The amyloid beta (A beta) peptide of Alzheimer's disease binds copper(II), and the peptide-bound metal may be a source of reactive oxygen species and neurotoxicity. To circumvent peptide aggregation and reduce redox activity, there is growing interest in using metal chelates as drug therapeutics for AD, whose design requires accurate data on the affinity of A beta peptides for copper(II). Reports on Cu2+ binding to A beta range from similar to 10(5) to similar to 10(9); these values' being obtained for different peptide lengths (1-16, 1-28, 1-40, 1-42) at varying pH. Herein, we report that Cu2+' s binding to A beta(1-40) at 37 degrees C occurs in a 1:1 stoichiometry with a pH-dependent binding constant: 1.1 (+/- 0.2) x 10(9) M-1 and 2.4 (+/-0.2) x 10(9) M-1 at pH 7.2 and 7.4, respectively. Under identical conditions, A beta(1-16) reveals a comparable binding constant, confirming that this portion of the peptide is the binding region. Several previously reported values can be reconciled with the current measurement by careful consideration of thermodynamics associated with the presence of competing ligands used to solubilize copper.