CD and Fluorescence Screening of α-Synuclein-Peptide Interactions.

α-Synuclein (AS), a natively unfolded protein, is the major components of the intracellular protein-aggregates, the Lewy bodies, found in the dopaminergic neurons of Parkinson's disease patients. The aggregates called "protofibrils," an intermediate in the fibrillogenesis process, are more cytotoxic than the amyloid-like fibrils in most of the proteins which generate fibrils. On one hand, aggregation inhibitors are expected to reduce AS cytotoxicity by preventing protofibril formation; on the other, an aggregation accelerator has recently been reported to reduce AS cytotoxicity, likely by causing protofibril precipitation. Therefore, amyloid aggregation modulating ligands are expected to serve as therapeutic medicines. In the present study, we evaluated the interaction of peptide ligands with AS by CD and fluorescence spectroscopies. For this purpose, we synthesized two peptides, H-RKVFYTW-NH2 and H-RGAVVTGR-NH2, named BB1 and BB2, respectively, and their all-D amino acid analogues. In addition, a rotamer-scan of the phenylalanine residue into the BB1 peptide was performed with the aim to evaluate the influence of the topography of this residue in the binding process. To this end, the Phe residue was replaced by L- or D-NMePhe, L- or D-Tic and Ala residues. Far-UV CD studies showed that AS conformation was strongly influenced by the interaction with these peptides. While the interaction with BB1 and BB2 induced an increase of the negative band at 198 nm, suggesting a corresponding increase of the unordered conformation of AS, other peptides caused a decrease of the same band. Surprisingly, L-NMePhe and L-Tic BB1 analogues did not interact with AS. The binding properties of the BB1 analogues was also confirmed by near-UV CD and fluorescence spectroscopies.