Immunotherapy Against N-Truncated Amyloid-β Oligomers

Nanostructure and dynamics of N-truncated copper amyloid-β peptides from advanced X-ray absorption fine structure

An X-ray absorption spectroscopy (XAS) electrochemical cell was used to collect high-quality XAS measurements of N-truncated Cu:amyloid-β (Cu:Aβ) samples under near-physiological conditions. N-truncated Cu:Aβ peptide complexes contribute to oxidative stress and neurotoxicity in Alzheimer's patients' brains. However, the redox properties of copper in different Aβ peptide sequences are inconsistent. Therefore, the geometry of binding sites for the copper binding in Aβ4-8/12/16 was determined using novel advanced extended X-ray absorption fine structure (EXAFS) analysis. This enables these peptides to perform redox cycles in a manner that might produce toxicity in human brains. Fluorescence XAS measurements were corrected for systematic errors including defective-pixel data, monochromator glitches and dispersion of pixel spectra. Experimental uncertainties at each data point were measured explicitly from the point-wise variance of corrected pixel measurements. The copper-binding environments of Aβ4-8/12/16 were precisely determined by fitting XAS measurements with propagated experimental uncertainties, advanced analysis and hypothesis testing, providing a mechanism to pursue many similarly complex questions in bioscience. The low-temperature XAS measurements here determine that CuII is bound to the first amino acids in the high-affinity amino-terminal copper and nickel (ATCUN) binding motif with an oxygen in a tetragonal pyramid geometry in the Aβ4-8/12/16 peptides. Room-temperature XAS electrochemical-cell measurements observe metal reduction in the Aβ4-16 peptide. Robust investigations of XAS provide structural details of CuII binding with a very different bis-His motif and a water oxygen in a quasi-tetrahedral geometry. Oxidized XAS measurements of Aβ4-12/16 imply that both CuII and CuIII are accommodated in an ATCUN-like binding site. Hypotheses for these CuI, CuII and CuIII geometries were proven and disproven using the novel data and statistical analysis including F tests. Structural parameters were determined with an accuracy some tenfold better than literature claims of past work. A new protocol was also developed using EXAFS data analysis for monitoring radiation damage. This gives a template for advanced analysis of complex biosystems.

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 Cu^II amyloid-β peptide (Aβ) 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 Aβ_1-16 and the N-truncated Aβ_4-9, Aβ_4-12, and Aβ_4-16 (Aβ_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^II_ATCUN) 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 Aβ_4-9/12/16 peptides not previously reported. Both XAS-EC and EPR measurements show that Cu^II:Aβ_4-16 can be reduced at mildly reducing potentials, similar to that of Cu^II:Aβ_1-16. Reduction of peptides lacking the H₁₃H₁₄ residues, Cu^II:Aβ_4-9/12, require far more forcing conditions, with metallic copper the only metal-based reduction product. The observations suggest that reduction of Cu^II_ATCUN 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 (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 Cu^II:Aβ_4-12/16 gives structural parameters consistent with crystallographic data for a five-coordinate Cu^III complex and the Cu^II_ATCUN complex. The structural results suggest that Cu^II and the oxidation product are both accommodated in an ATCUN-like binding site.