  One of the most fundamental hypotheses in astrochemistry and astrobiology
states that crucial biotic molecules like glycine (NH$_2$CH$_2$COOH) found in
meteorites and comets are inherited from early phases of star formation. Most
observational searches for glycine in the interstellar medium have focused on
warm, high-mass molecular cloud sources. However, recent studies suggest that
it might be appropriate to shift the observational focus to cold, low-mass
sources. We aim to detect glycine towards the so-called methanol hotspot in the
Barnard 5 dark cloud. The hotspot is a cold source ($T_\mathrm{gas}\approx 7.5$
K) with yet high abundances of complex organic molecules (COMs) and water in
the gas phase. We carried out deep, pointed observations with the Onsala 20m
telescope, targeting several transitions of glycine conformers I and II (Gly-I
and Gly-II) in the frequency range $70.2$-$77.9$ GHz. No glycine lines are
detected towards the targeted position, but we use a line stacking procedure to
derive sensitive abundance upper limits w.r.t. H$_2$ for Gly-I and Gly-II, i.e.
$\leq(2$-$5)\times10^{-10}$ and $\leq(0.7$-$3)\times10^{-11}$, respectively.
The obtained Gly-II upper limits are the most stringent for a cold source,
while the Gly-I upper limits are mostly on the same order as previously
measured limits. The measured abundances w.r.t. H$_2$ of other COMs at the B5
methanol hotspot range from $2\times10^{-10}$ (acetaldehyde) to
$2\times10^{-8}$ (methanol). Hence, based on a total glycine upper limit of
$(2$-$5)\times10^{-10}$, we cannot rule out that glycine is present but
undetected.
