Exploring the Mechanisms behind Non-aromatic Fluorescence with the Density Functional Tight Binding Method.

Area of Science:

• Biophysics
• Computational Chemistry
• Materials Science

Background:

• Non-aromatic fluorescence (NAF) is observed in aggregated biological systems without conjugated bonds.
• Previous studies identified carbonyl stretching and short hydrogen bonds (SHBs) as key factors in small model systems.
• Understanding NAF in realistic biological environments requires advanced simulation techniques.

Purpose of the Study:

• To investigate the mechanism of NAF in the crystal structure of l-pyroglutamine-ammonium.
• To compare NAF in l-pyroglutamine-ammonium with the nonfluorescent l-glutamine.
• To validate the density functional tight-binding (DFTB) method combined with non-adiabatic molecular dynamics (NAMD) for simulating NAF in biological systems.

Main Methods:

• Density functional tight-binding (DFTB) method.
• Non-adiabatic molecular dynamics (NAMD).
• Mixed quantum/molecular mechanics (QM/MM) approach.

Main Results:

• The DFTB/NAMD/QM/MM approach successfully simulates NAF in the l-pyroglutamine-ammonium crystal structure.
• The findings confirm the importance of carbonyl stretching and SHBs in NAF.
• The computational cost allows for better sampling of nonradiative events at conical intersections.

Conclusions:

• The proposed DFTB method is efficient and robust for NAF studies in biological systems.
• This work provides a deeper understanding of NAF mechanisms in realistic environments.
• The method is applicable to complex biological systems like amyloid aggregates and non-aromatic proteins.