Vacuum Ultraviolet Absorption Spectra of Polypeptides and their Dependence on Conformation

Optical absorption spectra of poly-L-leucine, poly-DL-leucine, poly-L-alanine, poly-DL-alanine, poly-L-methionine, poly-L-valine, poly-L-serine, poly-L-tyrosine, poly-L-phenylalanine, Nylon 3, Nylon 4, Nylon 6 and polyglycine were measured with special regard to their conformations in the wavelength region from 2500Å to 1200Å. On the assumption that the absorption spectra of polypeptides at random conformation could be approximated by that of Nylons, the dependence of the absorption spectra of polypeptides on the conformation was estimated by comparing them with that of Nylons. It is noticed that α-helical polypeptides in general have a significant absorption peak near 1650Å and this band shows the dominant hyperchromism. The origin of this 1650Å absorption band may be the same as that observed between NV 1 and NV 2 transition bands for peptide monomers, and it is plausible that σ orbitals are related to this absorption band. The sum of the oscillator strength up to 10 eV was ca. 1 for polyglycine. Since the total sum of the oscillator strength is expected to be 33, the dominant part of the absorption of polypeptides will appear in the energy range higher than 10 eV.