Thermodynamic parameters of helix‐coil transition in polypeptide chains I. Poly‐( L ‐glutamic acid)

The helix‐coil transitions for poly( L ‐glutamic acid) (PGA) in 0.2 M NaCl and in its mixture with dioxane were studied by the methods of spectropolarimetry, viscometry, and potentiometric titration at different temperatures from 8 to 50°C. The enthalpy and entropy differences between the helical and coillike states of uncharged PGA molecules were determined from the curves of potentiometric titration. The temperature dependence of the cooperativity parameter σ was determined by two methods: from the sharpness of transition and from the dependence of the intrinsic viscosity on the helical content in the transition region. In 0.2 M NaCl, σ= (2.5 ± 0.5) × 10 −3 and practically does not depend on temperature, i.e., the cooperativity of the helix‐coil transition is connected mainly with the entropy decrease in initiating helical regions (Δ S i ≈ −12 is mole of helical regions). On the contrary, initiation of a helical region in the water‐organic solvent mixture is accompanied by a considerable enthalpy increase.