  Glycine is the simplest proteinaceous amino acid and is present in all
life-forms on Earth. In aqueous solutions, it appears mainly as zwitterion
glycine (+NH3CH2COO-); however, in solid phase, it may be found in amorphous or
crystalline (alpha, beta, and gamma) forms. This molecular species has been
extensively detected in carbonaceous meteorites and was recently observed in
the cometary samples returned to Earth by NASA's Stardust spacecraft. We
present an experimental study on the destruction of zwitterionic glycine
crystals at room temperature by 1 MeV protons, in which the dependence of the
destruction rates of the alpha-glycine and beta-glycine crystals on bombardment
fluence is investigated. The samples were analyzed in situ by FTIR spectrometry
at different proton fluences at under ultrahigh vacuum conditions at the Van de
Graaff accelerator lab at PUC-Rio, Brazil. The dissociation cross section of
alpha-glycine was observed to be 2.5E-14 cm^-2, a value roughly 5 times higher
than the dissociation cross section found for beta-glycine. The estimated
half-lives of alpha-glycine and beta-glycine forms extrapolated to the Earth
orbit environment are 9E5 and 4E6 years, respectively. In the diffuse
interstellar medium the estimated values are 1 order of magnitude lower. These
results suggest that pristine interstellar beta-glycine is the one most likely
to survive the hostile environments of space radiation. A small feature around
1650-1700 cm^-1, tentatively attributed to an amide functional group, was
observed in the IR spectra of irradiated samples, suggesting that cosmic rays
may induce peptide bond synthesis in glycine crystals. Combining this finding
with the fact that this form has the highest solubility among the other glycine
polymorphs, we suggest that beta-glycine is the one most likely to have
produced the first peptides on primitive Earth.
