(A) Cu(II) binding curves: molar ellipticity at 570 nm with increasing amounts of Cu(II), pH 7.5. ●, 2-His peptide, PrP(51–75) (0.34 mM). ⧫, 3-His peptide, PrP(66–91) (0.021 mM). ▴, 4-His peptide, PrP(58–91) (0.033 mM). (B) pH dependence of the ellipticity at 570 nm for PrP(58–91). The pH dependence curve has been fitted to the following equation: Δɛobs = {Δɛacid[H+]n + Δɛbase[H+] Kan }/ [H+]n + Kan}, where n = Hill coefficient and Ka = acid dissociation constant for the transition. The midpoint of the transition is pH 6.7.