Vibrational analysis of peptides, polypeptides, and proteins: Characteristic amide bands of beta-turns.

Normal vibration calculations have been done for a type I beta-turn of CH(3)-CO-(Ala)(4)-NH-CH(3) and a type II beta-turn of CH(3)-CO-(Ala)(2)-Gly-Ala-NH-CH(3). The force field was the one we refined for beta-sheet and alpha-helical structures. A calculation was also done for CH(3)-O-CO-Gly-(Ala)(2)-Gly-O-CH(3), 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 beta-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 beta-turns as well as of beta-sheets. We also find that a band near 1665 cm(-1) is characteristic of type II turns.