  The largest non-cyclic molecules detected in the interstellar medium (ISM)
are organic with a straight-chain carbon backbone. We report an interstellar
detection of a branched alkyl molecule, iso-propyl cyanide (i-C3H7CN), with an
abundance 0.4 times that of its straight-chain structural isomer. This
detection suggests that branched carbon-chain molecules may be generally
abundant in the ISM. Our astrochemical model indicates that both isomers are
produced within or upon dust grain ice mantles through the addition of
molecular radicals, albeit via differing reaction pathways. The production of
iso-propyl cyanide appears to require the addition of a functional group to a
non-terminal carbon in the chain. Its detection therefore bodes well for the
presence in the ISM of amino acids, for which such side-chain structure is a
key characteristic.
