Dissecting two-dimensional ultraviolet spectra of amyloid fibrils into beta-strand and turn contributions.

We present an analysis of the contributions of various secondary structure elements of the amyloid β-protein to the two-dimensional far ultraviolet (2DFUV) signal of an amyloid fibril model. The contributions of the turns and the β-strands are affected by the geometry of the backbone peptide amide π → π* transition dipoles, the backbone interamide coupling in the excited state, and the exciton delocalization. These contributions are clearly distinguishable in the xyxy-xyyx pulse polarization configuration. The differences are attributed to the smaller splitting of the exciton energies and the larger fluctuations of the geometry of the peptide amide π → π* transition dipoles at the turns, while making the 2DFUV signal sensitive to the secondary structure. This signal may be used to determine the proportion of turns and β-strands.