[en] The dependence of the damage caused to biomolecules by a swift ion beam on the target molecule's orientation with respect to the beam has implications for radiation protection and therapy. At the most basic level, it is first necessary to understand energy deposition in these systems. As the material constants describing energy deposition are the target mean excitation energies (I₀) we report here the directional components of the mean excitation energies of the two simplest amino acids, glycine and alanine. It is found that the directional components of the mean excitation energies are similar in the two amino acids, and that both have a preferred direction for energy absorption. (authors)