Reference:
W.T. Franks, K.D. Kloepper, B.J. Wylie and C.M. Rienstra (2007) J. Biomol. NMR 39 107-131. (Link to Article)

Minimum labelling: ¹⁵N, ¹³C

Dimensions: 4 (but often recorded as a CANcoCX 3D)

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. Finally a PDSD step is used to transfer the magnetisation to any other ¹³C nuclei nearby. The chemical shift is evolved on the ¹³Cα, ¹⁵N and ¹³CO nuclei and then detected on ¹³C. Lower dimensionalities are also possible by eliminating one or both of the ¹⁵N and ¹³CO evolution periods.

 

This experiment is extremely useful for assignment, providing useful quasi-through-bond links between two sequential residues with a further, e.g. Cα_i-N_i-CO_i-1-Cα_i-1 and decreasing overlap problems on account of its being a 4D experiment. As with the CANCO, 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 anything at the end of this experiment. However, if you do get this to work, then a small protein can be assigned very easily using this experiment (see Franks et al. 2007).