Figure 1.

(A) The primary structural constraints for the polypeptide backbone are derived from the ¹⁵N–¹H and ¹⁵N–¹³C dipolar interactions, the C_α–²H quadrupolar interaction, and the anisotropic ¹⁵N chemical shift. The initial structure is developed by determining each peptide-plane orientation with respect to the magnetic field axis with two dipolar interactions. The relative orientations of the peptide planes is then determined (i.e., the φ and ψ angles) for a diplane structure. (B) The initial structure is assembled sequentially with overlapping diplanes. The initial structure is not a unique structure because of chirality ambiguities; however, the molecular fold, hydrogen-bonding pattern, helical sense, and residues per turn are uniquely defined.