Heteronuclear single quantum coherence

The HSQC (Heteronuclear Single Quantum Coherence) experiment is used frequently in NMR spectroscopy of organic molecules and is of particular significance in the field of protein NMR. The resulting spectrum is two-dimensional with one axis for 1H and other for a heteronuclear most often 13C or 15N. (A heteronucleus in NMR terminology is a nucleus other than proton.) The spectrum contains a peak for each unique proton attached to the heteronucleus being considered. Thus, if the chemical shift of a specific proton is known, the chemical shift of the coupled heteronucleus can be determined, and vice versa.

HSQC in protein NMR
For full article see protein NMR.

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.