Earliest events in alpha-synuclein fibrillation probed with the fluorescence of intrinsic tyrosines

The fluorescence of the four tyrosines of alpha-synuclein (Syn) was used for probing the earliest events preceding the fibrillation of Syn, during the onset of the so-called lag-time of fibrillation. Steady-state fluorescence experiments revealed an increase in the fluorescence intensity (FI) for Syn solutions at pH values 3 and 2, in comparison with pH 7, and fluorescence decays indicated that the FI increase did not result from suppression of excited-state proton transfer from the tyrosines to aspartates and glutamates, exposure of tyrosines to more hydrophobic environments, or reduction of homo-energy transfer. Instead, the FI increase was due to changes in the population of the tyrosine rotamers at low pH values. Stopped-flow experiments (pH-jumps) showed that the FI enhancement involves two processes: a fast (sub 7 ms) intramolecular (concentration-independent) process, which we assign to the protein collapse at low pH, and a slower intermolecular (concentration-dependent) process of protein dimerization/oligomerization, starting at 4-10 s after acidification. To the best of our knowledge, this is the first work on the experimental detection of these earliest processes in the fibrillation of Syn. (C) 2015 Elsevier B.V. All rights reserved.