Short hydrogen bonds enhance non-aromatic protein-related fluorescence

Here we show experimentally and computationally that the single amino acid L-glutamine, by undergoing a chemical transformation leading to the formation of a short hydrogen bond, displays optical properties that are significantly enhanced compared to L-glutamine itself. We highlight the importance of the direct local environment on the extent of the Stokes shift observed in fluorescent non-aromatic protein-like structures. Ab initio molecular dynamics simulations reveal that these short hydrogen bonds are characterised by proton transfer events. State-of-the-art excited state simulations demonstrate that systems with short hydrogen bonds lend themselves to lower non-radiative transition probabilities due to the inhibition of vibrational modes involving the ring and carbonyl groups. We further show that short hydrogen bonds significantly decrease the chance of a conical intersection in the excited state. We thus conclude that specific hydrogen bonding networks significantly enhance non-aromatic protein-related fluorescence.