Tyrosine Rotamer States in Beta Amyloid: Signatures of Aggregation and Fibrillation.

During the early stages of β amyloid (Ab) peptide aggregation, toxic oligomers form which have been recognized as a likely cause of Alzheimer's disease. In this work, we use fully atomistic molecular dynamics simulation to study the amorphous aggregation of the peptide as well as model β-sheet protofibril structures. In particular, we study the rotamer states of the single fluorescent tyrosine (Tyr) residue present in each Ab. We find that the occupation of the four previously identified rotamers is different for monomeric and amorphous aggregates because of the differing environments of the Tyr side-chains. Surprisingly, we also identify two new rotamers that uniquely appear for the β-sheet structures, so that together the rotamers provide distinct signatures for the different stages of aggregation and fibrillation. We propose that these rotamers could be identified in fluorescence spectroscopy, with each rotamer having a distinct fluorescence lifetime because of its different exposures to the solvent. The identification of the two new rotamers therefore provides a new means to probe amyloid formation kinetics and to monitor the effect of additives including prospective drugs.