Difference in Cu(II)-binding modes of Aβ(1–16) between pH 7.4 and pH 6.0 suggested by ESR spectroscopy. While His6 accounts for approximately 50% of the Cu(II)–histidine coordination in Component I at pH 6.0, the contribution of His14 is at least as significant as that of His6 at pH 7.4. At pH 7.4, Component I is composed of three subcomponents, Subcomponent IA, IB, and IC, in each of which two imidazole rings from two different histidine residues coordinate to Cu(II).