Ab initio study of the excited-state deactivation pathways of protonated tryptophan and tyrosine

In recent experiments, the excited-state lifetimes of protonated aromatic amino acids (TrpH(+) and TyrH(+)) have been recorded by means of pump-probe photodissociation technique. The lifetime of TyrH(+) is much longer than that of TrpH(+), which has been initially rationalized on the basis of a simple phenomenological model. Besides, specific photofragments including the formation of radical cation after hydrogen loss are observed for TrpH(+) that are not found for TyrH(+). The ab initio calculations reported here for TrpH(+) and TyrH(+) using a coupled-cluster method are meant to track the rich photochemistry of these protonated amino acids following UV excitation.