Nature of adsorption of amino acids and precursors on interstellar amorphous solid water

ABSTRACT Hundreds of interstellar molecules have been identified in interstellar molecular clouds by radio telescope observations. Although aminoacetonitrile, one of the glycine precursors, has been identified in molecular clouds, amino acids have not been detected. Therefore, the presence of amino acids in molecular clouds has not been confirmed yet. This study elucidates the nature of the adsorption of amino acids and their precursors on interstellar amorphous solid water (ASW) from the perspective of the binding energy. To calculate the binding energies of the adsorbed molecules on model ASW, machine learning potential-based molecular dynamics simulations and quantum chemical calculations were performed. For amino nitriles, precursors of amino acids, we found that the amino group forms stronger hydrogen bonds with ASW than the nitrile group, owing to the larger negative partial charge on the nitrogen in the amino group. We also revealed that the presence of methyl groups enhances the stability of adsorption, since methyl groups act as stronger electron donors than hydrogen atoms. Additionally, our results suggest that the adsorption strength of amino acids is larger than that of typical interstellar complex organic molecules, such as methanol, indicating that the detection of amino acids using radio telescopes will be challenging. In contrast, 2-aminopropionitrile, an alanine precursor, has binding energies comparable to those of aminoacetonitrile. These results suggest that 2-aminopropionitrile could be desorbed into the gas phase and has a high probability of being detected in molecular clouds by telescopic observations.