Controlled and Selective Photo-oxidation of Amyloid-β Fibrils by Oligomeric p-Phenylene Ethynylenes

Photodynamic therapy (PDT) has been explored as a therapeutic strategy to clear toxic amyloid aggregates involved in neurodegenerative disorders such as Alzheimer’s disease. A major limitation of PDT is off-target oxidation, which can be lethal for the surrounding cells. We have shown that a novel class of oligo- p -phenylene ethynylene-based compounds (OPEs) exhibit selective binding and fluorescence turn-on in the presence of pre-fibrillar and fibrillar aggregates of disease-relevant proteins such as amyloid-β (Aβ) and α-synuclein. Concomitant with fluorescence turn-on, OPE also photosensitizes singlet oxygen under illumination through the generation of a triplet state, pointing to the potential application of OPEs as photosensitizers in PDT. Herein, we investigated the photosensitizing activity of an anionic OPE for the photo-oxidation of toxic Aβ aggregates and compared its efficacy to the well-known but non-selective photosensitizer methylene blue (MB). Our results show that while MB photo-oxidized both monomeric and fibrillar conformers of Aβ40, OPE oxidized only Aβ40 fibrils, targeting two histidine residues on the fibril surface and a methionine residue located in the fibril core. Oxidized fibrils were shorter and more dispersed, but retained the characteristic β-sheet rich fibrillar structure and the ability to seed further fibril growth. Importantly, the oxidized fibrils displayed low toxicity. We have thus discovered a class of novel theranostics for the simultaneous detection and oxidization of amyloid aggregates. Importantly, the selectivity of OPE’s photosensitizing activity overcomes the limitation of off-target oxidation of currently available photosensitizers, and represents a significant advancement of PDT as a viable strategy to treat neurodegenerative disorders. ### Competing Interest Statement The authors have declared no competing interest. * AAA : amino acid analysis Aβ : amyloid β CD : circular dichroism CTAB : cetyltrimethylammonium bromide DMEM : Dulbecco’s Modified Eagle’s Medium DMSO : dimethyl sulfoxide DNPH : 2,4-Dinitrophenylhydrazine ESI-MS : electrospray ionization mass spectrometry FBS : fetal bovine serum GAFF : generalized force field HCl : hydrochloride acid RP-HPLC : reverse phase high performance liquid chromatography MB : methylene blue MD : molecular dynamics NaCl : sodium chloride OPE : oligo-p-phenylene ethynylene PDB : protein databank PDT : photodynamic therapy PS : penicillin-streptomycin TPBS : pH 7.4 phosphate buffered saline containing 0.1% Tween 20 TEM : transmission electron microscopy TFA : trifluoroacetic acid