The helix-coil transit.iotis for poly(L-glutamic acid) (PGA) in 0.2M NaCl and ill 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 ciirves of potentiometric titration. The temperature dependence of the cooperativity parameter u 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 0.2M NaC1, u = (2.5 =! = 0.5) X 10-3 and practically does riot depend on temperature, i.e., the cooperativity of the helix-coil transition is connect,ed mainly with the entropy decrease in initiating helical regions ( M i 'v -12 eu/mole of helical regions). On the contrary, initiation of a helical region in the waterorganic solvent mixkire is accompanied by a considerable eiithalpy increase. 2161 0 1971 by Joliii Wilcy & 8o115, In(*. 2IGY UYCHKOVA, PTITSYN, AND BARSKAYA
The AFo value for uncharged molecules of poly(L-glutamic acid) (PGA) was reported in a number of as well as the temperature dependence of AF0,3,5J,8 i.e., ' changes of the enthalpy AHo and entropy ASo with the increase of the chain helical region by one monomer unit. The AFi value for PGA molecules in aqueous medium was also estimated in a number of p a p e r ~; ~~~-' ~ however, the temperature dependence of this value up to now has not been studied. At the same time, only the simultaneous study of the enthalpy and entropy contributions to the AFo and AFi values may throw some light on the nature of the energetic and entropy factors influencing the stability of the helical state of the polypeptide chain.
The authors investigate the temperature dependence of the AFO and AFi values for PGA molecules in an aqueous medium and (for comparison) in water-dioxane mixtures.