Experimental study of the conformation of two stereoregular polymethionines: Poly( D ‐methionyl‐ L ‐methionine) and poly( L ‐methionyl‐ D ‐methionyl‐ L ‐methionine)

Infrared spectroscopy, X‐ray diffraction, and nuclear magnetic resonance spectroscopy have been used in investigating the conformation of two stereoregular polymethionines, poly( D ‐methionyl‐ L ‐methionine) and poly( L ‐methionyl‐ D ‐methionyl‐ L ‐methionine). When dissolved in a helicogenic solvent, such as chloroform and hexafluoroisopropanol, the polytripeptide is in an α‐helical conformation. A helix‐to‐coil transition can then be induced by addition of trifluoroacetic acid. On the other hand, it appears that the most stable conformation of poly( D ‐Met‐ L ‐Met) is a β antiparallel folded structure in which the linear polypeptide segments are near to the planar extension. This structure has been evidenced through X‐ray examination of oriented films, casted from solutions in chloroform. It has also been identified in solution in the same solvent, by use of infrared spectroscopy and by measuring the δ H α chemical shift which characterizes the H α proton in the peptide units. This δ H α value is found equal to 5.4 ppm and differs significantly from those which are usually attributed to the α‐helical conformation (δ H α = 4.2 ppm) and to the random coil (δ H α = 4.6 ppm). The β folded conformation of the poly( D ‐Met‐ L ‐Met) appears to be comparatively less stable than the α‐helical one for the poly( L ‐Met) macromolecular stereoisomer since hexafluoroisopropanol is a helicogenic solvent for this last solute and a destabilizing one for the poly( D ‐Met‐ L ‐Met) β folded conformer. X‐ray examinations carried out with stretched films, casted from a solution of poly( D ‐Met‐ L ‐Met) in chloroform, result in several data concering the cross β structure of this stereoregular polypeptide in the solid state.