Quantum yields of decomposition and homo-dimerization of solid L-alanine induced by 7.2 eV Vacuum ultraviolet light irradiation: an estimate of the half-life of L-alanine on the surface of space objects

Space Exposure of Amino Acids and Their Precursors during the Tanpopo Mission

Amino acids have been detected in extraterrestrial bodies such as carbonaceous chondrites (CCs), which suggests that extraterrestrial organics could be the source of the first life on Earth, and interplanetary dust particles (IDPs) or micrometeorites (MMs) are promising carriers of extraterrestrial organic carbon. Some amino acids found in CCs are amino acid precursors, but these have not been well characterized. The Tanpopo mission was conducted in Earth orbit from 2015 to 2019, and the stability of glycine (Gly), hydantoin (Hyd), isovaline (Ival), 5-ethyl-5-methylhydantoin (EMHyd), and complex organics formed by proton irradiation from CO, NH₃, and H₂O (CAW) in space were analyzed by high-performance liquid chromatography and/or gas chromatography/mass spectrometry. The target substances showed a logarithmic decomposition over 1-3 years upon space exposure. Recoveries of Gly and CAW were higher than those of Hyd, Ival, and EMHyd. Ground simulation experiments showed different results: Hyd was more stable than Gly. Solar ultraviolet light was fatal to all organics, and they required protection when carried by IDPs/MMs. Thus, complex amino acid precursors (such as CAW) were possibly more robust than simple precursors during transportation to primitive Earth. The Tanpopo 2 mission is currently being conducted to expose organics to more probable space conditions.

Optical Absorption Cross-Section of DNA Bases—Thymine and Guanine—in the Energy Region from 3.1 to 250 eV (5–400 nm)

(1) Background: Optical absorption cross-section—the absolute absorption intensity specific to each molecule—of nucleic acid bases enables us to estimate the reaction yields of DNA lesions induced by the exposure to not only photons but also ionizing radiations. However, it was unknown in the energy region exceeding ~10 eV (wavelength < ~120 nm). (2) Methods: Thin films of DNA bases—thymine and guanine—were prepared using a vacuum sublimation technique. Absorption spectra of these films were measured in the energy region from 3.1 to 250 eV (5–400 nm) at the synchrotron radiation facility UVSOR. (3) Results: The absorption spectra of both bases exhibited prominent absorption peaks around 20 eV and smaller peaks in the energy region below 10 eV. The determined optical oscillator strength distribution was verified to be reasonable based on the Thomas–Reiche–Kuhn oscillator strength sum rule. (4) Conclusion: Most of the oscillator strength distribution was positioned in the measured energy region, and therefore the absorption spectra significantly contributed to the quantitative study for the photo and radiation-chemical reactions of DNA.

Environmental Data and Survival Data of Deinococcus aetherius from the Exposure Facility of the Japan Experimental Module of the International Space Station Obtained by the Tanpopo Mission

The Tanpopo mission has two objectives: (1) test the panspermia hypothesis and (2) test whether organic compounds may have been transferred to Earth before the origin of life. We developed an exposure panel (EP) designed to expose microbes and organic compounds to the space environment and a capture panel designed to capture high-velocity particles on the International Space Station (ISS) using aerogel contained in an aluminum container. The panels returned after 1 year of exposure at the Exposure Facility of the Japan Experimental Module, ISS. In this communication, we report the measurements of temperature, radiation dosimeter and vacuum ultraviolet dosimeter in the EP, and survival data of Deinococcus aetherius. The environmental data are consistent with survival data of microbes and organic compounds, which will be presented elsewhere in detail.

Infrared Evaluation of Enantiometric Amount and Application to Racemization at the Interface Between L- and D-Alanine

Amino acids, except for glycine, have two types of molecular structures, L- and D-forms, which are enantiomers. As the L- and D-form molecules have identical chemical and physical properties, they cannot be easily identified by conventional methods. In this study, we found that the amount and the spatial distribution of D-enantiomers in an L-alanine (CH₃CH(COOH)NH₂) matrix in evaporated thin films, which is one of amino acid molecules, can be evaluated quantitatively using an infrared micro spectroscopy with synchrotron radiation. This method was applied to the evaluation of the volume fraction of a racemic component appearing at the interface between L- and D-alanine thin films.

Polycondensation of Asparagine-comprising Dipeptides in Aqueous Media-A Simulation of Polypeptide Formation in Primordial Earth Hydrosphere

Asparagine and aspartic acid might have mutually transformed in the primordial hydrosphere of the earth, if ammonia and aspartic acid had existed in equilibrium. These amino acids seem to contribute to polypeptides, while the simple amino acids glycine and alanine easily form cyclic dipeptides and do not achieve long peptide chains. Asparagine-comprising dipeptides contribute some kinds of activation forms of dipeptides because these can polymerize faster than asparagine only. The new finding of polypeptide formation suggests a pathway of sequential polypeptides to evolve a diversity of polypeptides.

On the physical basis of asymmetry and homochirality

Mirror symmetry breaking is ubiquitous in our visible universe taking place in elementary particles, atoms, and molecules. Molecular chirality is not biogenic in itself, although its detection is often considered a biosignature, a conjecture inferred from the fact that we do not know life devoid of homochirality. The question of whether there is a connection between the cosmic preference for one enantiomer, as imposed by the weak force, and the single chirality displayed on Earth is vividly debated. This article gives a glimpse on the origin of asymmetry from a cosmological perspective and on physical transformations that lead to an enantiomeric imbalance, leaving chemical reactions essentially aside. These processes are more plausible as sources of prebiotic chirality than asymmetric amplifications requiring unnatural substrates and conditions and fighting against racemization. The latter may actually be a friend, not foe, and a driving force for enantioselection.