References:
Y. Li, D.A. Berthold, H.L. Frericks, R.B. Gennis, C.M. Rienstra (2007) Chem. Biochem. 8 434-442. (Link to Article)

Minimum labelling: ¹⁵N, ¹³C

Dimensions: 3

Magnetisation is transferred from ¹H to ¹³Cα via cross polarisation and then selectively to the ¹⁵N using specific cross polarisation. A further specific cross polarisation step is used to transfer the magnetisation onto ¹³CO nuclei.The chemical shift is evolved on the ¹³Cα nuclei and ¹⁵N nuclei and then detected on the ¹³CO nuclei, resulting in a 3D spectrum. A 2D version of this experiment is also possible, in which the ¹⁵N evolution time is omitted.

This experiment is a useful addition to the NCACX and NCOCX during assignment, providing useful quasi-through-bond links between two sequential residues (i.e. Cα_i-N_i-CO_i-1). The main problem with this experiment is its low signal-to-noise. The three CP transfers tend to have fairly low transfer efficiency. On a sample which suffers from low signal-to-noise anyway, there may not be enough signal left to detect at the end of this experiment.