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

Abstract

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 nπ * states with excitation localized on the amide groups. The strong stabilization of the nπ * states with respect to the amide group deplanarization and the concomitant increase of the oscillator strength makes excitation in the near UV possible. With respect to emission, our dynamics simulations revealed that the amyloid cross β 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 nπ * states, fluorescence takes place in the visible (green) part of the electromagnetic spectrum.

On-off transition and ultrafast decay of amino acid luminescence driven by modulation of supramolecular packing

“…The second decay component was characterized by 12.6 ps decay. The presence of the sub-picosecond fluorescence decay is in agreement with the calculation of the recent published work 35 , where the spectral diffusion and relaxation was observed on a few hundred picoseconds time scale.…”

“…This paradigm was extended in the recent work of Grisanti et al, where, using ab initio calculations, fluorescence properties of model peptides aggregates were assessed. 35 Besides the prediction of the emission in the visible spectral range, a conclusion was made on the time-resolved behavior of the aggregation induced fluorescence. Namely, the presence of an ultrafast fluorescence decay and accompanying spectral diffusion, i.e.…”

“…In the work of Grisanti et al 35 , the ab initio nonadiabatic dynamics simulations were used to reveal characteristic properties of the excited electronic states of model amyloid-like peptides.…”

“…The ultrafast relaxation occurs on the time scale of ~100 fs, corresponding to the non-radiative decay rate of ~10 12 -10 13 s -1 , which is, however, lower than in the case of monomeric proteins, where the lack of stabilization of the nπ* states results in the absence of near-UV and visible absorption and blue-green fluorescence. 35 Moreover, the presence of the ultrafast decay may result in low fluorescence quantum yield of the aggregationinduced emission of proteins (~0.01). 11 Further relaxation from the lowest energy nπ*min state may then occur on a longer time scale due to the stabilization of the geometry of the aggregate, and this relaxation corresponds to the 1-2 ns fluorescence lifetime, which was observed for the self-assembly induced fluorescence in this paper (Figure 4d) and other previous works.…”

“…21 Hence, by detecting the ultrafast decay of the fibrillar structures formed as the result of phenylalanine self-assembly, the model of Grisanti et al can be extended from peptides to amino acids, therefore elucidating the origin of their enigmatic fluorescence emission upon packing. 35…”