In the last years the phenomenon of green-blue luminescence of fibrillar aggregates of proteins has received constant attention from experimental and theoretical sides. Besides their unique structural, chemical, and optical properties, amyloids feature near-UV excitation and characteristic blue-green luminescence which photophysical mechanism is particularly hard to address in amyloid systems devoid of aromatic amino acids.<br/>We designed and computationally investigated model amyloid-like peptides, and identified the molecular ingredients that are able to originate such behaviour. We used ab initio static calculations and nonadiabatic dynamics simulations to study the excited electronic states of these systems. We discovered that combinations of electronic features and atomic motions are able to shape the photophysics of lowest-lying carbonyl nπ* excitations. In particular, the dense set of nπ* excited states, localized on the amide groups, results to be strongly stabilized by the amide deplanarization and the concomitant increase of the oscillator strengths is compatible with the observed excitations in the near-UV.  Our dynamics simulations revealed that photoexctation is coupled to a marked C=O elongation, suggesting that in the emission process the amyloid cross β arrangement effectively hinders the nonradiative relaxation channels, usually operative in proteins. Besides, we observed that after relaxation of the photoexcited peptides fluorescence should take place in the visible (green) part of the electromagnetic spectrum.

Deciphering optical properties of amyloids: A reaction path and nonadiabatic dynamics study

ACS Spring Meeting 2021

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