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) on the structural stability of a 33-residue peptide corresponding to the leucine zipper region of GCN4 have been studied by high-sensitivity differential scanning calorimetry. These experiments have allowed an estimation 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 approximately 0.5 M, depending 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, judging from the lack of a significant salt dependence of the transition enthalpy. The salt dependence of the stability of the peptide is complex, suggesting the presence of specific salt effects at high salt concentrations in addition to the nonspecific electrostatic effects that are prevalent at lower salt concentrations. The data is consistent with the existence of specific interactions between anions and peptide with an affinity that follows a reverse size order (F- > Cl- > Br-). Under all conditions studied, the coiled coil undergoes reversible thermal unfolding that can be well represented by a reaction of the form N2<==>2U, indicating that the unfolding is a two-state process in which the helices are only stable when they are in the coiled coil conformation.