Solvent and pH effects on the ionization energies of bio-organic molecules: a case study of alanine and glycine

Abstract

The ionization energies (IEs) and double ionization energies (DIEs) of the amino acids alanine and glycine have been studied in the gas phase and a polarizable continuum model (PCM) to account for solvent effects. Amino acids undergo structural modifications, particularly in their functional groups, in response to changes in their chemical environment, such as pH. Four possible different structural forms, neutral (NH₂–R–COOH), zwitterionic (NH₃⁺–R–COO⁻), deprotonated (NH₂–R–COO⁻), and protonated (NH₃⁺–R–COOH), were generated for each amino acid. In this study, we use these four forms to explore the effects of protonation and deprotonation (pH effect) on the IEs of inner-valence states. In amino acids, IEs and DIEs are essential for evaluating the energetic viability of inner-valence-initiated non-radiative decay processes, such as interatomic Coulombic decay (ICD). All IE and DIE values were calculated using the complete active space perturbation theory (CASPT2) method. A comparison is made across gas-phase and PCM environments to assess the impact of solvation. The observed variations in IE and DIE values indicate a lot about alanine and glycine's electronic structure and chemical reactivity under different physiological and environmental conditions (pH effect and solvation), hence improving our understanding of their functional roles in biochemical systems.