The random coil → β transition of copolymers of L‐lysine and L‐valine: Potentiometric titration and circular dichroism studies

A series of copolymers of L ‐lysine and L ‐valine [poly( L ‐lysine f L ‐valine 100‐ f )] containing 0–13% L ‐valine have been studied, in 0.10 M KF solution, using potentiometric titration and circular dichroism spectroscopy. Incorporation of increasing amounts of valine into the copolymers favors β‐sheet formation over α‐helix formation at high pH and room temperature. The titrations were analyzed using the method of Zimm and Rice and the partial free energy (Δ G 0 cβ ) for the coil‐to‐β‐sheet transition for valine is estimated at 900 cal/mole at 25°C. From the temperature dependence of the free energy, the partial enthalpy, Δ H 0 cβ , and entropy, Δ S 0 cβ , of the transition for valine is estimated to be 854 cal/mole and 6.0 e.u., respectively. The corresponding partial thermodynamic parameters for L ‐lysine are in agreement with published results. The fraction of β‐sheet versus pH has been calculated for poly( L ‐lysine 86.8 L ‐valine 13.2 ) at 25.0°C using the titration data; data obtained from circular dichroism spectroscopy for the same copolymer are in good accord. It is concluded from these results that L ‐valine is a very strong β‐sheet forming amino acid. Furthermore, these results indicate that the Zimm–Rice method is applicable to transitions between the coil and β‐sheet states for a polypeptide containing two different residues.