The structure of synthetic polypeptides. II. Investigation with polarized infra-red spectroscopy

Methods of producing oriented films of synthetic polypeptides when prepared from various solvents are described. The polarized infra-red spectra of a number of these films have been measured. It is shown, in agreement with earlier work, that the polypeptides can exist in a folded (α) and an extended (β) configuration. In the α form the N—H and C═O stretching modes of the polypeptide chain show parallel dichroism while the N—H deformation mode shows perpendicular dichroism. In the β form these conditions are reversed. The α form has characteristic frequencies of about 1660 and 1560 cm.-1, while the β form has characteristic frequencies of about 1640 and 1530 cm.-1. When highly dispersed in an inert polymer or liquid solvent it is shown that all the hydrogen bonds of the polypeptide chain can be internally satisfied. It is concluded that in the folded state the chain is maintained in the form of seven-membered rings by intra-molecular hydrogen bonds.

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