The vicinal amide proton-C α proton spin-spin coupling constants, 3 J HNα in the globular protein basic pancreatic trypsin inhibitor (BPTI) have been measured using phase-sensitive correlated spectroscopy at high digital resolution. In conjunction with the crystal structure of BPTI, these data were used to calibrate the correlation between 3 J HN α and the dihedral angle φ. The resulting “BPTI curve” is 3J HNα = 6.4 cos 2 θ − 1.4 cos θ + 1.9 (θ = ¦φ − 60° ¦) . It is further shown that measurement of the spin-spin couplings 3 J HN α presents an independent, reliable method for identification of the location of helical structure in the amino acid sequence of proteins.