Photo-induced dissociation of protonated tryptophan TrpH+:: A direct dissociation channel in the excited states controls the hydrogen atom loss

Protonated tryptophan ions (TrpH(+)) are generated by electrospray ionization and dissociated by irradiation with a UV laser. Different photo-fragments are observed among which a new photo-induced dissociation channel leading to the loss of a hydrogen atom that is not observed in conventional collision-induced dissociation. A tryptophan radical cation (Trp(+)) is produced in this process that subsequently leads to the m/z = 130 fragment through a C-alpha-C-beta bond cleavage, a typical fragmentation product of the Trp(+) radical cation generated either by electron impact or by photo-ionization. These results can be understood considering the excited states of protonated tryptophan: UV excitation of TrpH(+) produces a mixed pipi*/pisigma* state, the pipi* state being mainly located on the indole chromophore while the pisigma* is mainly on the protonated terminal amino group. This pisigma* state is repulsive along the N-H bond coordinate and leads either to hydrogen atom detachment producing a Trp(+) radical cation that undergoes further fragmentations or to internal conversion to the ground state of the protonated TrpH(+) ion.