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Glycine is the simplest proteinaceous amino acid and is present in alllife-forms on Earth. In aqueous solutions, it appears mainly as zwitterionglycine (+NH3CH2COO-); however, in solid phase, it may be found in amorphous orcrystalline (alpha, beta, and gamma) forms. This molecular species has beenextensively detected in carbonaceous meteorites and was recently observed inthe cometary samples returned to Earth by NASA's Stardust spacecraft. Wepresent an experimental study on the destruction of zwitterionic glycinecrystals at room temperature by 1 MeV protons, in which the dependence of thedestruction rates of the alpha-glycine and beta-glycine crystals on bombardmentfluence is investigated. The samples were analyzed in situ by FTIR spectrometryat different proton fluences at under ultrahigh vacuum conditions at the Van deGraaff accelerator lab at PUC-Rio, Brazil. The dissociation cross section ofalpha-glycine was observed to be 2.5E-14 cm^-2, a value roughly 5 times higherthan the dissociation cross section found for beta-glycine. The estimatedhalf-lives of alpha-glycine and beta-glycine forms extrapolated to the Earthorbit environment are 9E5 and 4E6 years, respectively. In the diffuseinterstellar medium the estimated values are 1 order of magnitude lower. Theseresults suggest that pristine interstellar beta-glycine is the one most likelyto survive the hostile environments of space radiation. A small feature around1650-1700 cm^-1, tentatively attributed to an amide functional group, wasobserved in the IR spectra of irradiated samples, suggesting that cosmic raysmay induce peptide bond synthesis in glycine crystals. Combining this findingwith the fact that this form has the highest solubility among the other glycinepolymorphs, we suggest that beta-glycine is the one most likely to haveproduced the first peptides on primitive Earth.

The Influence of Crystallinity Degree on the Glycine Decomposition Induced by 1 MeV Proton Bombardment in Space Analog Conditions