The paper presents theoretical treatment of a screw-sense reversal of tightly hydrogen-bonded alpha-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, sigma) 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 sigma 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(beta-phenethyl L-aspartate) (PPLA) (Mv = 6 x 10(4)) observed in the lyotropic liquid crystalline state in 1,1,2,2-tetrachloroethane has been reproduced with the instability parameter sigma = 0.001-0.0001. Attempts were further extended to interpret the thermally-induced helix-helix transition behaviors of the PRA homopolymers and random copolymers involving benzyl L-aspartate residues observed under various conditions. (C) 2010 Elsevier Ltd. All rights reserved.