Conformational Studies of Poly(β-phenethyl L-aspartate)

Abstract

Poly(β-phenethyl L-aspartate) was synthesized and its conformational transitions in chloroform–dichloroacetic acid and chloroform–trifluoroacetic acid mixed solvents were examined by optical rotatory dispersion, circular dichroism and viscosity measurements. Poly(β-phenethyl L-aspartate), having a right-handed α-helical form in chloroform, undergoes a transition to a left-handed α-helical form by the addition of 2.0% dichloroacetic acid at 21°C. In the range of dichloroacetic acid content between 1.2% and 2.0%, the polymer exhibits a thermally-induced inversion of the helix sense from the right-handed to the left-handed. The transition temperature is dependent on the solvent composition. Furthermore, in the chloroform–trifluoroacetic acid system, the polymer exhibits both the helix-sense inversion and the helix–coil transition when the composition of the mixed solvent is varied. The thermally- and acid-induced inversions of the helix sense are discussed in terms of the change in the side chain conformation.