Reference:
R.T. Clubb, V. Thanabal and G. Wagner (1992) J. Magn. Reson. 97 213-217. (Link to Article)

Minimum labelling: ¹⁵N, ¹³C

Dimensions: 3

The Magnetisation is transferred from ¹H to ¹⁵N and then via the N-Cα J-coupling to the ¹³Cα. From there it is transferred to the ¹³CO via the ¹³Cα-¹³CO J-coupling. For detection the magnetisation is transferred back the same way: from ¹³CO to ¹³Cα, ¹⁵N and finally ¹H. The chemical shift is only evolved on ¹H, ¹⁵N and ¹³CO and not on the ¹³Cα. The result is a three-dimensional spectrum. Because the amide nitrogen is coupled both to the Cα of its own residue and that of the preceding residue, both these transfers occur and transfer to both ¹³CO nuclei occurs. Thus for each NH group, two carbonyl groups are observed in the spectrum. But because the coupling between N_i and Cα_i is stronger than that between N_i and Cα_i-1, the H_i-N_i-CO_i peak generally ends up being more intense than the H_i-N_i-CO_i-1 peak.

This experiment can be useful for backbone assignment when used in conjunction with the HNCA, HN(CO)CA and HNCO if the CBCANNH and CBCA(CO)NNH spectra are of bad quality.

An overlay of the HNCO and HN(CA)CO spectra makes it very easy to distinguish between CO_i and CO_i-1 for each NH group.