Examining the Amine Functionalization in Dicarboxylates: Photoelectron Spectroscopy and Theoretical Studies of Aspartate and Glutamate

Aspartate (Asp2–) and glutamate (Glu2–), two doubly charged conjugate bases of the corresponding amino acids, were investigated using low-temperature negative ion photoelectron spectroscopy (NIPES) and ab initio calculations. The effect of amine functionalization was studied by a direct comparison to the parent dicarboxylate species (−CO2–(CH2)n–CO2 –, DC n 2–), succinate (DC2 2–) and propionate (DC3 2–). Experimentally, the addition of the amine group for the n = 2 case (DC2 2–, Asp2–) significantly improves the stability of the resultant Asp2– dianionic species, albeit that NIPES shows only a small increase in adiabatic electron detachment energy (ADE) (+0.05 eV). In contrast, for n = 3 (DC3 2–, Glu2–), a much larger ADE increase is observed (+0.15 eV). Similar results are obtained through ab initio calculations. The latter indicates that increased stability of Asp2– can be attributed to the lowering of the energy of the singlet dianion state due to hydrogen bonding effects. The effect of the amino group on the doublet monoanion state is more complicated and results in the weakening of the binding of the adjacent carboxylate group due to electronic structure resonance effects. This conclusion is confirmed by the analysis of NIPES results that show enhanced production of near-zero kinetic energy electrons observed experimentally for amine-functionalized species.