The conformation fo poly‐L‐alanine in hexafluoroisopropanol

High‐molecular‐weight poly‐ L ‐alanine dissolved in hexafluoroisopropanol exhibits infrared, ultraviolet, circular dichroism, and optical rotatory dispersion spectra which are unique and unlike any other previously reported polypeptide spectra. Strong evidence that a helical conformation is present is shown by the high degree of hypochromism in the 187mμ absorption peak and by the positions of the amide infrared bands. The CD and ORD spectra are also similar to those of α‐helical polypeptides, though important qualitative and qualitative differences are observed. To explain the novel spectra, which are not mixtures of the spectra of previously reported polypeptide conformations, a new α‐helix‐like conformation is proposed. The postulated conformation (a doubly hydrogen‐bonded helix) is a distorted α‐helix in which the peptide carbonyl groups point slightly out from the helix axis and are hydrogen bonded simul taneously both to the NH of the fourth peptide residue to the carboxyl terminal side (as in the classical α‐helix), as well as to a solvent molecule's hydroxyl hydrogen.