Conformer‐selective Photodynamics of TrpH(+)−H(2)O

The photodynamics of protonated tryptophan and its mono hydrated complex TrpH(+)−H(2)O has been revisited. A combination of steady‐state IR and UV cryogenic ion spectroscopies with picosecond pump‐probe photodissociation experiments sheds new lights on the deactivation processes of TrpH(+) and conformer‐selected TrpH(+)−H(2)O complex, supported by quantum chemistry calculations at the DFT and coupled‐cluster levels for the ground and excited states, respectively. TrpH(+) excited at the band origin exhibits a transient of less than 100 ps, assigned to the lifetime of the excited state proton transfer (ESPT) structure. The two experimentally observed conformers of TrpH(+)−H(2)O have been assigned. A striking result arises from the conformer‐selective photodynamics of TrpH(+)−H(2)O, in which a single water molecule inserted in between the ammonium and the indole ring hinders the barrierless ESPT reaction responsible for the ultra‐fast deactivation process observed in the other conformer and in bare TrpH(+).