Tryptophan Fluorescence Reveals Structural Features of α-Synuclein Oligomers

Oligomeric alpha-synuclein (alpha S) is considered to be the potential toxic species responsible for the onset and progression of Parkinson's disease, possibly through the disruption of lipid membranes. Although there is evidence that oligomers contain considerable amounts of secondary structure, more detailed data on the structural characteristics and how these mediate oligomer-lipid binding are critically lacking. This report is, to our knowledge, the first study that aimed to address the structure of oligomeric alpha S on a more detailed level. We have used tryptophan (Trp) fluorescence spectroscopy to gain insight into the structural features of oligomeric alpha S and the structural basis for oligomer-lipid interactions. Several single Trp mutants of alpha S were used to gain site-specific information about the microenvironments of monomeric alpha S, oligomeric alpha S and lipid-bound oligomeric alpha S. Acrylamide quenching and spectral analyses indicate that the Trp residues are considerably more solvent protected in the oligomeric form compared with the monomeric protein. In the oligomers, the negatively charged C-terminus was the most solvent exposed part of the protein. Upon lipid binding, a blue shift in fluorescence was observed for alpha S mutants where the Trp is located within the N-terminal region. These results suggest that, as in the case of monomeric alpha S, the N-terminus is critical in determining oligomer-lipid binding. (C) 2009 Elsevier Ltd. All rights reserved.