Raman and normal-mode studies of the extended-helix conformation in polypeptide chains This is paper number 36 in a series on vibrational analysis of peptides, polypeptides, and proteins, of which Ref. 23 is paper number 35.

Abstract

The nature of the local main-chain conformation of polypeptides with charged side chains has been the subject of considerable discussion since Tiffany and Krimm first proposed [(1968) Biopolymers 6 , 1379–1381] that, rather than being random, the chain is locally relatively regular, with conformations similar to that of a left-handed threefold helix. Such structures, referred to as “extended-helix” (EH) conformations, have now been studied in a charged poly( L -glutamic acid) system by a combination of Raman spectroscopy and normal-mode analysis. Calculations were done for EH conformations with 3.0 and 2.4 residues/turn, using force fields refined for Α-helix, 3 1 -helix, and Β-sheet structures. Together with previous results on the Α-helix and Β-sheet forms, an interesting new correlation emerged: the frequency of the C Α C stretch skeletal mode, usually found in the 900–1000 cm −1 region of the Raman spectrum, is essentially linearly correlated with the value of the φ angle. Applying this relationship to the observed frequencies of the Α-helix and Β-sheet forms of poly( L -glutamic acid), we find that an observed sharp band in the spectrum of crystals of the calcium salt of poly( L -glutamic acid) (which is close to the frequency observed for the charged form in solution) corresponds to an EH conformation very close to that predicted from energy calculations. These studies thus provide very strong support for our proposal that charged polypeptide chains are not random but adopt local conformations of the EH type.