Heteronuclear single quantum correlation
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A HSQC spectrum is an experiment used frequently in NMR spectroscopy, and is particularly useful in the field of protein NMR spectroscopy. The acronym stands for Heteronuclear Single Quantum Correlation. The spectrum obtained in the experiment has two axis, a proton axis and a heteronuclei axis. In the NMR terminology a heteronucleus is another nucleus other than protons, but most often 13C and 15N. The spectrum contains a peak for each unique proton attached a heteronucleus, though only one kind of heteronucleus can be recorded at the time. Thus, if the chemical shift of either the protons are known, the chemical shift of corresponding heteronuclei 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) have 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 amount of peaks in the spectrum should match the amount of residues in the protein, (plus the sidechains, that contains nitrogen bound protons). If this is not the case it will probably prove difficult to solve the structure of the protein. 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.
See also
References
Protein NMR Spectroscopy : Principles and Practice (1995) John Cavanagh, Wayne J. Fairbrother, Arthur G. Palmer III, Nicholas J. Skelton, Academic Press
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