A two-stranded a-helical coiled coil was prepared having a Cys(4) metal-binding site within its hydrophobic interior. The addition of Cd2+ results in the incorporation of 2 equiv of metal ion, which is accompanied by a conformational change of the peptide, as observed by circular dichroism (CID) spectroscopy. Isothermal titration calorimetry (ITC) shows that the addition of Cd2+ is accompanied by two thermodynamic events. A comparison of the time dependence of the ITC behavior with those of the UV absorption and CID behavior allows the assignment of these events to a preliminary endothermic metal-binding step followed by a slower exothermic conformational change.