Gas-Phase Structure and Dissociation Chemistry of Protonated Tryptophan Elucidated by Infrared Multiple-Photon Dissociation Spectroscopy

Abstract

We report infrared multiple-photon dissociation (IRMPD) spectra of protonated tryptophan, TrpH+, as well as the dissociation products from NH3 loss, [Trp+H−NH3]+, and subsequent CH2CO loss, [Trp+H−NH3−CH2CO]+. These results were obtained using a custom-built mass spectrometer, where the mass-isolated ions were photodissociated by a tunable optical parametric oscillator laser in a reduced-pressure (i.e., 10−5 mbar) “Paul-type” 3D ion trap. A comparison to computed spectra of putative conformations for TrpH+ shows that the protonated amino group, NH3+, interacts with the carboxylic acid carbonyl oxygen and the aromatic indole side chain. Ammonia loss is confirmed to occur via nucleophilic attack from C3 on the indole side chain, as opposed to C2 or C4, resulting in a spirocyclopropane structure with the charge located on the indole side chain. For the consecutive [Trp+H−NH3−CH2CO]+ product, the agreement with theory is slightly less persuasive, even if the qualitative trends are in accordance with the proposed structure.