Premium
Calorimetric measurement of enthalpy change in the isothermal helix–coil transition of poly‐ L ‐lysine in aqueous solution
Author(s) -
Chou Peter Y.,
Scheraga Harold A.
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.360100406
Subject(s) - chemistry , optical rotatory dispersion , enthalpy , lysine , aqueous solution , random coil , helix (gastropod) , crystallography , titration , polymer , isothermal microcalorimetry , heat capacity , calorimetry , isothermal process , isothermal titration calorimetry , analytical chemistry (journal) , polymer chemistry , thermodynamics , circular dichroism , organic chemistry , amino acid , ecology , biochemistry , physics , biology , snail
The heat Δ H ° for converting an uncharged lysine residue from a coil to an α‐helical state in poly‐ L ‐lysine in 0.1 N KCl has been determined calorimetrically to be −1200 cal/mole at both 15°C and 25°C. Essentially the same value has been obtained for the conversion of an uncharged residue from a coil to a β‐pleated sheet state. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute Δ H °, the observed Calorimetric heat was corrected for the heat of breaking the sample cell, the heal of dilution of HCl, the heat of neutralization of OH − ion, and the heat of ionization of the ε‐amino group in the random coil. The latter was obtained from similar Calorimetric measurements on poly‐ D , L ‐lysine, which was shown to be a good model for the random coil form of poly‐ L ‐lysine. The measured transition heat was ∼0.7 cal., which is only 7% of the total heat liberated when a 40 ml solution of 0.25% w/v poly‐ L ‐lysine is brought, from pH 11 to pH 7; nevertheless it could be determined with a precision of ±8%. The conformation of poly‐ L ‐lysine at pH 11 appears to be completely helical at 15°C, but a mixture of 90% α‐helix, 5% β form, and 5% coil at 25°C. Since Δ H ° ∼ 0 for the α ⇌ β conversion, the polymer behaves like one of 95% α‐helix and 5% coil in the calorimeter at 25°C. At neutral pH, poly‐ L ‐lysine is an extended coil, like poly‐ D , L ‐lysine.