# EPR spectroscopy reveals different Cu(ii) coordination in APP142–172 and APP145–170 peptide fragments of amyloid precursor protein

## TL;DR

This study shows how small changes in a protein fragment affect copper binding and redox activity, which may influence Alzheimer's disease processes.

## Contribution

The study reveals how peptide length affects copper coordination and redox activity in APP fragments.

## Key findings

- APP142–172 adopts a single N2O2 coordination at physiological pH.
- APP145–170 exists in two coordination modes: N2O2 and 4N, depending on pH.
- Only the N2O2 species shows ferroxidase activity, while the 4N species is redox-silent.

## Abstract

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 APP142–172 and APP145–170 using electron paramagnetic resonance (EPR) and UV-Vis spectroscopy. At physiological pH, Cu(ii)–APP142–172 adopted a single N2O2 coordination, whereas Cu(ii)–APP145–170 existed in two distinct coordination modes identified by spectral simulation: the same N2O2 form (component I) as in Cu(ii)–APP142–172, 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.

Subtle APP sequence variations influence copper coordination: APP142–172 adopts redox-active N2O2 configuration while APP145–170 exhibits both redox-active N2O2 and redox-silent 4N modes.