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Thermodynamic parameters of helix–coil transition in polypeptide chains. II. Poly‐ L ‐lysine
Author(s) -
Barskaya T. V.,
Ptitsyn O. B.
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.360101112
Subject(s) - chemistry , cooperativity , enthalpy , helix (gastropod) , potentiometric titration , viscometer , crystallography , dipole , entropy (arrow of time) , thermodynamics , analytical chemistry (journal) , viscosity , chromatography , organic chemistry , ecology , biochemistry , physics , electrode , biology , snail
The helix–coil transitions for poly‐ L ‐lysine (PL) were investigated by the methods of spectropolarimetry, viscometry and potentiometric titration in 0.2 M NaCl at different temperatures as well as in 0.2 M NaBr, 1 M KCl, and in mixtures of 0.2 M NaCl or NaBr with methanol at room temperature. The enthalpy and entropy differences between the helical and coillike states of uncharged PL molecules in 0.2. M NaCl were determined from the potentiometric titration curves. The cooperativity parameters σ for PL in different solvents were determined by two methods (from the sharpness of the transition and from the dependence of the intrinsic viscosity on the helical content in the transition region). In 0.2 M NaCl σ has a value of (2.3 ± 0.5) × 10 −4 and does not depend on temperature, i.e., the cooperativity of the helix‐coil transition, as for PGA, is mainly of an entropy origin (the initiating of the helical region is accompanied by the entropy decrease Δ S i = −12 eu/mole of helical regions). A comparison of the obtained results for PGA and PL with the molecular theories of the helix‐coil transitions shows that the role of dipole‐dipole interactions of nonneighboring peptide groups is greatly overestimated in these theories, leading to a considerable enthalpy contribution to the free energy of initiating helical regions which is not observed in the experiment.