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)₁ 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.