Thermal and charge‐induced coil to α‐helix transition of poly‐L‐glutamic acid and random L‐glutamic acid–L‐alanine copolymers

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Abstract
Thermal and charge induced random coil to α‐helix transitions of poly‐L‐glutamic acid (PGA) were measured by optical rotatory dispersion in various solvents. The data of PGA in 0.1M Nacl were analyzed by the Zimm‐Rice theory. Enthalpy and entropy changes for the coil‐to‐helix transition in the unionized state were obtained: ΔH° = −1020 ± 100 cal/residue mole; ΔH° = −3.0 ± 0.4 e.u./residue mole. The initiation parameter, σ, of the Zimm‐Rice theory was given by a value of 5 ± 1 × 10‐3. Random copolymers of L‐glutamic acid and L‐alanine containing 10, 30, and 40 molar percents of alanyl residue were synthesized. Stabilities of α‐helix of the copolymers were compared to that of PGA. In water and water‐ethanol solutions, stabilities of the polymers were almost equal after the simple correction about the ionized charge density of the polymers. In 0.1 M NaCl solution these copolymers showed some deviations from the transition curve of PGA, which would suggest the hydrophobic contribution of the alanyl residues.