Thiols in the ISM: first detection of HC(O)SH and confirmation of
  C$_2$H$_5$SH
  The chemical compounds carrying the thiol group (-SH) have been considered
essential in recent prebiotic studies regarding the polymerization of amino
acids. We have searched for this kind of compounds toward the Galactic Centre
quiescent cloud G+0.693-0.027. We report the first detection in the
interstellar space of the trans-isomer of monothioformic acid (t-HC(O)SH) with
an abundance of $\sim\,$1$\,\times\,$10$^{-10}$. Additionally, we provide a
solid confirmation of the gauche isomer of ethyl mercaptan (g-C$_2$H$_5$SH)
with an abundance of $\sim\,$3$\,\times\,$10$^{-10}$, and we also detect methyl
mercaptan (CH$_3$SH) with an abundance of $\sim\,$5$\,\times\,$10$^{-9}$.
Abundance ratios were calculated for the three SH-bearing species and their
OH-analogues, revealing similar trends between alcohols and thiols with
increasing complexity. Possible chemical routes for the interstellar synthesis
of t-HC(O)SH, CH$_3$SH and C$_2$H$_5$SH are discussed, as well as the relevance
of these compounds in the synthesis of prebiotic proteins in the primitive
Earth.
