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

Abstract

One of the leading hypotheses regarding the origin of prebiotic molecules on primitive Earth is that they formed from inorganic molecules in extraterrestrial environments and were delivered by meteorites, space dust and comets. To evaluate the availability of extraterrestrial amino acids, it is necessary to examine their decomposition and oligomerization rates as induced by extraterrestrial energy sources, such as vacuum ultraviolet (VUV) and X-ray photons and high energy particles. This paper reports the quantum yields of decomposition ((8.2 ± 0.7) × 10(-2) photon(-1)) and homo-dimerization ((1.2 ± 0.3) × 10(-3) photon(-1)) and decomposition of the dimer (0.24 ± 0.06 photon(-1)) of solid L-alanine (Ala) induced by VUV light with an energy of 7.2 eV. Using these quantum yields, the half-life of L-Ala on the surface of a space object in the present earth orbit was estimated to be about 52 days, even when only photons with an energy of 7.2 eV emitted from the present Sun were considered. The actual half-life of solid L-Ala on the surface of a space object orbit around the present day Earth would certainly be much shorter than our estimate, because of the added effect of photons and particles of other energies. Thus, we propose that L-Ala needs to be shielded from solar VUV in protected environments, such as the interior of a meteorite, within a time scale of days after synthesis to ensure its arrival on the primitive Earth.

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

“…However, slight differences in relative peak intensities were observed; they might have emerged due to the difference between the film and pellets, i.e., the thickness and density of the thin films are different from those of the pellets. In previous circular dichroism (CD) experiments of amino acids in the vacuum-UV and soft X-ray regions by Izumi et al., 5,6 the same evaporation method as ours has been adopted, and they have obtained spectra that are consistent with powder samples. These results also suggest that the thermal degradation can be ignored.…”

“…Another method that can reveal the chirality is the circular dichroism spectroscopy for the vacuum-ultraviolet (UV) and soft X-ray spectral regions. 5,6 However, as these methods require a very high sensitivity owing to the very low dichroism signal, the concentration of D-amino acid molecules and their spatial distribution cannot be measured. To detect the spatial distribution of D-amino acids gives us the information of the deformation position of peptides and proteins owing to racemization, which is a sign of aging diseases.…”

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

“…The thickness of the films was roughly estimated to be tens of nanometers using a thin-film thickness monitor (CRTM-5000, ULVAC, Inc., Kanagawa, Japan). Uncertainty of the thickness was 10-20% depending on the positions of the substrates on our experience [15,16].…”

On the Physical Basis of Asymmetry and Homochirality

“…Even though the extraterrestrial origin of organic matter in space objects may be indisputable, decomposition reactions before delivery would also alter the initial composition. With quantum yields of vacuum UV light, solid L‐alanine deposited on the surface of a meteorite would have a half‐life of about 50 days, even when only considering the energy from solar photons …”

“…With quantum yields of vacuum UV light, solid L-alanine deposited on the surface of a meteorite would have a half-life of about 50 days, even when only considering the energy from solar photons. 47 Together with meteorites and interstellar dust, comets could have seeded the abiotic chemistry prior to life. This idea flourished several decades ago and has been since a recurring theme in astrochemical research.…”