SPECTROSCOPIC AND COMPUTATIONAL ANALYSIS OF THREONINE: A REVIEW

Threonine is an essential proteinogenic amino acid whose molecular structure combines amino, carboxyl, hydroxyl, and methyl groups. This combination makes it an informative model for studying the relationship between molecular conformation, hydrogen bonding, protonation state, and vibrational spectra. This review surveys experimental and computational approaches used to investigate L-threonine in crystalline, isolated-molecule, and aqueous environments. The spectroscopic methods discussed include infrared absorption, Fourier-transform infrared (FTIR), Raman, far-infrared, vibrational circular dichroism (VCD), rotational microwave spectroscopy, and inelastic incoherent neutron scattering. Computational approaches include molecular mechanics conformational searches, density functional theory (DFT), post-Hartree-Fock calculations, implicit and explicit solvation, normal-mode analysis, and periodic calculations for crystals. Published work has demonstrated that threonine cannot be represented adequately by a single geometry in every environment: multiple conformers may contribute, and cationic, zwitterionic, and anionic forms differ in their bonding and spectra. The low-frequency region can be particularly informative about torsional and skeletal motions, while VCD adds sensitivity to molecular chirality. In the solid state, intermolecular hydrogen bonding and crystal packing affect vibrational and electronic properties. The review also considers limitations in frequency prediction, solvent modelling, conformer populations, and comparisons between isolated molecules and crystals. Finally, it identifies opportunities for reproducible workflows that combine measured spectra, explicit solvent modelling, conformational sampling, and uncertainty-aware assignments.