EPR spectroscopy reveals different Cu(ii) coordination in APP_142-172 and APP_145-170 peptide fragments of amyloid...

Amyloid precursor protein (APP) is central to Alzheimer's disease pathogenesis, yet the coordination chemistry and functional impact of core peptide fragments within its copper binding domain (CuBD) remain elusive. Here, we characterised the copper coordination environments and redox properties of two CuBD fragments APP_142-172 and APP_145-170 using electron paramagnetic resonance (EPR) and UV-Vis spectroscopy. At physiological pH, Cu(ii)-APP_142-172 adopted a single N₂O₂ coordination, whereas Cu(ii)-APP_145-170 existed in two distinct coordination modes identified by spectral simulation: the same N₂O₂ form (component I) as in Cu(ii)-APP_142-172, and a nitrogen-rich 4N form (component II). Moreover, EPR-monitored pH titrations revealed that the 4N species predominated at alkaline pH and the N₂O₂ species at acidic pH. Although both Cu(ii)-APP complexes could promote Fe(ii) oxidation, only the N₂O₂ species (component I) exhibited ferroxidase activity, whereas the 4N species (component II) was redox-silent. These observations demonstrate that subtle changes in peptide length act as a structural switch for Cu(ii) coordination and redox activity, thereby affecting the copper-mediated regulation of neuronal redox processes.