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 (HCONH2) 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 (similar to 8 K) halo region, only the 1(01) - 0(00) transition of formamide appears to be exclusively from the cold halo; the 3(12) - 3(13) 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 similar to 0.1 to similar 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.