Minimum labelling: 15N, 13C
The magnetisation is passed from 1H to 15N and then to 13CO. From here it is transferred to 13Cα and the chemical shift is evolved. The magnetisation is then transferred back via 13CO to 15N and 1H for detection. The chemical shift is only evolved for the 1HN, the 15NH and the 13Cα, but not for the 13CO. This results in a spectrum which is like the HNCA, but which is selective for the Cα of the preceding residue.
This experiment can be useful for backbone assignment when used in conjunction with the HNCA, HNCO and HN(CA)CO if the CBCANNH and CBCA(CO)NNH spectra are of bad quality.