick experiment, the HSQC is useful to screen candidates for structure determination by NMR. The number of peaks in the spectrum should match the number of residues in the protein (though sidechains with nitrogen-bound protons will add additional peaks). It will probably be difficult to solve the structure of the protein if this is not the case. The labour-intensive process of structure determination is usually not undertaken until a good HSQC is obtained.
It is not possible to assign the HSQC spectrum by itself, or in other words to determine which peaks correspond to a particular residue in the protein. This process can be done in different ways as outlined in the protein NMR article. The assignment of the spectrum is usually the first step in a structure determination, and is essential for a meaningful interpretation of more advanced NMR experiments.
The HSQC experiment is also useful for detecting interactions with ligands, such as other proteins or drugs. By comparing the HSQC of the free protein with the one bound to the ligand, it is possible to find the changes in the chemical shifts of the peaks, which is most likely to occur in the binding interface.
Resurrecting Abandoned Proteins with Pure Water: CD and NMR Studies of Protein Fragments Solubilized in Salt-Free Water
Dec 01, 2006; ABSTRACT Many proteins expressed in Escherichia coli cells form inclusion bodies that are neither refoldable nor soluble in...