Periodic Reporting for period 2 - A-LIFE (The asymmetry of life: towards a unified view of the emergence of biological homochirality) Periodo di rendicontazione: 2020-10-01 al 2022-03-31 Sintesi del contesto e degli obiettivi generali del progetto How did life begin? There can hardly be a greater question for humanity. It is widely accepted that Earth was formed about 4.5 billion years ago, and that life must have appeared on the early Earth anywhere between 4.1 and 3.8 billion years ago. However, one of the oddest characteristics of life on Earth which puzzles chemists, biologists and physicists alike, is nature’s choice of using only one of two possible mirror images for building up its molecular machinery. Each of the monomers constituting life’s biopolymers have a mirror image twin. Just like our left hand mirrors our right hand, but can never be superimposed on the same space, they exist in both left- and right-handed forms.Proteins—the workhorses in all living cells—are made up of hundreds of thousands of amino acids, which are all left-handed. While ribose, the chiral sugar subunit upon which nucleotides build the structure of DNA and RNA is always right-handed. This phenomenon of selecting only one molecular handedness is called homochirality of life. Understanding how this selection initially arose is a key question of my research. The asymmetry of life may have begun in interstellar space, where circularly polarized light (CPL) could have catalyzed the generation of asymmetrically biased molecules. This scenario is strengthened by the observation of circularly polarized radiation in molecular clouds and the discovery of an excess in the left-handed form of amino acids found in meteorites.My ERC project therefore aims in investigating the original source of the asymmetry in all monomeric building blocks building up life’s fundamental macromolecules – proteins, nucleic acids, and lipids – to understand prebiotic selection and genetic evolution. A major goal is to shed light on the asymmetric mechanism for the formation of chiral proteome, genome and lipidome precursor molecules based on their interaction with CPL with emphasis on absolute asymmetric synthesis. The significance of this project arises due to the current lack of experimental demonstration that amino acids, sugars and lipids can simultaneously and asymmetrically be synthesized by a universal physical selection process. The ultimate goal of this project will be to demonstrate how the diverse set of heterogeneous enantioenriched molecules, available from meteoritic impact, assembles into homochiral pre-biopolymers, by simulating the evolutionary stages on early Earth. Lavoro eseguito dall’inizio del progetto fino alla fine del periodo coperto dalla relazione e principali risultati finora ottenuti We simulated the organic complexity of interstellar ices in the laboratory to address the abiotic formation of phosphorus organic compounds. The transfer of phosphate groups is an essential biochemical reaction of numerous phosphate-containing molecules including ATP and nucleic acids, and so far, there have been no laboratory experiments carried out to investigate the specific role of phosphorus in interstellar ices. In addition to detecting phosphate and alkylphosphonic acids in interstellar ice analogues, we identified glycerol and carboxylic acids – two ingredients of membrane phospholipids – in electron irradiated interstellar analogue ices.To shed light on the asymmetric interactions of interstellar chiral photons with organic molecules, we have studied the chiroptical properties of hydroxycarboxylic acids, considered to be co-building blocks of ancestral proto-peptides along with amino acids. Our results indicate that the same handedness of amino acid and hydroxycarboxylic acid enantiomers can be selected in interstellar ices following the interaction with broadband stellar CPL. We have furthermore investigated the interaction of chiral photons with amino acids in the gas phase, with implications on possible chiral photon interactions during the life cycle of interstellar dust grains. Finally, we have studied the impact of chiral photons on amorphous solid-state sugar molecules. These novel results on chiral sugar compounds in the isotropic solid-state reveal significant asymmetric interactions that are encouraging in terms of efficient chirality transfer from CPL to chiral pre-RNA subunits envisaged in future asymmetric photolysis experiments.First experiments have been performed on our ERC-funded in-house developed polarization- and VUV wavelength-tuneable circularly polarized laser set-up that allows for the in-depth investigation of optimal photolysis parameters to achieve highest enantiomeric enrichment. We are currently investigating the potential role of CPL on triggering a chiral bias into the amino acid isovaline. Isovaline stands out amongst most other detected meteoritic organic compounds due to its large measured excess in the left-handed form. The original cause of L-isovaline enrichments in diverse carbonaceous chondrites is still unknown but its detection suggests that amino acids delivered by meteorites could have biased Earth’s prebiotic inventory with left-handed proteome precursors. Progressi oltre lo stato dell’arte e potenziale impatto previsto (incluso l’impatto socioeconomico e le implicazioni sociali più ampie del progetto fino ad ora) Our recent results on the formation on alkylphosphonic acids propose an alternative – exogeneous – source of phosphorus-bearing organic molecules that could overcome the current challenges in geochemically linking insoluble abiotic phosphorus sources ‘trapped’ in terrestrial minerals with prebiotic chemistry on early Earth. Alkylphosphonic acids, brought to the early Earth through diverse impact events, may have served as a prebiotic source of soluble phosphorus, while the detection of methylphosphate in our samples demonstrates the ability of P(+V)─O─C linkages to form in interstellar ices and acts as a prototype for the P─O─C backbone moiety found in contemporary biomolecules such as ATP and DNA/RNA.We have advanced our studies on the asymmetric interaction of chiral photons with chiral molecules to the gas and solution phase that further improves our understanding of a more realistic CPL scenario covering different stages and locations of UV-illuminated interstellar ices within dense molecular clouds and protoplanetary disks and the associated morphological changes of water impacting the chiroptical properties of solvated and solvent-free enantiomers. Due to an interstellar dust cycling process, by which organic molecules such as amino acids can reversibly desorb and condense back onto the surface of cometary and pre-cometary dust particles, chiral photon-molecule interactions are assumed to have – at least partly – taken place in the gas phase of interstellar environments. Such desorption-condensation cycles may have been crucial for an ee amplification via asymmetric photolysis of interstellar ices intimately connected with the surrounding gas, i.e. starting from small g values in the gas and in the solid state, to a few percent ee as identified in carbonaceous chondrites. We therefore propose for the origin and evolution of biomolecular asymmetry a two-step process, with an initial mirror-symmetry breaking event in the life cycle of interstellar gas and dust during cloud evolution followed by nonequilibrium reaction networks on the early Earth driving prebiotic molecular systems toward a homochiral state.Our future experiments will address the impact of more realistic broad-band CPL on H2O-dominated ices. Besides gaining deeper insights into the conformational dependence of amino acids, hydroxy acids, and sugar molecules on the surrounding environmental conditions and therefore their chiroptical response to CPL, our research will allow for better understanding the potential role of stellar CPL for the systematic generation of enantiomeric excesses across molecular families. erc-alife.png
