An infrared study of unordered poly-L-proline in CaCl 2 solutions.

Infrared spectra have been obt,aiiied of poly-Lproline in aqueous CaCL 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, 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 interpretatiori of these changes, particularly in the CHZ bending modes, is that disordering is associated primarily with an increase in the range of accessible C"-C' (=O) rotation angles rather than with the random introduction of cis imide bonds in the chain.