X-ray beam effects on metallo-peptides reflect redox events: insights from X-ray absorption spectroscopy measurements on metal complexes of truncated amyloid-β peptides

The interaction between transition-metal ions and amyloid-β (Aβ) peptides is linked to the pathogenesis of Alzheimer's disease. X-ray absorption spectroscopy is widely used to investigate the coordination of these metal-peptide complexes, but exposure to synchrotron radiation can induce artefacts due to interaction with the X-ray beam. In this work, we examine the effects of X-ray irradiation on Cu(I), Cu(II) and Zn(II) complexes with two truncated Aβ fragments, Aβ_1-6 and Aβ_1-16. Experiments performed at 10 K reveal a marked photoreduction-associated effect: while the spectra of Cu(I)- and Zn(II)-bound peptides remain unchanged, Cu(II) complexes undergo significant spectral modifications. To probe structural relaxation following reduction, we exposed samples at 10 K, raised the temperature to 200 K and then collected additional spectra upon re-cooling. These experiments reveal that higher temperatures promote relaxation processes that are otherwise kinetically limited, and that the extent of relaxation, depending on the metal-binding mode, differs for Aβ_1-6 and Aβ_1-16. Overall, our experiments show that major structural modifications only take place in the presence of X-ray-induced metal reduction and that they are modulated by temperature. Thus, X-ray irradiation can be exploited not only as a probe but also as a trigger to study the redox-associated structural dynamics of copper-Aβ complexes and beyond.