Premium
The random coil → β transition of copolymers of L ‐lysine and L ‐isoleucine: Potentiometric titration and circular dichroism studies
Author(s) -
Walter Bert,
Fasman Gerald D.
Publication year - 1977
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.1977.360160103
Subject(s) - potentiometric titration , chemistry , isoleucine , circular dichroism , enthalpy , titration , random coil , titration curve , crystallography , analytical chemistry (journal) , thermodynamics , organic chemistry , leucine , amino acid , biochemistry , physics , electrode
Copolymers of L ‐lysine and L ‐isoleucine [poly( L ‐Lys f , L ‐Val 1 − f )] containing 4–15% isoleucine were investigated using potentiometric titration and circular dichroism (CD) spectroscopy. With increasing isoleucine content, β‐sheet formation is favored over α‐helix formation at high pH and room temperature. The fraction of β‐sheet present, as a function of pH, calculated from titrations of poly( L ‐Lys 85.2 , L ‐Ile 14.8 ), agreed well with data obtained from CD studies for the same copolymer. Thermodynamic parameters were determined from titrations using the method of Zimm and Rice; the partial free energy (Δ G ° C → β ) at 25° for the coil‐to‐β‐sheet transition for isoleucine was estimated to be −515 cal/mol; from the temperature dependence of free energy, the partial entropy (Δ S ° c β ), and the partial free enthalpy (Δ H ° c → β ) of the coil → β transition for isoleucine is estimated to be 2.6 e.u. and 260 cal/mol, respectively. The partial thermodynamic parameters obtained for lysine are in good agreement with literature values. It is concluded from these studies that isoleucine has a very high potential for a β‐sheet formation.