A Density Functional Theory Investigation of the Fragmentation Mechanism of Deprotonated α-Alanine

Abstract

In this paper, density functional theory (DFT) was applied to study the isomerization process and fragmentation mechanism of deprotonated α-alanine ions ([α-Ala–H]–) at B3LYP/6-311++G(2df,2pd) level. Taking a2, the lowest energy of the [α-Ala–H]– isomers as the zero point, the potential energy profiles of the [α-Ala–H]– in isomerization, H2O- and NH3-loss reaction pathways were plotted using Relative Gibbs free Energies (R.G.) relative to a2 as the reference point. In the isomerization process, the reaction barrier (ΔG≠) required for hydrogen transfer is higher than other ones. From the perspective of chemical reaction thermodynamics, Path 11 of deamination to p3 is the only positive spontaneous pathway among all H2O- and NH3-loss pathways; and from the perspective of reaction kinetics, the dominant path is Path 2 of H2O-loss pathways ( = 53.65 kcal•mol-1). In addition, the effect of temperature on reaction pathways and dissociative products were also investigated. The results show that temperature has little effect on H2O- and NH3-loss pathways.