An infrared study of unordered poly‐L‐proline in CaCL2 solutions

Abstract Infrared spectra have been obtained of poly‐ L ‐proline in aqueous CaCl 2 solutions. As the salt concentration is increased, the CO stretching band develops a component at the frequency found in the solid state while the CH 2 bending band broadens to higher frequency. Since circular dichroism spectra indicate progressive disordering of the chain with increasing salt concentration, we associate the infrared spectral changes with the same phenomenon. Our interpretation of these changes, particularly in the CH 2 bending modes, is that disordering is associated primarily with an increase in the range of accessible C α –C′ (O) rotation angles rather than with the random introduction of cis imide bonds in the chain.