If there is a large cluster of severely overlapped peaks around the middle of the spectrum, that would indicate the presence of significant unstructured elements in the protein. If the protein is folded, the peaks are usually well-dispersed, and most of the individual peaks can be distinguished. The 15N HSQC is normally the first heteronuclear spectrum acquired for the assignment of resonances where each amide peak is assigned to a particular residue in the protein. The backbone amide peaks of glycine normally appear near the top of the spectrum. The sidechain amine peaks from tryptophan are usually shifted downfield and appear near the bottom left corner. In a typical HSQC spectrum, the NH 2 peaks from the sidechains of asparagine and glutamine appear as doublets on the top right corner, and a smaller peak may appear on top of each peak due to deuterium exchange from the D 2O normally added to an NMR sample, giving these sidechain peaks a distinctive appearance. In addition to the backbone amide resonances, sidechains with nitrogen-bound protons will also produce peaks. Normally the N-terminal residue (which has an NH 3 + group attached) is not readily observable due to exchange with solvent. Each residue (except proline) therefore can produce an observable peak in the spectra, although in practice not all the peaks are always seen due to a number of factors. The HSQC provides the correlation between the nitrogen and amide proton, and each amide yields a peak in the HSQC spectra. Such labelled proteins are usually produced by expressing the protein in cells grown in 15N-labelled media.Įach residue of the protein, with the exception of proline, has an amide proton attached to a nitrogen in the peptide bond. The HSQC experiment can be performed using the natural abundance of the 15N isotope, but normally for protein NMR, isotopically labeled proteins are used. The 15N HSQC experiment is one of the most frequently recorded experiments in protein NMR. Each peak in the spectrum represents a bonded N-H pair, with its two coordinates corresponding to the chemical shifts of each of the H and N atoms. 1H– 15N HSQC spectrum of a fragment of an isotopically labeled protein NleG3-2. Main article: protein NMR 1H- 15N HSQC 1H– 15N HSQC polarization scheme for a protein/amino acid. The 1H signal is detected in the directly measured dimension in each experiment, while the chemical shift of 15N or 13C is recorded in the indirect dimension which is formed from the series of experiments. In HSQC, a series of experiments is recorded where the time delay t 1 is incremented. After a time delay ( t 1), the magnetization is transferred back to the proton via a retro-INEPT step and the signal is then recorded. The basic scheme of this experiment involves the transfer of magnetization on the proton to the second nucleus, which may be 15N, 13C or 31P, via an INEPT (Insensitive nuclei enhanced by polarization transfer) step. The HSQC experiment is a highly sensitive 2D-NMR experiment and was first described in a 1H- 15N system, but is also applicable to other nuclei such as 1H- 13C and 1H- 31P. The 2D HSQC can also be combined with other experiments in higher-dimensional NMR experiments, such as NOESY-HSQC or TOCSY-HSQC. The spectrum contains a peak for each unique proton attached to the heteronucleus being considered. The resulting spectrum is two-dimensional (2D) with one axis for proton ( 1H) and the other for a heteronucleus (an atomic nucleus other than a proton), which is usually 13C or 15N. The experiment was first described by Geoffrey Bodenhausen and D. ![]() The heteronuclear single quantum coherence or heteronuclear single quantum correlation experiment, normally abbreviated as HSQC, is used frequently in NMR spectroscopy of organic molecules and is of particular significance in the field of protein NMR.
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