Vibrational analysis of peptides, polypeptides, and proteins: Characteristic amide bands of β-turns*
Abstract
Normal vibration calculations have been done for a type I β-turn of CH3-CO-(Ala)4-NH-CH3 and a type II β-turn of CH3-CO-(Ala)2-Gly-Ala-NH-CH3. The force field was the one we refined for β-sheet and α-helical structures. A calculation was also done for CH3-O-CO-Gly-(Ala)2-Gly-O-CH3, which is an appropriate model for two tetrapeptides for which infrared data are available. The agreement between observed and calculated frequencies in this case is good, thus supporting the conclusions drawn from the above β-turn calculations. The most important result of the calculations is the prediction of bands near 1690 cm-1, a region heretofore associated only with the antiparallel-chain pleated sheet structure. This means that bands observed in proteins near 1690 cm-1 should be associated with the presence of β-turns as well as of β-sheets. We also find that a band near 1665 cm-1 is characteristic of type II turns.
