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 þ ) 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 þ ) is produced in this process that subsequently leads to the m/z ¼ 130 fragment through a C a -C b bond cleavage, a typical fragmentation product of the Trp þ 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 þ produces a mixed pp*/ps* state, the pp* state being mainly located on the indole chromophore while the ps* is mainly on the protonated terminal amino group. This ps* state is repulsive along the N-H bond coordinate and leads either to hydrogen atom detachment producing a Trp þ radical cation that undergoes further fragmentations or to internal conversion to the ground state of the protonated TrpH þ ion.

UV photoinduced dynamics in protonated aromatic amino acid

“…The observation of the m/z=204 channel (H loss) (see figure 2) and ab-initio calculations have been the key to understand the possible deactivation pathways 6,12,13,17 . At the same energy as the locally excited state where the excited electron is in a π* orbital on the indole ring, there is a πσ* state where the electron is localized on the protonated amino group.…”

Photo-Induced Dissociation of Protonated Peptide Ions in a Quadrupole Ion Trap Time-of-Flight Mass Spectrometer

“…22 These fragment ions including those of m/z 132 and 146 were also observed in the previous PID of WH + , 9 which indicates that these ions are from dissociation of the tryptophan residue of GWH + . The fragment ions of m/z 216 and 244 that have strong intensity in the CID spectrum are absent in the PID spectrum.…”

“…The PID of peptide ions has been studied by irradiating IR 2,8 or UV beams onto the ions in the trap. 9 The photodissociation dynamics of multiply charged ions, ligated/solvated transition metal dications and metal cluster ions were investigated using optical spectroscopy. 10,11 The dissociation dynamics of single amino acid or peptide ions 12 were studied using nanosecond and femtosecond laser pulses.…”

Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces

“…Reactions occurring in excited electronic states are also examples in which the dynamics often proceeds far from the minimum energy path and non-equilibrium phenomena often observed [69][70][71][72][73][74]. These dynamics may have different origins, but an important issue is that excited state potential energy barriers are often very small or non-existent, such that the system reacts much before statistical equilibration of the original ground state nuclear wave-packet.…”