Production of doubly-charged highly reactive species from the long-chain amino acid GABA initiated by Ar9+ ionization

Abstract

We present a combined experimental and theoretical study of the fragmentation of multiply-charged γ-aminobutyric acid molecules (GABA^z+, z = 2, 3) in the gas phase. The combination of ab initio molecular dynamics simulations with multiple-coincidence mass spectrometry techniques allows us to observe and identify doubly-charged fragments in coincidence with another charged moiety. The present results indicate that double and triple electron capture lead to the formation of doubly-charged reactive nitrogen and oxygen species (RNS and ROS) with different probabilities due to the different charge localisation and fragmentation behaviour of GABA²⁺ and GABA³⁺. The MD simulations unravel the fast (femtosecond) formation of large doubly charged species, observed on the experimental microsecond timescale. The excess of positive charge is stabilised by the presence of cyclic X-member (X = 3–5) ring structures. 5-Member cyclic molecules can sequentially evaporate neutral moieties, such as H₂, H₂O and CO₂, leading to smaller doubly charged fragments as those observed in the experiments.