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

X-ray absorption spectroscopy of Cu-II amyloid-beta peptide (A beta) under in situ electrochemical control (XAS-EC) has allowed elucidation of the redox properties of Cu-II bound to truncated peptide forms. The Cu binding environment is significantly different for the A beta(1-16) and the N-truncated A beta(4-9), A beta(4-12), and A beta(4-16) (A beta(4-9/12/16)) 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 Cu-II ATCUN-type binding sites (Cu-ATCUN(II)) 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 A beta(4-9/12/16) peptides not previously reported. Both XAS-EC and EPR measurements show that Cu-II:A beta(4-16) can be reduced at mildly reducing potentials, similar to that of Cu-II:A beta(1-16). Reduction of peptides lacking the H13H14 residues, Cu-II:A beta(4-9/12), require far more forcing conditions, with metallic copper the only metal-based reduction product. The observations suggest that reduction of Cu-ATCUN(II) 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 Cu-I (H-13,H-14 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 AB Cu-II:A beta(4-12/16) gives structural parameters consistent with crystallographic data for a five-coordinate Cu-III complex and the Cu-ATCUN(II) complex. The structural results suggest that Cu-II and the oxidation product are both accommodated in an ATCUN-like binding site.