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Thermodynamic parameters of helix‐coil transition in polypeptide chains I. Poly‐( L ‐glutamic acid)
Author(s) -
Bychkoya V. E.,
Ptitsyn O. B.,
Barskaya T. V.
Publication year - 1971
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360101111
Subject(s) - chemistry , cooperativity , enthalpy , potentiometric titration , helix (gastropod) , viscometer , intrinsic viscosity , entropy (arrow of time) , crystallography , solvent , thermodynamics , viscosity , organic chemistry , polymer , biochemistry , ecology , physics , electrode , biology , snail
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.