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Minimum labelling: 15N, 13C
Here the magnetisation is passed from 1H to 15N and then via the N-Cα J-coupling to the 13Cα and then back again to 15N and 1H hydrogen for detection. The chemical shift is evolved for 1HN as well as the 15NH and 13Cα, resulting in a 3-dimensional spectrum. Since the amide nitrogen is coupled both to the Cα of its own residue and that of the preceding residue, both these transfers occur and peaks for both Cαs are visible in the spectrum. However, the coupling to the directly bonded Cα is stronger and thus these peaks will appear with greater intensity in the spectra.
This experiment can be useful for backbone assignment when used in conjunction with the HN(CO)CA, HNCO and HN(CA)CO if the CBCANNH and CBCA(CO)NNH spectra are of bad quality.
By overlaying the HN(CO)CA spectrum with the HNCA, it becomes even easier to identify and distinguish between all Cαi and Cαi-1 peaks.