Pyrolysis of amino acids: Recovery of starting materials and yields of condensation products

The present study relates to the problem of the thermal stability of small biomolecules during their extraterrestrial delivery. It was undertaken in order to make rough estimates of the temperatures under which amino acids can survive during atmospheric entry heating of organics-bearing space bodies in oxygen-free atmosphere. Amino acids tested in the present study were glycine (Gly), α- and β-alanine (Ala and β-Ala, respectively), α-aminoisobutyric acid (Aib), valine (Val), leucine (Leu), proline (Pro), phenylalanine (Phe) and glutamic acid (Glu). Off-line design of pyrolytic experiments was used, with extraction and HPLC quantification of residual amino acids and peptide-type products formed as a result of amino acid dehydration. The lower temperature limit used was 400 °C, repeating the experiments with 100 °C increments. Of the nine studied amino acids, Glu and β-Ala are the most unstable, pyrolyzing completely at 400 °C. At the other extreme was Ala which was still detected at 800 °C with 0.4% recovery. At a percent recovery level, Aib and Pro are able to survive 500 °C; Gly, 600 °C; Val and Leu, 700 °C. Formation of cyclic dipeptides piperazine-2,5-diones (PDs) was observed for Gly, Ala, Aib, Val, Leu and Pro; and pyroglutamic acid for Glu. Pro and Ala PDs form in especially high yields (about 80 and 48%, respectively, at 400 °C), but as a whole, PD yields are of the same order of magnitude as amino acid recovery. Thermal stability of PDs is also comparable with that of amino acids.