Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation.

The electronic spectroscopy and the electronic excited state properties of cold protonated phenylalanine and protonated tyrosine have been revisited on a large spectral domain and interpreted by comparison with ab initio calculations. The protonated species are stored in a cryogenically cooled Paul trap, maintained at ∼10 K, and the parent and all the photofragment ions are mass-analyzed in a time-of-flight mass spectrometer, which allows detecting the ionic species with an improved mass resolution compared to what is routinely achieved with a quadrupole mass spectrometer. These new results emphasize the competition around the band origin between two proton transfer reactions from the ammonium group toward either the aromatic chromophore or the carboxylic acid group. These reactions are initiated by the coupling of the locally excited ππ* state with higher charge transfer states, the positions and coupling of which depend on the conformation of the protonated molecules. Each of these reaction processes gives rise to specific fragmentation channels that supports the conformer selectivity observed in the photofragmentation spectra of protonated tyrosine and phenylalanine.