We report the first detection in the interstellar medium of a
C2H5O2N isomer: syn-glycolamide (NH2C(O)CH2OH). The exquisite
sensitivity at sub-mK levels of an ultra-deep spectral survey carried out with
the Yebes 40m and IRAM 30m telescopes towards the G+0.693-0.027 molecular cloud
have allowed us to unambiguously identify multiple transitions of this species.
We derived a column density of (7.4 ± 0.7)×1012 cm−2,
which implies a molecular abundance with respect to H2 of
5.5×10−11. The other C2H5O2N isomers, including the
higher-energy anti conformer of glycolamide, and two conformers of glycine,
were not detected. The upper limit derived for the abundance of glycine
indicates that this amino acid is surely less abundant than its isomer
glycolamide in the ISM. The abundances of the C2H5O2N isomers cannot
be explained in terms of thermodynamic equilibrium, and thus chemical kinetics
need to be invoked. While the low abundance of glycine might not be surprising,
based on the relative low abundances of acids in the ISM compared to other
compounds (e.g. alcohols, aldehydes or amines), several chemical pathways can
favour the formation of its isomer glycolamide. It can be formed through
radical-radical reactions on the surface of dust grains. The abundances of
these radicals can be significantly boosted in an environment affected by a
strong ultraviolet field induced by cosmic rays, such as that expected in
G+0.693-0.027. Therefore, as shown by several recent molecular detections
towards this molecular cloud, it stands out as the best target to discover new
species with carbon, oxygen and nitrogen with increasing chemical complexity.Comment: Accepted in The Astrophysical Journal Letter