  We present a high-resolution spectroscopic study of the envelope of the young
Class 0 protostar IRAM 04191+1522 in Taurus. N2H+(1-0) observations with the
Plateau de Bure Interferometer and the IRAM 30m telescope demonstrate that the
molecular ion N2H+ disappears from the gas phase in the inner envelope (r <
1600 AU, n(H2) > 4-7 * 1e5 cm-3). This may result from N2 depletion on polar
ice mantles and enhanced grain chemistry.
