  We present new kinetics data on the radiolytic destruction of amino acids
measured in situ with infrared spectroscopy. Samples were irradiated at 15,
100, and 140 K with 0.8-MeV protons, and amino-acid decay was followed at each
temperature with and without H$_2$O present. Observed radiation products
included CO$_2$ and amines, consistent with amino-acid decarboxylation. The
half-lives of glycine, alanine, and phenylalanine were estimated for various
extraterrestrial environments. Infrared spectral changes demonstrated the
conversion from the non-zwitterion structure NH$_2$-CH$_2$(R)-COOH at 15 K to
the zwitterion structure $^+$NH$_3$-CH$_2$(R)-COO$^-$ at 140 K for each amino
acid studied.
