The Role of Site-Specific Hydrogen Bonding Interactions in theSolvation Dynamics of N‑Acetyltryptophanamide

Measurements of the ultrafast broadband UVfluorescence of N-acetyltryptophanamide (NATA) providedetailed information on its relaxation patterns in three differentsolvents: methanol (MeOH), water and acetonitrile (ACN).Several processes leading to excited state solvation and coolingare found to occur on different characteristic time scales andare thoroughly analyzed. Comparison between protic MeOHand aprotic ACN allows one to single out a 12 ps componentin the former, which is attributed to the rearrangement of H-bonds existing between the protic solvent and excited NATA. Thissignificantly stabilizes the excited state and provides the molecule with an efficient cooling mechanism. The correspondingdynamics in water are much faster (<1.5 ps). Comparison with static spectroscopic properties stresses the role of site-specific Hbondingin controlling the fluorescence quantum yield of NATA in protic solvents. These findings are consistent with existingmodels that describe tryptophan quenching as a result of charge transfer from the indole to the amide assisted by H-bonding atthe carbonyl site.