Vibrational analysis of peptides, polypeptides, and proteins. VI. Assignment of β‐turn modes in insulin and other proteins

The normal modes have been calculated for structures having the dihedral angles of the four β‐turns of insulin. Frequencies are predicted in the amide I region near 1652 and 1680 cm −1 . The former overlaps the α‐helix band at 1658 cm −1 in the Raman spectrum, while the latter accounts for the hitherto unassignable band at 1681 cm −1 . Calculated amide III frequencies extend above 1300 cm −1 , providing a compelling assignment of the 1303‐cm −1 band in insulin and similar bands in other globular proteins.