Figure 3. Protein backbone.

Each amino acid is connected via a peptide bond between the carbonyl carbon of the first amino acid and the amide of the next amino acid, which is highlighted by the dashed square box. The ¹H-¹⁵N HSQC experiment detects protons directly coupled to nitrogen (designated by the circle), and the resulting spectrum contains one peak for every amino acid in the protein. The HNCO experiment (see Isotopic Labeling Techniques under Advances in NMR) correlates the carbonyl carbon of amino acid (i) to the amide proton of the proceeding amino acid (i-1).