We have monitored the formation of toxic β-amyloid oligomers leading to Alzheimer's disease by detecting changes in the fluorescence decay of intrinsic tyrosine. A new approach based on the non-Debye model of fluorescence kinetics resolves the complexity of the underlying photophysics. The gradual disappearance of nonmonotonic fluorescence decay rates, at the early stages of aggregation as larger, tighter-packed oligomers are formed, is interpreted in terms of tyrosine-peptide dielectric relaxation influencing the decay. The results demonstrate the potential for a new type of fluorescence lifetime sensing based on dual excited-state/dielectric relaxation, with application across a broad range of biological molecules. The results also reconcile previously conflicting models of protein intrinsic fluorescence decay based on rotamers or dielectric relaxation by illustrating conditions under which both are manifest.

The fluorescence decay of beta-amyloid’s (Ab) intrinsic fluorophore tyrosine has been used for sensing the oligomer formation of dye-labelled Ab monomers and the results compared with previously studied oligomerization of the non-labelled Ab peptides. It has been demonstrated that two different sized, covalently bound probes 7-diethylaminocoumarin-3-carbonyl and Hilyte Fluor 488 (HLF), alter the rate and character of oligomerization to different extents. The ability of HLF to inhibit formation of highly ordered structures containing beta-sheets was also shown. The implications of our findings for using fluorescence methods in amyloidosis research are discussed and the advantages of this auto-fluorescence approach highlighted

The aggregation of beta amyloid (Ab) protein is associated with the development of Alzheimer's disease. In this work we monitor Ab aggregation using fluorescence anisotropy, a technique that provides information on the rotational diffusion of the fluorescing tyrosine (Tyr) side chains. We also perform Monte Carlo (MC) and fully atomistic Molecular Dynamics (MD) simulations to interpret the experiments. The experimental results show that there are two different rotational timescales contributing to the anisotropy. Our MC simulation captures this behaviour in a coarse-scale manner, and, more importantly, shows that the Tyr side chains must have their movements restricted in order to reproduce the anisotropy. The MD simulations provide a molecular scale view, and indeed show that aggregation restricts the Try side chains to yield anisotropy in line with the experimental results. This combination of experiment and simulation therefore provides a unique insight into the aggregation process, and we suggest how this approach might be used to gain further information on aggregating protein systems.