  Interstellar molecules with a peptide link -NH-C(=O)-, like formamide
(NH$_2$CHO), acetamide (NH$_2$COCH$_3$) and isocyanic acid (HNCO) are
particularly interesting for their potential role in pre-biotic chemistry. We
have studied their emission in the protostellar shock regions L1157-B1 and
L1157-B2, with the IRAM 30m telescope, as part of the ASAI Large Program.
Analysis of the line profiles shows that the emission arises from the outflow
cavities associated with B1 and B2. Molecular abundance of
$\approx~(0.4-1.1)\times 10^{-8}$ and $(3.3-8.8)\times 10^{-8}$ are derived for
formamide and isocyanic acid, respectively, from a simple rotational diagram
analysis. Conversely, NH$_2$COCH$_3$ was not detected down to a relative
abundance of a few $\leq 10^{-10}$. B1 and B2 appear to be among the richest
Galactic sources of HNCO and NH$_2$CHO molecules. A tight linear correlation
between their abundances is observed, suggesting that the two species are
chemically related. Comparison with astrochemical models favours molecule
formation on ice grain mantles, with NH$_2$CHO generated from hydrogenation of
HNCO.
