Conformational analysis of thioflavin T bound to the surface of amyloid fibrils.

The interaction of small molecules with the surface of amyloid assemblies is important for the detection and inhibition of amyloid formation. Thioflavin T (ThT), a small molecular rotor, has been used for the detection of amyloid fibrils for over half a century. The basis for detection is simple in that in the presence of fibrils the fluorescence of ThT is dramatically enhanced. The mechanism for this enhancement is not well understood but may depend on the determination of the conformation of ThT bound to the fibril surface. Here, we first use solution-state (1)H NMR to show that the on-off binding of ThT to the surface of insulin amyloid fibrils correlates with the enhancement of ThT fluorescence. We then show that the conformation of surface-bound ThT is twisted. The implications of this result in light of recent experimental and computational studies of the binding of ThT to amyloid or amyloid-like assemblies are discussed.