Wavelength-Dependent Ultraviolet Photodissociation of Protonated Tryptamine

Ultraviolet photodissociation (UVPD) experiments of protonated tryptamine ([Tryp+H]⁺) have been implemented by a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer combined with a wavelength-tunable optical parametric oscillator (OPO) laser. UVPD mass spectra under different laser wavelengths have been obtained, in which the dependence of the yield of fragment ions on the laser wavelength was observed. The UVPD spectrum of [Tryp+H]⁺ has been obtained in the range of 210-310 nm. Besides the previously reported two competitive channels of H loss and NH₃ loss, two important channels of losing CH₂NH and CH₂NH₂ units were observed and further studied by UV-UV tandem mass spectrometry and theoretical calculations. Interestingly, results show that the pair of competitive channels of CH₂NH loss and CH₂NH₂ loss are both from the McLafferty-type rearrangement caused by ππ* electronic excited states. After the excitation, the two different dissociation pathways produce two different ion-neutral complexes, respectively. The wavelength-dependent dissociation and the existing competitive channels shown in this study reflect the diversity of UVPD processes of such organic molecules.