Low‐frequency infrared bands and chain conformations of polypeptides

Infrared spectra of polypeptides were measured in the region of 1800–400 cm −1 . For the α‐helical form, disordered form, and antiparallel‐chain β‐form, amide V band‐ arising from N‐H out‐of‐plane bending models were observed at 610–620, around 650, and 700–705 cm −1 , respectively, and amide V′ bands arising from N‐D out‐of‐plane bending modes were observed at 455–465, around 510, and a 515–530 cm −1 , respectively. These correlations are useful for conformation diagnoses, particularly for copolyamino‐acids or proteins which are not oriented. The nature of low‐frequency amide bands are discussed with reference to potential energy distributions calculated for the α‐helical form and β form.