Solvent and pH effects on the ionization potential of bioorganic molecules: A case study of Alanine and Glycine

Abstract

The ionization potentials (IP) and double ionization potentials (DIP) 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 the chemical environment, such as pH. Four distinct 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 by modifying their functional groups. This study uses these four forms to explore the influence of protonation and deprotonation states on IP and DIP. IP and DIP values were calculated for these forms using the complete active space perturbation theory (CASPT2) method and compared across gas phase and PCM environments to assess the impact of solvation. The observed variations in IP and DIP values tell a lot about alanine and glycine's electronic structure and chemical reactivity under different physiological and environmental conditions. Hence, improving our understanding of their functional roles in biochemical systems.