Steric structure of L‐proline oligopeptides. I. Infrared absorption spectra of the oligopeptides and poly‐L‐proline

Poly‐ L ‐prolines I and II were differentiated by the characteristic bands in the far infrared region. Form I showed two broad bands at about 280 and 160 cm −1 and form II two bands at, 400 and 670 cm. −1 . Furthermore, three broad bands at about 250, 200, and 100 cm. −1 were observed in the spectrum for form II. Infrared absorption bands of the pentamer, hexamer, and octamer of tert ‐amyloxycarbonyl‐ L ‐proline were almost similar to those of poly‐ L ‐proline II in the 1800–75 cm. −1 region. In the far‐infrared region, especially, the absorption bands of these three oligopeptides were in good agreement with that of poly– L –proline II. Accordingly we concluded that the molecules of pentamer, hexamer, and octamer had a helical structure of a left‐handed threefold screw axis. The tetrapeptide of tert ‐amyloxycarbonyl‐ L ‐proline might also have a left‐handed helix, probably one turn, since the tetramer clearly showed an absorption band at about 400 cm. −‐1 characteristic of poly– L –proline II.