The role of site-specific hydrogen bonding interactions in the solvation dynamics of N-acetyltryptophanamide

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