Furthering the Diverse Hydrogen Atom Transfer and Carbon Bond Dissociation of Amino Acids under Vacuum Ultraviolet

Abstract

AbstractHydrogen atom transfer (HAT) and bond dissociation of amino acids are studied utilizing a customized vacuum ultraviolet laser ionization mass spectrometer (VUV‐LIMS). Three typical molecules (glycine, threonine and cysteine) which have similar structures but a minor difference by having one or two bonds associated with Cα or having a sulfhydryl were select in this study. As results, glycine finds a simplex dissociation pathway by producing CNH4+ (m/z 30) indicative of direct bond cleavage of Cα‐C1 (C1 stands for the carbon atom of the carboxyl group here). Threonine bears two bonds associated with Cα and they both dissociate under VUV excitation, giving rise to three major mass peaks at m/z 30 (CNH4+), 44 and 75 respectively along with selective HAT process. Interestingly, cysteine also bears two dissociative bonds associated with Cα, however, the dominant HAT for producing NH3, CNH4+ or CO2 was found to be dramatically restrained by the production of H2S which is known as an important gasotransmitter in biological nervous system. Based on density functional theory (DFT) calculations, an in‐depth analysis is conducted to show the thermodynamics and reaction kinetics in forming these fragments with or without a HAT process. It is demonstrated that the HAT‐accommodated structure relaxation and incidental elongation of the bonds associated with Cα, depending on the residual energies after ionization, bring forth competitive and cooperative effect in determining the selectivity of alpha‐beta carbon chemistry.