Isotopic Dilution Provides No Evidence for Cu2+-Bridged β-Amyloid Homo-Oligomers

Abstract The oligomerization of many peptides is known to be influenced by the binding of transition metal ions, with intermolecular metal–ion bridges being one possible mechanism. In the case of copper ions, it is possible to identify Cu 2+ bridges between peptide monomers using EPR spectroscopy by analyzing the metal–ligand hyperfine structure in the spectra of a mixture of two peptides, each having the same amino acid sequence but a different isotopic content. This isotopic dilution method was previously used to show that the N-terminus of the misfolding peptide α-synuclein forms antiparallel Cu 2+ -bridged dimers, and such a quaternary structure was also proposed for the Cu 2+ -bound form of another misfolding peptide, β-amyloid (Aβ). To investigate this possibility, the isotopic dilution strategy was applied to the Aβ1–16 peptide, which contains the complete N-terminal Cu 2+ -binding domain of Aβ. Rigorous analysis of the superhyperfine structure in experimental and simulated low-frequency EPR spectra provided no conclusive evidence for Cu 2+ bridging of Aβ monomers in the pH range 6.5–8.5.