Micro Solvation Effect of Methanol on Excited-State Dynamics of Protonated Tryptophan and Dopamine.

The electronic and vibrational cryogenic ion spectroscopy of protonated tryptophan (TrpH) and dopamine (DAH) complexed with methanol has been recorded. These two biological chromophores exhibit ultrafast photochemistry due to excited-state proton transfer (ESPT). We have established the relationship between the structure of the complexes and their photodynamics and compared them with recent results obtained in hydrated complexes. For TrpH, there is no substantial change between methanol and water complexes; ESPT is hindered by a single solvent molecule. In the DAH(MeOH) complex, the most stable conformer adopts a structure that prevents the direct interaction of the ammonium group of the side chain with the catechol ring, thus blocking the ESPT reaction. Such a ring structure is indeed a very minor populated conformer in the single-hydrated complex. The change in conformal stability between water and methanol clusters is due to a weak CH-π attractive interaction of the methyl group of methanol with the catechol.