Sparse Labeling for Protein NMR
It is standard practice to enrich proteins to high levels of 13C enrichment in-order to make spectral assignments and to ultimately determine structure. However, a high level of uniform 13C enrichment creates scalar and dipolar coupling between the directly bonded 13C nuclei, which for dipolar-based sequences used in solid state NMR, creates large signals which often obscures low-intensity, information-rich signals produced from long range coupling. For scalar-based sequences used in solution state NMR, the presence of directly bonded 13C nuclei will degrade spectral resolution and provides an unwanted dipolar relaxation mechanism for the alpha carbon.
Reduction of 13C-13C dipolar coupling is made possible by sparse 13C labeling. 13C sparse labeling is where the expressed protein does not contain adjacent 13C nuclei. This can be accomplished to a large degree by using selectively 13C-labeled carbon sources for the protein expression. There are several different substrates available for 13C sparse labeling, including glucose, glycerol, and pyruvate.