Toward Understanding Optical Properties of Amyloids: A Reaction Path and Nonadiabatic Dynamics Study

Amyloids have unique structural, chemical, and optical properties. Although much theoretical effort has been directed toward understanding amyloid nucleation, the understanding of their optical properties has remained rather limited. In particular, the photophysical mechanisms leading to near-UV excitation and characteristic blue-green luminescence in amyloid systems devoid of aromatic amino acids have not been resolved. We use ab initio static calculations and nonadiabatic dynamics simulations to study the excited electronic states of model amyloid-like peptides. We show that their photophysics is essentially governed by the multitude of n pi* states with excitation localized on the amide groups. The strong stabilization of the n pi* states with respect to the amide group deplanarization and the concomitant increase of the oscillator strength make excitation in the near-UV possible. With respect to emission, our dynamics simulations revealed that the amyloid cross beta arrangement effectively hinders the nonradiative relaxation channels usually operative in proteins. Finally, we show that after relaxation of the photoexcited peptides toward the minimum of the different n pi* states, fluorescence takes place in the visible (green) part of the electromagnetic spectrum.