Gas-phase reactions of protonated tryptophan

Abstract

The gas phase reactions of protonated tryptophan have been examined in a quadrupole ion trap using a combination of collision induced dissociation, hydrogen-deuterium exchange, regiospecific deuterium labeling and molecular orbital calculations (at the B3LYP/6-31G* level of theory). The loss of ammonia from protonated tryptophan is observed as the primary fragmentation pathway, with concomitant formation of a [M + H - NH(3)](+) ion by nucleophilic attack from the C3 position of the indole side chain. Hydrogen-deuterium exchange and regiospecific deuterium labeling reveals that scrambling of protons in the C2 and C4 positions of the indole ring, via intramolecular proton transfer from the thermodynamically preferred site of protonation at the amino nitrogen, precedes ammonia loss. Molecular orbital calculations have been employed to demonstrate that the activation barriers to intramolecular proton transfer are lower than that for NH(3) loss.

UV photoinduced dynamics in protonated aromatic amino acid

“…The fragmentation channels will thus be similar to the ones observed in CID 3,19,22 . However, one might expect that the branching ratio between the different fragmentation channels changes drastically.…”

“…Such structure is thought to give rise to the formation of the immonium ion as demonstrated by Ohair 19 . If the proton moves back to the amino group before fragmentation, then ammonia loss happens along with its secondary fragments at masses m/z= 146 and 144.…”

“…This process occurs in the ground state after a complex rearrangement 19 and has a barrier of 1 eV 3 . This is the lowest energy channel.…”

Photodissociation of protonated tryptamine and its supramolecular complex with 18-crown-6 ether: Dissociation times and channels, absorption spectra, and excited states calculations

“…We cannot exclude the formation of TryH + since there is large discrimination against storage of lower mass ions in this experiment. The loss of NH 3 and CH 3 NH 2 is intriguing since the CE is initially bound to the ammonium group; the isomer where CE is bound to protonated indole is higher in energy by 1 eV according to theoretical modeling [27,28] during dissociation along the C-NH 3 coordinate, the CE has time to rearrange. Likely an ion-molecule complex is formed from which either NH 3 or CH 3 NH 2 escapes.…”

On the role of dissociative πσ* states in the photochemistry of protonated tryptamine and tryptophan: An ab initio study

“…For all conformers, the protonation site is the amino group; other protonation sites lead to structures that are much higher in energy or are separated by large energy barriers [8,15]. The low-energy conformers preselected in this way were further optimized at the RI/MP2 level, using the Turbomole package with the SV(P) basis set for all atoms.…”