Chasing the Elusive “In‐Between” State of the Copper‐Amyloid <i>β</i> Complex by X‐ray Absorption through Partial Thermal Relaxation after Photoreduction

The redox activity of Cu ions bound to the amyloid-β (Aβ) peptide is implicated as a source oxidative stress in context Alzheimer's disease. In order explain efficient cycling between CuII-Aβ (distorted square-pyramidal) and CuI-Aβ (digonal) resting states, existence low-populated “in-between” state, prone bind both oxidation has been postulated. Here, we exploited partial X-ray induced photoreduction at 10 K, followed by thermal relaxation 200 trap characterize Absorption Spectroscopy (XAS) partially reduced Cu-Aβ1–16 species different from states. Remarkably, XAS spectrum well-fitted previously proposed model hence providing first direct spectroscopic characterization an intermediate state. present approach could be used explore identify catalytic intermediates other relevant metal complexes.