Intramolecular hydrogen transfer in the ionization process of α-alanine

Abstract

Photoionization and dissociative photoionization of α-alanine have been studied with infrared laser desorption/tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry (IR LD/VUV PIMS) and theoretical calculations. By scanning photoionization efficiency (PIE) spectra, appearance energies (AEs) of the fragments CH₃ĊHNH₃⁺ (m/z = 45, doublet state) and CH₃CH = NH₂⁺ (m/z = 44, singlet state) are measured to be 9.53 and 9.21 ± 0.05 eV, respectively, which are consistent with the theoretical values of 9.61 and 9.37 eV with ab initio G3B3 calculations. Formation of the CH₃ĊHNH₃⁺ ion produced via intramolecular hydrogen transfer and subsequent decarboxylation processes is competitive to that of the CH₃CHNH₂⁺ ion derived from a direct C–C(O) bond cleavage. The photoionization mass spectrum obtained at the photon energy of 10.0 eV shows that the intensity of CH₃CHNH₂⁺ is much stronger than that of the CH₃ĊHNH₃⁺ fragment. The formation of the CH₃ĊHNH₃⁺ and CH₃CHNH₂⁺ ions is thought to be thermodynamically and kinetically favorable, respectively. Detailed formation pathways for the two cations have been proposed by using ab initio calculations. The calculated results provide a clear picture of the photoionization and dissociative photoionization processes of the α-alanine cation.