Cu and Zn have been shown to accumulate in the brains of Alzheimer's disease patients. We have previously reported that Cu2+ and Zn2+ bind amyloid beta (A beta), explaining their enrichment in plaque pathology. Here we detail the stoichiometries and binding affinities of multiple cooperative Cu2+-binding sites on synthetic A beta 1-40 and A beta 1-42. We have developed a ligand displacement technique (competitive metal capture analysis) that uses metal-chelator complexes to evaluate metal ion binding to A beta, a notoriously self-aggregating peptide. This analysis indicated that there is a very-high-affinity Cu2+-binding site on A beta 1-42 (log K-app = 17.2) that mediates peptide precipitation and that the tendency of this peptide to self-aggregate in aqueous solutions is due to the presence of trace Cu2+ contamination (customarily similar to 0.1 mu M). In contrast, A beta 1-40 has much lower affinity for Cu2+ at this site (estimated log K-app = 10.3), explaining why this peptide is less self-aggregating. The greater Cu2+-binding affinity of A beta 1-42 compared with A beta 1-40 is associated with significantly diminished negative cooperativity, The role of trace metal contamination in inducing A beta precipitation was confirmed by the demonstration that A beta peptide (10 mu M) remained soluble for 5 days only in the presence of high-affinity Cu2+-selective chelators.