  We present ESO - Very Large Telescope and ESA - Infrared Space Observatory 3
to 4 $\mu$m spectra of Seyfert 2 nuclei as compared to our galactic center
lines of sight. The diffuse interstellar medium probed in both environments
displays the characteristic 3.4 $\mu$m aliphatic CH stretch absorptions of
refractory carbonaceous material. The profile of this absorption feature is
similar in all sources, indicating the CH$_2$/CH$_3$ ratios of the carbon
chains present in the refractory components of the grains are the same in
Seyfert 2 inner regions. At longer wavelengths the circumstellar contamination
of most of the galactic lines of sight precludes the identification of other
absorption bands arising from the groups constitutive of the aliphatics seen at
3.4 $\mu$m. The clearer continuum produced by the Seyfert 2 nuclei represents
promising lines of sight to constrain the existence or absence of strongly
infrared active chemical groups such as the carbonyl one, important to
understand the role of oxygen insertion in interstellar grains. The Spitzer
Space Telescope spectrometer will soon allow one to investigate the importance
of aliphatics on a much larger extragalactic sample.
