R.T. Clubb, V. Thanabal and G. Wagner (1992) J. Magn. Reson. 97 213-217. (Link to Article)
Minimum labelling: 15N, 13C
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 Ni and Cαi is stronger than that between Ni and Cαi-1, the Hi-Ni-COi peak generally ends up being more intense than the Hi-Ni-COi-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 COi and COi-1 for each NH group.