CD and fluorescence screening of alpha-synuclein-peptide interactions

alpha-Synuclein (AS) is the major component of the intracellular protein-aggregates found in the dopaminergic neurons of Parkinson's disease patients. A critical step in the aggregation of AS is the production of oligomers, which are more cytotoxic than the amyloid-like fibrils. Aggregation inhibitors are expected to reduce AS cytotoxicity by preventing oligomer formation; on the other hand, an aggregation accelerator has recently been reported to reduce AS cytotoxicity, likely by causing oligomer precipitation. Therefore, ligands that modulate amyloid aggregation may have a therapeutic potential. For this purpose, we synthesized two peptides, named BB1 and BB2, respectively, and their all-D amino acid analogues as potential amyloid aggregation modulating ligands. In addition, a rotamer-scan of the Phe4 residue into the BB1 peptide was performed with the aim to evaluate the influence of the topography of this residue in the binding process. Constraint-producing amino acid substitutions are typically used as modifier of the peptide/residue topography. The Tic residue is characterized by only two allowed side-chain orientations, the g(-) or g(+) conformations. The preferred conformation of the D-Tic residue into peptide chain is g(-), which corresponds to g(+) for an L-residue in the same position. For the L-NMePhe residue, all three side-chain conformations are possible in principle, but mostly the t and g(-) conformations are observed in peptides. The presence of a methyl group on the nitrogen constrains the psi angle of the preceding residue to a large value typical of those found in extended or beta-structures. In addition to providing topography restrictions, constraint residues can impart other properties to peptides, such as increased resistance to proteolysis.