The product of NH3 loss from gas phase protonated tyrosine

Abstract

The lowest energy unimolecular dissociation product channel of protonated tyrosine, [Tyr + H]⁺, is loss of NH₃. The structure of the [Tyr − NH₃ + H]⁺ ion is still debated; past calculations suggest that the global minimum benzyl cation form is only accessible via a relatively high barrier and that a lower energy pathway to formation of the higher-energy phenonium isomer is likely to occur via collision-induced dissociation (CID). To resolve this open question, [Tyr − NH₃ + H]⁺ was studied computationally and experimentally using ion mobility spectrometry, ultraviolet photodissociation (UVPD) spectroscopy, and infrared ion spectroscopy (IRIS). Traveling wave ion mobility spectrometry (TWIMS) yields a collision cross section of Ω_N2 = 130.0 ± 1.4 Ų, which compares favorably with computed values of and for the benzyl cation and phenonium products, respectively. Differential mobility spectrometry and mass spectrometry were used to mobility- and mass-select [Tyr + H]⁺ prior to producing [Tyr − NH₃ + H]⁺via CID and subsequently measuring its UVPD spectrum. Similarly, [Tyr − NH₃ + H]⁺ was produced via CID prior to measuring its IRIS spectrum. The UVPD and IRIS spectra indicate that the phenonium ion is the major product formed via CID.