  We present a high-resolution millimeter study of the very young Class 0
protostar IRAM 04191+1522 in the Taurus molecular cloud. N2H+(1-0) observations
with the IRAM Plateau de Bure Interferometer and 30m telescope demonstrate that
the molecular ion N2H+ disappears from the gas phase in the inner part of the
protostellar envelope (r < 1600 AU, n(H2) > 5 x 10^5 cm^-3). This result
departs from the predictions of current chemical models. It suggests either
that N2 is more depleted than the models predict, owing to a higher binding
energy on polar ice or an enhanced grain chemistry transforming N2 to less
volatile species, or that strong deuterium fractionation enhances N2D+ to the
detriment of N2H+.
