Structural Insight into Redox Dynamics of Copper Bound N-Truncated Amyloid-β Peptides from in Situ X-ray Absorption Spectroscopy.

X-ray absorption spectroscopy of CuII amyloid-β peptide (Aβ) under in situ electrochemical control (XAS-EC) has allowed elucidation of the redox properties of CuII bound to truncated peptide forms. The Cu binding environment is significantly different for the Aβ1-16 and the N-truncated Aβ4-9, Aβ4-12, and Aβ4-16 (Aβ4-9/12/16) peptides, where the N-truncated sequence (F4R5H6) provides the high-affinity amino-terminal copper nickel (ATCUN) binding motif. Low temperature (ca. 10 K) XAS measurements show the adoption of identical CuII ATCUN-type binding sites (CuIIATCUN) by the first three amino acids (FRH) and a longer-range interaction modeled as an oxygen donor ligand, most likely water, to give a tetragonal pyramid geometry in the Aβ4-9/12/16 peptides not previously reported. Both XAS-EC and EPR measurements show that CuII:Aβ4-16 can be reduced at mildly reducing potentials, similar to that of CuII:Aβ1-16. Reduction of peptides lacking the H13H14 residues, CuII:Aβ4-9/12, require far more forcing conditions, with metallic copper the only metal-based reduction product. The observations suggest that reduction of CuIIATCUN species at mild potentials is possible, although the rate of reduction is significantly enhanced by involvement of H13H14. XAS-EC analysis reveals that, following reduction, the peptide acts as a terdentate ligand to CuI (H13, H14 together with the linking amide oxygen atom). Modeling of the EXAFS is most consistent with coordination of an additional water oxygen atom to give a quasi-tetrahedral geometry. XAS-EC analysis of oxidized CuII:Aβ4-12/16 gives structural parameters consistent with crystallographic data for a five-coordinate CuIII complex and the CuIIATCUN complex. The structural results suggest that CuII and the oxidation product are both accommodated in an ATCUN-like binding site.