Polydepsipeptides. 7. Conformational Analysis of Poly(L-alanyl-L-alanyl-L-lactic acid)

The conformational properties of poly[(L-Ala)2-L-Lac] are described. Circular dichroism studies indicate this polymer is α helical in chloroform solutions but undergoes a helix-to-coil transition by solvent or temperature denaturation. The transition occurs near 50 °C. Infrared dichroism indicates that the polymer, in polyoxyethylene films, exists as a distorted α helix with the same average structure as poly[γ-ethyl L-glutamate]. The nonhydrogen-bonded ester groups are transverse to the helical axis, distorting the helix. A statistical thermodynamic theory for depsipeptide melting is developed and applied to this polymer. The theory indicates that melting for the parent peptide poly[L-Ala] should occur above 200 °C.