Although the Cu2+-binding sites of the prion protein have been well studied when the protein is fully saturated by Cu2+, the Cu2+-loading mechanism is just beginning to come into view. Because the Cu2+-binding modes at low and intermediate Cu2+ occupancy necessarily represent the highest-affinity binding modes, these are very likely populated under physiological conditions, and it is thus essential to characterize them in order to understand better the biological function of copper-prion interactions. Besides binding-affinity data, almost no other thermodynamic parameters (e.g., Delta H and Delta S) have been measured, thus leaving undetermined the enthalpic and entropic factors that govern the free energy of Cu2+ binding to the prion protein. In this study, isothermal titration calorimetry (ITC) was used to quantify the thermodynamic parameters (K, Delta G, Delta H, and T Delta S) of Cu2+ binding to a peptide, PrP(23-28, 57-98), that encompasses the majority of the residues implicated in Cu2+ binding by full-length PrP. Use of the buffer N-(2-acetomido)-aminoethanesulfonic acid (ACES), which is also a well-characterized Cu2+) chelator, allowed for the isolation of the two highest affinity binding events. Circular dichroism spectroscopy was used to characterize the different binding modes as a function of added Cu2+. The Kd values determined by ITC, 7 and 380 nM, are well in line with those reported by others. The first binding event benefits significantly from a positive entropy, whereas the second binding event is enthalpically driven. The thermodynamic values associated with Cu2+ binding by the A beta peptide, which is implicated in Alzheimer's disease, bear striking parallels to those found here for the prion protein.