Speaker
Description
Protonated glycine constitutes one of the most extensively investigated model systems in gas-phase ion chemistry due to its fundamental relevance for understanding proton localization, conformational flexibility, as well as intra- and intermolecular hydrogen-bonding interactions in biomolecular ions. Owing to its structural simplicity and rich vibrational behavior, it provides an ideal platform for developing and validating theoretical methodologies aimed at interpreting high-resolution infrared spectroscopic data. In particular, the combination of computational chemistry and infrared multiple-photon dissociation (IRMPD) spectroscopy has emerged as a powerful framework for establishing detailed relationships between structure and spectrum in isolated molecular ions.
In the present study, a comprehensive theoretical investigation of protonated glycine and its weakly bound complexes with helium and molecular hydrogen is performed with the objective of elucidating the influence of molecular tagging on structural stability, vibrational dynamics, and predicted IRMPD spectral signatures. Special attention is devoted to the limitations of the harmonic approximation and the necessity of incorporating anharmonic effects for an accurate description of highly localized X–H stretching vibrations. The vibrational dynamics is analyzed beyond the purely harmonic regime, enabling a more realistic representation of frequency shifts, mode coupling, and energy-transfer pathways that contribute to multiphoton dissociation mechanisms. To account for the coexistence of multiple low-energy conformers, Boltzmann-weighted spectral averaging is applied, allowing thermodynamic populations to be incorporated into the theoretical spectra.
Comparative evaluation of the calculated spectra reveals that helium preserves the native structural characteristics of the ion to a large extent, whereas hydrogen induces measurable perturbations that are particularly pronounced in vibrational modes associated with protonated and hydrogen-bonded sites. Beyond the specific system investigated, the study illustrates the broader role of theoretical chemistry in supporting spectroscopic characterization of isolated biomolecular ions and in guiding future studies of structurally related systems.
Keywords: protonated glycine, quantum chemistry, IRMPD spectroscopy, anharmonic vibrations, conformational analysis, molecular tagging, potential energy surface, vibrational dynamics.