Ice chemistry of acetaldehyde reveals competitive reactions in the first step of the Strecker synthesis of alanine: formation of HO–CH(CH3)–NH2 vs. HO–CH(CH3)–CN

Abstract

The understanding of compound formation in laboratory simulated astrophysical environments is an important challenge in obtaining information on the chemistry occurring in these environments. We here investigate by means of both laboratory experiments and quantum chemical calculations the ice-based reactivity of acetaldehyde (CH 3 CHO) with ammonia (NH 3) and hydrogen cyanide (HCN) in excess of water (H 2 O) promoted by temperature. A priori, this study should give information on alanine (2 HN–CH(CH 3)–COOH) formation (the simplest chiral amino acid detected in meteorites), since these reactions concern the first steps of its formation through the Strecker synthesis. However, infrared spectroscopy, mass spectrometry with HC 14 N or HC 15 N isotopologues and B3LYP-D3 results converge to indicate that an H 2 O-dominated ice containing CH 3 CHO, NH 3 and HCN not only leads to the formation of α-aminoethanol (2 HN–CH(CH 3)–OH, the product compound of the first step of the Strecker mechanism) and its related polymers (2 HN–(CH(CH 3)–O) n –H) due to reaction between CH 3 CHO and NH 3 , but also to the 2-hydroxypropionitrile (HO–CH(CH 3)–CN) and its related polymers (H–(O–CH(CH 3)) n –CN) from direct reaction between CH 3 CHO and HCN. The ratio between these two species depends on the initial NH 3 /HCN ratio in the ice. Formation of α-aminoethanol is favoured when the NH 3 concentration is larger than HCN. We also show that the presence of water is essential for the formation of HO–CH(CH 3)–CN, contrarily to 2 HN–CH(CH 3)–OH whose formation also takes place in absence of H 2 O ice. As in astrophysical ices NH 3 is more abundant than HCN, formation of α-aminoethanol should consequently be favoured compared to 2-hydroxypropionitrile, thus pointing out α-aminoethanol as a plausible intermediate species for alanine synthesis through the Strecker mechanism in astrophysical ices.