Molecular chirality in meteorites and interstellar ices, and the chirality experiment on board the ESA cometary Rosetta mission

Life, as it is known to us, uses exclusively L-amino acid and D-sugar enantiomers for the molecular architecture of proteins and nucleic acids. This Minireview explores current models of the original symmetry-breaking influence that led to the exogenic delivery to Earth of prebiotic molecules with a slight enantiomeric excess. We provide a short overview of enantiomeric enhancements detected in bodies of extraterrestrial origin, such as meteorites, and interstellar ices simulated in the laboratory. Data are interpreted from different points of view, namely, photochirogenesis, parity violation in the weak nuclear interaction, and enantioenrichment through phase transitions. Photochemically induced enantiomeric imbalances are discussed more specifically in the topical context of the "chirality module" on board the cometary Rosetta spacecraft of the ESA. This device will perform the first enantioselective in situ analyses of samples taken from a cometary nucleus.

Photoreaction of rac-leucine in ice by circularly polarized synchrotron radiation: temperature-induced mechanism switching from Norrish Type II to deamination

The delivery of extraterrestrial organics to primitive Earth is considered to have triggered the origin and subsequent evolution of life. Indeed, enantiomerically enriched amino acids of nonterrestrial origin have been found in carbonaceous meteorites, and enantioselective photodecomposition by circularly polarized light (CPL) in outer space has been proposed to have played some role in the initial enantiomeric bias. To experimentally examine this possibility and elucidate the photoreaction mechanisms, we have studied the photolysis of racemic leucine (rac-Leu) in acidic and neutral ice/water media at 21-298 K with left- and right-CPL in an attempt to detect enantiomerically enriched D- and L-Leu, respectively. Comprehensive product analyses revealed that the CPL-induced deracemization of Leu proceeds in both acidic and neutral ice matrices even at 21 K, and that the main mechanism switches from Norrish-type II γ-hydrogen abstraction to SN i deamination on lowering the temperature. The potential role of the CPL-induced photodecomposition of amino acids as a source of the enantiomer imbalance in meteorites is discussed.

Chirality, photochemistry and the detection of amino acids in interstellar ice analogues and comets

The primordial appearance of chiral amino acids was an essential component of the asymmetric evolution of life on Earth. In this tutorial review we will explore the original life-generating, symmetry-breaking event and summarise recent thoughts on the origin of enantiomeric excess in the universe. We will then highlight the transfer of asymmetry from chiral photons to racemic amino acids and elucidate current experimental data on the photochemical synthesis of amino and diamino acid structures in simulated interstellar and circumstellar ice environments. The chirality inherent within actual interstellar (cometary) ice environments will be considered in this discussion: in 2014 the Rosetta Lander Philae onboard the Rosetta space probe is planned to detach from the orbiter and soft-land on the surface of the nucleus of comet 67P/Churyumov-Gerasimenko. It is equipped for the in situ enantioselective analysis of chiral prebiotic organic species in cometary ices. The scientific design of this mission will therefore be presented in the context of analysing the formation of amino acid structures within interstellar ice analogues as a means towards furthering understanding of the origin of asymmetric biological molecules.