The Perseus Exobiology mission on MIR: behaviour of amino acids and peptides in Earth orbit.

Leucine, alpha-methyl leucine and two peptides were exposed to space conditions on board the MIR station during the Perseus-Exobiology mission. This long duration space mission was aimed at testing the delivery of prebiotic building blocks. During this mission, two amino acids (leucine and alpha-methyl leucine) and two peptides (leucine-diketopiperazine and trileucine thioethylester) were exposed in Earth orbit for three months. Basalt, clay and meteorite powder were also mixed with the samples in order to simulate the effects of potential meteorite protection. Analysis of the material after the flight did not reveal any racemization or polymerisation but did provide information regarding photochemical pathways for the degradation of leucine and of the tripeptide. Amino acids appeared to be more sensitive to UV radiation than peptides, the cyclic dipeptide being found to be as particularly resistant. Meteorite powder which exhibits the highest absorption in Vacuum UltraViolet (VUV) afforded the best protection to the organic molecules whereas montmorillonite clay, almost transparent in VUV, was the least efficient. By varying the thickness of the meteorite, we found that the threshold for efficient protection against radiation was about 5 microm. The possible exogenous origin of biological building blocks is discussed with respect to the stability to the molecules and the nature of the associated minerals.