Interstellar ice embedded glycine response to H⁺/proton irradiation. A theoretical study

The glycine molecule is the simplest amino acid molecule which is expected to be easily formed on ice grains in the interstellar medium. Assessing its stability under interstellar irradiation is therefore crucial in a prebiotic context. In this work we investigate the ionisation mechanism of ice-embedded glycine under H⁺ irradiation at kinetic energies of 1, 10, and 100 keV by studying charge and energy deposition dynamics. To this aim, real time-time dependent-density functional theory combined with a quantum mechanics/molecular mechanics approach is used. Charge distribution analysis reveals ebb-and-flow effects at both molecular and atomic scales, with collisions at nuclei inducing stronger polarization and dipole dynamics than impacts along chemical bonds. While electronic excitation occurs locally, glycine-water interactions can collectively enhance, reduce or leave unchanged the energy deposition, depending on the projectile trajectory and kinetic energy. The charge transfer between the glycine molecule and its surrounding environment does not exceed 0.4 electrons once the projectile is gone. When the projectile does not directly intersect the glycine backbone, the deposited energy on the glycine molecule is negligible and glycine remains neutral. Consequently, the degree of molecular perturbation is governed not only by the projectile's energy, but also by its microscopic trajectory and the surrounding ice environment.