Mechanism of the screw-sense reversal of tightly hydrogen-bonded α-helical network triggered by the side-chain conformation

The paper presents theoretical treatment of a screw-sense reversal of tightly hydrogen-bonded α-helical polypeptide chains. The transition is triggered by the conformational free-energy difference (s) of the side chain flanking the chiral (right(r)- or left(ℓ)-handed) backbone. The two-parameter scheme (s, σ) corresponding to a zipper-type transition model starting from one terminal has been constructed and applied to the thermally-induced helix–helix transition characteristic of polyaspartic acid esters observed in the helicoidal solution. The weighting parameter σ takes care of the instability associated with the ℓ/r junction, i.e., the unfolded site where at least three successive hydrogen-bonds should become free to change their partner. The enthalpy of transition (ca. 2 kJ/mol) of poly(β-phenethyl l-aspartate) (PPLA) (Mv = 6 × 104) observed in the lyotropic liquid crystalline state in 1,1,2,2-tetrachloroethane has been reproduced with the instability parameter σ = 0.001–0.0001. Attempts were further extended to interpret the thermally-induced helix–helix transition behaviors of the PPLA homopolymers and random copolymers involving benzyl l-aspartate residues observed under various conditions.