Detection of Acetamide (CH₃CONH₂): The Largest Interstellar Molecule with a Peptide Bond

Acetamide (CH₃CONH₂) has been detected in emission and absorption toward the star-forming region Sagittarius B2(N) with the 100 m Green Bank Telescope (GBT) by means of four A-species and four E-species rotational transitions. All transitions have energy levels less than 10 K. The Sgr B2(N) cloud is known to have a cold halo with clumps of gas at several different velocities. Absorption features are largely characterized by local standard of rest (LSR) velocities that are typical of the two star-forming cores with systemic LSR velocities of +64 and +82 km s^-1. Continuum sources embedded within the star-forming cores give rise to the absorption from the molecular gas halo surrounding the cores. Emission features are seen at an approximate intermediate LSR velocity of +73 km s^-1 that characterizes the widespread molecular halo that has a spatial scale of a few arcminutes. Two low-energy transitions of formamide (HCONH ₂) were also observed with the GBT toward Sagittarius B2(N) since formamide is the potential parent molecule of acetamide; both molecules are the only interstellar species with an NH₂ group bound to a CO group, the so-called peptide bond, that provides the linkage for the polymerization of amino acids. While the acetamide transitions observed appear to be confined to the cold (~8 K) halo region, only the 1₀₁-0 ₀₀ transition of formamide appears to be exclusively from the cold halo; the 3₁₂-3₁₃ transition of formamide is apparently contaminated with emission from the two hot cores. The relative abundance ratio of acetamide to formamide is estimated to be in the range of ~0.1 to ~0.5 in the cold halo. The exothermic neutral-radical reaction of formamide with methylene (CH₂) may account for the synthesis of interstellar acetamide in the presence of shock phenomenon in this star-forming region.