HSQC-NOESY

The 3D [1H, 13C]-HSQC-NOESY experiment is specifically designed to obtain carbon-edited NOESY spectra of carbon 13 labeled biomolecules and protein-ligand complexes from which homonuclear ¹H-¹H NOEs can be clearly assigned even in overcrowded regions (the mechanism involves heteronuclear transfer via ¹J_CH followed by a ¹H-¹H NOE spatial exchange step). In that way it is different to NOESY-HSQC because the heteronuclear exchange via ¹J_CH does not produce a diagonal. Thus when the ¹H-¹H NOE spatial exchange step is done the interference from the NOESY autocorrelation diagonal is much smaller. The problem occurs because each hydrogens sees a lot of water molecules so the water signal is quite large, if water suppression is not done properly (or sample dissolved in D₂O).

To start with, magnetization is transferred from ¹H to neighboring ¹³C nuclei and back to ¹H, encoding the ¹J_CH as well as the ¹H chemical shift. We obtain cross peaks for each carbon attached proton (a typical HSQC spectra).

After HSQC the magnetization is exchanged between all hydrogens using the NOE. The NOESY experiment resolves the 2D HSQC cross peaks in the extra 1H dimension producing two different types of signals: a.) autocorrelation diagonal peaks which don’t expand into the carbon dimension, and b.) off-diagonal cross-peaks correlating hydrogens close to each other. NOESY spectra are represented in phase-sensitive mode in order to distinguish exchange cross-peaks and unwanted anti-phase COSY contributions.

Each strip in the H-H plane contains NOE’s from one CH group to all other hydrogens nearby. A double set of peaks arises from CH2 groups in which the two hydrogen atoms have different chemical shifts. The resolution is greatest in the NOE plane as this is the detected dimension. This experiment yields constraints that can be used to calculate the 3D protein structure after carbon assignation has been done using other NMR methods.

Reference:

Majumdar, A. N. A. N.Y. A., & Zuiderweg, E. R. P. (1993). Improved 13 C-Resolved HSQC-NOESY Spectra in H 2 O, Using Pulsed Field Gradients.Journal of Magnetic Resonance, Series B, 102(2), 242-244.