Gamma-Ray-Induced Amino Acid Formation during Aqueous Alteration in Small Bodies: The Effects of Compositions of Starting Solutions

Abstract

Organic compounds, such as amino acids, are essential for the origin of life, and they may have been delivered to the prebiotic Earth from extra-terrestrial sources, such as carbonaceous chondrites. In the parent bodies of carbonaceous chondrites, the radioactive decays of short-lived radionuclides, such as ²⁶Al, cause the melting of ice, and aqueous alteration occurs in the early stages of solar system formation. Many experimental studies have shown that complex organic matter, including amino acids and high-molecular-weight organic compounds, is produced by such hydrothermal processes. On the other hand, radiation, particularly gamma rays from radionuclides, can contribute to the formation of amino acids from simple molecules such as formaldehyde and ammonia. In this study, we investigated the details of gamma-ray-induced amino acid formation, focusing on the effects of different starting materials on aqueous solutions of formaldehyde, ammonia, methanol, and glycolaldehyde with various compositions, as well as hexamethylenetetramine. Alanine and glycine were the most abundantly formed amino acids after acid hydrolysis of gamma-ray-irradiated products. Amino acid formation increased with increasing gamma-ray irradiation doses. Lower amounts of ammonia relative to formaldehyde produced more amino acids. Glycolaldehyde significantly increased amino acid yields. Our results indicated that glycolaldehyde formation from formaldehyde enhanced by gamma rays is key for the subsequent production of amino acids.

Keywords: amino acids; aqueous alteration; carbonaceous chondrites; prebiotic chemistry.

Figure 9

The Ala/Gly and β-Ala/Ala ratios of AAs from the FAW and HMT samples compared to the literature values of carbonaceous chondrites. Data from * Glavin et al. (2011) [73], ** Glavin et al. (2006) [72], *** Martins et al. (2007) [74], and **** Burton et al. (2012) [8].

Figure 1

HPLC chromatograms of FAW(10) after gamma-ray irradiation at 1.5 kGy/h × 600 h, FAW(10) after heating at 80 °C for 168 h after acid hydrolysis, and AA standard solution.

Figure 2

Yields of (a) total AAs, (b) Gly, and (c) Ala with gamma-ray doses of the gamma-ray irradiated samples. Right: enlarged to show the ranges shown in dotted squares.

Figure 3

Results of AA formation for each gamma-ray radiation dose: (a) 1.5 kGy/h × 600 h = 900 kGy, (b) 1.5 kGy/h × 60 h = 90 kGy, (c) 0.15 kGy/h × 600 h = 90 kGy, and (d) 0.15 kGy/h × 60 h = 9 kGy. Black lines indicate the AA concentrations in the control samples. The largest values are shown if there are multiple control samples (the largest values from either 0 h or 600 h) from the same starting solutions.

Figure 4

Carbon yield (%) of (a) total AAs, (b) Gly, and (c) Ala by gamma-ray irradiation at 90 kGy (1.5 kGy/h × 60 h) and control samples (600 h at room temperature) with the HCHO/NH₃ ratios of the starting solutions. (d) Each AA yields from the samples shown in (a–c) and 600 h control samples of the same starting solution.

Figure 5

Yields of (a) total AAs, (b) Gly, and (c) Ala from HMT by gamma-ray irradiation and control experiments.

Figure 6

AA yields from the gamma-ray irradiation experiments from FGAWCa, FGAW(1), and FGAW (5) compared to FAW(5), FAW(2), and (FAW(0.5).

Figure 7

AA yields from the gamma-ray irradiation experiments compared to the results of the heating experiments and controls. (a) FAW(5), (b) FAW(10), (c) FAW(5/10), (d) HMT/HMT(rt), and (e) FGAWCa.

Figure 8

Gly, Ala, and total AAs yields from samples heated at 150 °C for 24, 48, and 72 h and controls (0 h) of (a) FAW(5), (b) HMT, and (c) FGAWCa.