  Understanding the origin of the composition of solar system cosmomaterials is
a central question, not only in the cosmochemistry and astrochemistry fields,
and requires various approaches to be combined. Measurements of isotopic ratios
in cometary materials provide strong constraints on the content of the
protosolar nebula. Their relation with the composition of the parental dark
clouds is, however, still very elusive. In this paper, we bring new constraints
based on the isotopic composition of nitrogen in dark clouds, with the aim of
understanding the chemical processes that are responsible for the observed
isotopic ratios. We have observed and detected the fundamental rotational
transition of C$^{15}$N towards two starless dark clouds, L1544 and L1498. We
were able to derive the column density ratio of C$^{15}$N over $^{13}$CN
towards the same clouds, and obtain the CN/C$^{15}$N isotopic ratios, which
were found to be $500\pm75$ for both L1544 and L1498. These values are
therefore marginally consistent with the protosolar value of 441. Moreover,
this ratio is larger than the isotopic ratio of nitrogen measured in HCN. In
addition, we present model calculations of the chemical fractionation of
nitrogen in dark clouds, which make it possible to understand how CN can be
deprived of $^{15}$N and HCN can simultaneously be enriched in heavy nitrogen.
The non-fractionation of N2H+, however, remains an open issue and we propose
some chemical way of alleviating the discrepancy between model predictions and
the observed ratios.
