Theoretical studies of infrared signatures of proton-bound amino acid dimers with homochiral and heterochiral moieties

Proton-bound homochiral and heterochiral dimers, X-H⁺-X, of five amino acids (X = Ser, Ala, Thr, Phe, and Arg) are investigated theoretically using quantum chemical density functional theory (DFT) calculations and molecular dynamics simulations with the aim to unveil diastereomer-specific mid-infrared (mid-IR) absorption bands in the spectral range of 1000 to 1800 cm⁻¹. The theoretical calculations performed in this work imply that all systems, except Ala₂H⁺, have distinct mid-IR absorption bands in homochiral and heterochiral configurations, which make them appropriate systems to be studied experimentally with mid-IR spectroscopy. We show that intermolecular interaction with the side chain, in the form of hydrogen bonding or cation-π interaction, is necessary for chiral effects to be present in the mid-IR spectra of proton-bound dimers of amino acids. We also report new conformers for Ala₂H⁺, Thr₂H⁺, Phe₂H⁺, and Arg₂H⁺, which were not found in earlier studies of these dimers.