DFT calculations were carried out using the Orca program at the wB97X(23)-D3BJ(24, 25) /def2-TZVPP (26) level of theory, which employs the RIJCOSX approximation for Coulomb and Exchange integrals . Preoptimization in the gas phase was done using the xTB tool . To better connect the species calculated with the dehydrated AA form present in peptide chains, which lacks both the caroxylate and ammonium groups, AAs were studied as their corresponding diamide forms, i.e., C(O)NC(R)C(O)ON, where R is the side group. Geometries were optimized for the singlet state of each modified AA and the corresponding doublet states of the (singly) oxidized and reduced forms. Dipole moments and isotropic polarizabilities were also obtained for the singlet state species. Solvation was treated with Orca's C-PCM variant of the Minnesota SMD model . IE values of solvated AAs were calculated as: