  Recent experimental findings reveal non-conventional fluorescence emission in
biological systems devoid of conjugated bonds or aromatic compounds, termed
\textit{Non-Aromatic Fluorescence} (NAF). This phenomenon is exclusive to
aggregated or solid states, remaining absent in monomeric solutions. Previous
studies focused on small model systems in vacuum show that the carbonyl
stretching mode along with strong interaction of short hydrogen bonds (SHBs)
remain the primary vibrational mode explaining NAF in these systems. In order
to simulate larger model systems taking into account the effects of the
surrounding environment, in this work we propose using the density functional
tight-binding (DFTB) method in combination with non-adiabatic molecular
dynamics (NAMD) and the mixed quantum/molecular mechanics (QM/MM) approach. We
investigate the mechanism behind NAF in the crystal structure of
L-pyroglutamine-ammonium, comparing it with the related non-fluorescent amino
acid L-glutamine. Our results extend our previous findings to more realistic
systems, demonstrating the efficiency and robustness of the proposed DFTB
method in the context of NAMD in biological systems. Furthemore, due to its
inherent low computational cost, this method allows for a better sampling on
the non-radiative events at the conical intersection which is crucial for a
complete understanding of this phenomenon. Beyond contributing to the ongoing
exploration of NAF, this work paves the way for future application of this
method in more complex biological systems such as amyloid aggregates,
biomaterials and non-aromatic proteins.
