  Molecular line emission from protoplanetary disks is a powerful tool to
constrain their physical and chemical structure. Nevertheless, only a few
molecules have been detected in disks so far. We take advantage of the enhanced
capabilities of the IRAM 30m telescope by using the new broad band correlator
(FTS) to search for so far undetected molecules in the protoplanetary disks
surrounding the TTauri stars DM Tau, GO Tau, LkCa 15 and the Herbig Ae star MWC
480. We report the first detection of HC3N at 5 sigma in the GO Tau and MWC 480
disks with the IRAM 30-m, and in the LkCa 15 disk (5 sigma), using the IRAM
array, with derived column densities of the order of 10^{12}cm^{-2}. We also
obtain stringent upper limits on CCS (N < 1.5 x 10^{12} cm^{-3}). We discuss
the observational results by comparing them to column densities derived from
existing chemical disk models (computed using the chemical code Nautilus) and
based on previous nitrogen and sulfur-bearing molecule observations. The
observed column densities of HC3N are typically two orders of magnitude lower
than the existing predictions and appear to be lower in the presence of strong
UV flux, suggesting that the molecular chemistry is sensitive to the UV
penetration through the disk. The CCS upper limits reinforce our model with low
elemental abundance of sulfur derived from other sulfur-bearing molecules (CS,
H2S and SO).
