Modification of amyloid-beta peptide aggregation via photoactivation of Ru(II) polypyridyl complexes

Alzheimer’s disease (AD) is a chronic neurodegenerative disease characterized by progressive and irreversible damage to the brain. One of the hallmarks of the disease is the presence of both soluble and insoluble aggregates of the amyloid beta (Ab) peptide in the brain. In this work we investigate how photoactivation of three Ru(II) polypyridyl complexes [Ru(6,6’-dimethyl-2,2’-dipyridyl)2(2-thiophen-2-yl-1H-imidazo(4,5-f) (1,10)phenanthroline)] (Ru1), [Ru(6,6’-dimethyl-2,2’-dipyridyl)2(2-phenyl-1H-imidazo (4,5-f)(1,10)phenanthroline)] (Ru2), and [Ru(6,6’-dimethyl-2,2’-dipyridyl)2(2,2’-bipyridine)] (Ru3), alters the aggregation profile of the Ab peptide. Both Ru1 and Ru2 contain an extended planar (4,5-f)(1,10)phenanthroline ligand, as compared to a 2,2’-bipyridine ligand for Ru3, and we show that the presence of the phenanthroline ligand leads to a greater effect on peptide aggregation. The ability of photoactivated Ru1-3 to bind to the Ab peptide was evaluated by Nuclear Magnetic Resonance (NMR) which indicated the loss of the 6,6’-dimethyl-2,2’-bipyridyl (6,6’-dmb) ligand for all three complexes and the formation of a covalent bond with the Ab peptide via His residue shifts for Ru1 and Ru2. By comparison, no shift in His residues was observed for Ru3, or for the unactivated Ru1-3 samples. The influence of Ru1-3 on peptide aggregation was investigated using gel electrophoresis / Western blot, Transmission electron microscopy (TEM) and a Bicinchoninic acid assay (BCA assay). Upon photoactivation, the Ab aggregation was greatly enhanced in the presence of Ru1 and Ru2 relative to Ru3, in agreement with initial binding studies by 1H NMR. However, the three complexes resulted in a similar aggregate size distribution at 24 h, forming mostly insoluble amorphous aggregates. Excitingly, the complexes also changed Ab1-42 fibrils to amorphous aggregates upon photoactivation. The unactivated Ru1 and Ru2 complexes exhibited a much stronger binding affinity for Ab (via Tyr10 fluorescence) in comparison to Ru3, further indicating the important role of hydrophobic interactions between the Ru complexes and the insoluble fibrillar peptide aggregates. Overall, our results show that upon photoactivation the extended planar ligand of Ru1 and Ru2 promotes immediate covalent binding and formation of soluble high molecular weight Ab aggregates in comparison to Ru3, however similar aggregate size and morphology is observed after 24 h for all three Ru complexes.