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

Abstract

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 Cα–Cβ 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 ππ*/πσ* state, the ππ* state being mainly located on the indole chromophore while the πσ* is mainly on the protonated terminal amino group. This πσ* 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.