Ultracomplex Amino Acid Precursors Formed from Interstellar Media Analogs by High Energy Particles Irradiation

Since 1950's, a large number of experiments have been conducted to abiotically synthesize amino acids and other organics of biological importance. Miller detected amino acids in his spark discharge products from a mixture of methane, ammonia, hydrogen and water [1]. Urey and Miller suggested that these amino acids were produced via the Strecker synthesis, which is a reaction among hydrogen cyanide, aldehydes and ammonia [2]. On the other hand, a wide variety of amino acids have been detected in water-extract of carbonaceous chondrites (CCs) [3], and it is suggested that such extraterrestrial amino acids had important roles for the generation of the first life on the Earth. Since meteoritic amino acids increased after acid-hydrolysis, there should be amino acid precursors in the extract of CCs. These amino acids might have been formed either interstellar ices [4] in molecular clouds and/or interiors of meteorite parent bodies [5]. Experiments simulating interstellar and asteroid environments showed that amino acid precursors, rather than free amino acids, were formed in such prebiotic reactions [5, 6]. Amino acid precursors, however, have not been characterized well. It has often been insisted that $\alpha$-amino acids found in CCs were formed by the Strecker-type reactions. If so, aminonitriles (NH2-CHR-CN) are precursors of a-amino acids, but the evidence has not been shown, nor aminonitriles have not been detected in CCs. Shimoyama et al. [7] detected hydantoin in extracts of CCs, which is one of possible precursors of glycine. We irradiated possible interstellar media, such as mixture of carbon monoxide (or methanol), ammonia and water to simulate reactions among interstellar media in dense clouds. The irradiation products were analyzed by HPLC and mass spectrometry to characterize amino acid precursors in the products. Experiment: A mixture of CO and NH$ _{3}$ (350 Torr each) and water (5 mL) was sealed in a Pyrex glass tube with liquid water. The gaseous mixture was irradiated via a Havar foil window with 2.5 MeV protons from a Tandem accelerator (Tokyo Institute of Technology, Japan). Total quantity of electricity was 2 mC. The irradiation products are hereafter referred to as CAW. A mixture of CH3OH, NH3 and H2O (molar ratio 1:1:2.8) was sealed in a Pyrex glass tube, frozen in liquid nitrogen, and irradiated with 290 MeV/u carbon ions from HIMAC accelerator (NINS, QST, Japan). The irradiation products are hereafter referred to as MeAW. Aminoacetonitrile, a precursor of glycine, in CAW and MeAW was determined by cation-exchange HPLC after derivatized with orthophthalaldehyde and N-acetyl-L-cysteine. Hydantoin in the both products were determined by reversed-phase HPLC. Then both products were fractionated by gel filtration chromatography (column: Shodex OHpak SB-802.5 HQ) and/or ultrafiltration (Pall Ultrafiltration device 3K). After these fractionations, each fraction was acid-hydrolyzed and subjected to amino acids analysis by the cation-exchange HPLC. Both products were also analyzed by LC/Orbitrap-MS. Results and Discussion: Without hydrolysis, CAW yielded only trace of amino acids, which drastically increased after acid-hydrolysis. Glycine was predominant in both cases. It can be said that amino acid precursors, not free amino acids, were formed in these reactions. It was shown that neither aminoacetonitrile nor hydantoin were major glycine precursors in both products. We should consider unknown precursors of amino acids instead. Gel filtration chromatography of CAW had large peaks in the region of high molecular compounds whose molecular weights were some thousands (calibrated by protein standards). The fraction contained such peaks yielded amino acids after hydrolysis. Thus, we can say that complex amino acid precursors, other than aminonitriles and hydantoins, were major amino acid precursors. Ultrafiltration separation showed that amino acid precursors whose molecular weights were over 3000 were predominant. Peaks of m/z = 1000 or over were detected by mass spectrometry. Characterization of MeAW will also be reported. These results strongly suggested that complex amino acid precursors with large molecular weights could be directly generated from small molecules such as CO and NH$ _{3}$ in molecular cloud environments. If so, the Strecker-type reaction would be not a major pathway of amino acid formation in molecular clouds. We must consider non-conventional synthetic pathways of complex organics in space. The present work was partly supported by JSPS KAKENHI Grant Numbers JP17H02991 and 19K21895. [1] S. L. Miller, Science 118, 528 (1953). [2] S. L. Miller and H. C. Urey, Science 130, 245 (1959). [3] K. A. Kvenvolden et al., Nature 228, 923 (1970). [4] T. Kasamatsu et al., Bull. Chem. Soc. Jpn. 70, 1021 (1997). [5] Y. Kebukawaet al., Sci. Adv. 3 (3), e1602093 (2017). [6] Y. Takano et al., Bull. Chem. Soc. Jpn. 77, 779 (2004). [7] A. Shimoyama and R. Ogasawara, Orig. Life Evol. Biosph., 32, 165-179 (2002).