A CALORIMETRIC CHARACTERIZATION OF THE SALT DEPENDENCE OF THE STABILITY OF THE GCN4 LEUCINE-ZIPPER

The effects of different salts (LiCl, NaCl, ChoCl, KF, KCl, and KBr) o n the structural stability of a 33-residue peptide corresponding to th e leucine zipper region of GCN4 have been studied by high-sensitivity differential scanning calorimetry. These experiments have allowed an e stimation of the salt dependence of the thermodynamic parameters that define the stability of the coiled coil. Independent of the nature of the salt, a destabilization of the coiled coil is always observed upon increasing salt concentration up to a maximum of similar to 0.5 M, de pending on the specific cation or anion. At higher salt concentrations , this effect is reversed and a stabilization of the leucine zipper is observed. The effect of salt concentration is primarily entropic, jud ging from the lack of a significant salt dependence of the transition enthalpy. The salt dependence of the stability of the peptide is compl ex, suggesting the presence of specific salt effects at high salt conc entrations in addition to the nonspecific electrostatic effects that a re prevalent at lower salt concentrations. The data is consistent with the existence of specific interactions between anions and peptide wit h an affinity that follows a reverse size order (F- > Cl- > Br-). tind er all conditions studied, the coiled coil undergoes reversible therma l unfolding that can be well represented by a reaction of the form N-2 <-> 2U, indicating that the unfolding is a two-state process in which the helices are only stable when they are in the coiled coil conforma tion.