Stopped-flow kinetics reveal multiple phases of thioflavin T binding to Alzheimer beta (1-40) amyloid fibrils.

The benzothiazole dye thioflavin T (ThT) is a classical amyloid stain for senile plaques containing beta/A4 peptide in Alzheimer's disease brain. ThT also binds rapidly and specifically to the anti-parallel beta-sheet fibrils formed from synthetic beta (1-40) peptide, but does not bind to monomer or oligomeric intermediates. The fibrillar beta-sheet-bound dye species undergoes a characteristic 120 nm red shift of its excitation spectrum that may be selectively excited at 450 nm, resulting in a fluorescence signal at 482 nm. Mixing of preformed beta (1-40) amyloid fibrils with ThT in a stopped-flow spectrophotometer, monitoring fluorescence emission at > 475 nm while exciting at 450 nm, distinguished multiple kinetic phases of roughly equivalent amplitude with tau's in the ranges of 0.007, 0.05, 0.75, and 10-20 s. The fastest reaction appears to reflect a bimolecular dye binding event while the remaining reactions are rate-limited by protein tertiary or quaternary conformational changes. The high activation energies of the three slower reactions support this interpretation. The ThT concentration dependence of the reaction rates at different ratios of ThT/beta (1-40) amyloid fibrils rules out a rate-limiting conformational change occurring prior to ligand binding. ThT is a useful probe for the aggregated fibrillar state of beta (1-40) amyloid fibrils as the amyloid-specific fluorescence reports only fibrillar species. The binding of ThT does not interfere with the aggregation of this peptide into amyloid fibrils. The putative conformational changes detected by the ThT fluorescence suggest that small pharmacologic ligands can perturb and possibly dissociate A beta amyloid fibrils.