Metal ions have been shown to play a critical role in β-amyloid (Aβ) neurotoxicity, thus prompting an intense investigation into the formation of metal-Aβ complexes. Isothermal titration calorimetry (ITC) has been widely used to determine binding constants (K) for a variety of metal-protein interactions, including those in metal-Aβ complexes. In this study, ITC was used to more fully quantify the thermodynamics (K, ΔG, ΔH, and TΔS) of Cu(2+) binding to Aβ16, N-acetyl-Aβ16, Aβ28, N-acetyl-Aβ28, and Aβ28 variants (H6A, H13A, H14A) at pH 7.4 and 37 °C. After deconvolution of competing reactions, K for Aβ16 was found to be 1.1 (±0.13) × 10(9) and is in strong agreement with literature values measured under similar conditions. Further, a similar K value was obtained at two additional concentrations of competing ligand, suggesting that ternary complex formation is not significant. The acetylated peptide analogs reveal a marked decrease in the overall free energy upon binding, which is the result of less favorable enthalpic and entropic contributions. Circular dichroism spectroscopy shows conformational changes that are consistent with these results. Most importantly, data for Aβ28 variants lacking a potential Cu(2+)-binding histidine residue reveal that the overall free energy of binding remains constant, which is the result of entropy/enthalpy compensation. These data provide fundamental thermodynamic evidence for coordination plasticity in Cu(2+) binding to Aβ and other intrinsically disordered peptides.