The presence of organic molecules in extraterrestrial environments has been widely accepted thanks to recent successes in the in situ detection of cometary molecules toward 67P/ Churyumov-Gerasimenko, as well as long-standing astronomical observations and analyses of carbonaceous meteorites in laboratories. However, despite extensive studies on the formation of organic molecules in various extraterrestrial environments such as molecular clouds, protosolar nebula, and asteroids, it still remains under debate when, where, and how such extraterrestrial molecules were abiotically formed.
A key molecule to solve the problems is hexamethylenetetramine, which is a polyheterocyclic organic molecule. Based on laboratory experiments simulating photochemical and thermal reactions of interstellar and cometary ice analogues (at about 10 degrees Kelvin) initially made of observed molecules, such as water, ammonia, and methanol, hexamethylenetetramine is in general a significant product (up to 60% by weight) in the total organic products. Although the composition of products varies depending on the experimental conditions, hexamethylenetetramine is generally abundant especially when methanol is used as an initial reactant. Since methanol is abundant in interstellar ices, the hexamethylenetetramine formation is likely to take place in the interstellar medium and become incorporated into solar system ices similar to other interstellar molecules.
Yet hexamethylenetetramine has not been observed toward any extraterrestrial environments. Owing to its symmetric tetrahedral structure, hexamethylenetetramine does not possess a permanent dipole moment, which precludes its remote observational detection by rotational spectroscopy. Though hexamethylenetetramine is an infrared active molecule, its detection in the presence of deep nitrogen–hydrogen, carbon–hydrogen, and carbon–nitrogen bands in ices, as well as the presence of a strong silicate band at 10 μm, would complicate its definitive identification, so it is also not surprising that it has not yet been observed in interstellar or planetary ices. However, hexamethylenetetramine has been postulated to be one of the extended sources of ammonia and hydrogen cyanide in comets. Besides the lack of astronomical detection, there has also been no report on the detection of hexamethylenetetramine in any extraterrestrial materials including carbonaceous meteorites, interstellar dust particles, and cometary return samples.
Since hexamethylenetetramine is susceptible to degradation by 100°C water and acid hydrolysis methods traditionally used in meteoritic soluble organic analyses; a different method to extract hexamethylenetetramine from meteorites was developed. In a paper published in the journal Nature Communications on 7 December 2020, Yasuhiro Oba of the Institute of Low Temperature Science at Hokkaido University, Yoshinori Takano of the Biogeochemistry Research Center at the Japan Agency for Marine-Earth Science and Technology, Hiroshi Naraoka of the Department of Earth and Planetary Sciences and Research Center for Planetary Trace Organic Compounds at Kyushu University, Yoshihiro Furukawa of the Department of Earth Science at Tohoku University, Daniel Glavin and Jason Dworkin of the Solar System Exploration Division at NASA's Goddard Space Flight Center, and Shogo Tachibana of the University of Tokyo Organization for Planetary and Space Science and the Institute of Space and Astronautical Science at the Japan Aerospace Exploration Agency, present the results of a study in which they extracted relatively large portions (masses ranging from 0.5 to 2 g) of interior samples of three carbonaceous chondrites, Murchison, Murray, and Tagish Lake, under mild conditions which utilised neither concentrated acidic solutions nor high temperatures for the extraction processes. The aqueous extracts were purified using cation-exchange chromatography and were then analysed using a high-resolution mass spectrometer coupled with a high-performance liquid chromatograph. Hexamethylenetetramine was successfully detected from Murchison, Tagish Lake, and Murray meteorite extracts at parts-per-billion levels.
Oba et al. produced mass chromatograms of the Murchison, Murray, and Tagish Lake meteorites at the mass-to-charge ratio of 141.1135 ± 0.0004, which corresponds to the protonated ion of hexamethylenetetramine formed by electrospray ionisation, analysed by a high-performance liquid chromatograph equipped with an InertSustain PFP analytical column. One sharp peak was observed for each chromatogram at about 20.5 min, which was consistent with hexamethylenetetramine standard reagent and far above the blank detection level. The similar consistency was also observed when the sample was analysed under different analytical conditions where Hypercarb or InertSustain Amide was used as a separation column for high-performance liquid chromatograph analysis. Based on the retention time and mass accuracy (within 3 parts per million of the theoretical mass-to-charge ratio), even under the different analytical conditions, the observed peak can be confidently assigned to hexamethylenetetramine. The observed consistency in the fragmentation pattern of hexamethylenetetramine by tandem mass spectrometry experiments between the Murchison extract and the standard reagent further supports this conclusion. The concentrations of hexamethylenetetramine in the three meteorites were 846 ± 37, 29 ± 9, and 671 ± 9 parts per billion for Murchison, Murray, and Tagish Lake, respectively. Mass peaks attributable to the deuterium, carbon¹³, and nitrogen¹⁵-substituted isotopologues of hexamethylenetetramine were also identified in the mass spectra of the Murchison extract. Oba et al. have confirmed that the loss of hexamethylenetetramine is negligible and that there is no hydrogen isotopic fractionation of hexamethylenetetramine during their analytical procedure.
Oba et al. also observed several peaks with the mass-to-charge ratio values well consistent with the hexamethylenetetramine derivatives methyl-hexamethylenetetramine, amino-hexamethylenetetramine, hydroxy-hexamethylenetetramine, and hydroxymethyl-hexamethylenetetramine, in the mass chromatograms at the mass-to-charge ratios of 155.1291, 156.1244, 157.1084, and 171.1240, respectively, as each protonated ion formula in Murchison. The mass-to-charge ratio 171.1240 trace shows at least three peaks, which might be derived from hydroxymethyl-hexamethylenetetramine and its structural isomers methoxy-hexamethylenetetramine and monohydroxy-monomethyl-hexamethylenetetramine. No authentic standards were available, so these assignments are the most likely but other isomers (e.g. ethyl-pentamethylene tetramine instead of methyl-hexamethylenetetramine) cannot be excluded. The absence of these species on the mass chromatograms for the hexamethylenetetramine standard reagent indicates that these are likely not formed during workup or clusters or N-functionalisations formed by electrospray ionisation and so should be indigenous to the meteorite samples. Without authentic standards, an estimate of their possible abundances assumed the same ionization efficiency as hexamethylenetetramine; the most abundant derivative was methyl-hexamethylenetetramine (2% of hexamethylenetetramine), followed by hydroxymethyl-hexamethylenetetramine or its isomers (less than 0.6%), hydroxy-hexamethylenetetramine (0.2%), and amino-hexamethylenetetramine (0.03%).
The negligible amounts of hexamethylenetetramine in the blank and control samples compared to the elevated concentrations of hexamethylenetetramine measured in the meteorite extracts argue that hexamethylenetetramine is indigenous to the meteorites. In addition, the likely detection of several hexamethylenetetramine-derivatives also bolsters this conclusion; unlike hexamethylenetetramine itself, to Oba et al.'s best knowledge, these hexamethylenetetramine-derivatives are commercially unavailable and their presence in terrestrial environments has not been reported. However, these hexamethylenetetramine-derivatives have been identified in organic residues produced by photolysis of interstellar ice analogues followed by warming to room temperatures, which mimics the processes of molecular evolution toward star formation. Furthermore, the estimated relative abundances of these hexamethylenetetramine-derivatives in the organic residues (orders of magnitudes less abundant than hexamethylenetetramine) are in reasonable agreement with those of the meteoritic hexamethylenetetramine-derivatives.
The concentration of hexamethylenetetramine in Murchison (846 ± 37 parts per billion) is within the range of individual water-extractable and acid-produced amino acids (200–5000 parts per billion) and higher than that of sugars (below 180 parts per billion) and nucleobases (less than about 70 parts per billion) in the Murchison Meteorite. In the Tagish Lake Meteorite, the concentration of hexamethylenetetramine (671 ± 9 parts per billion) is also in the range of individual amino acid concentrations identified in acid hydrolysed water extracts of the Tagish Lake Meteorite (less than 14 parts per billion: Tagish Lake 11i, less than 1000 parts per billion: Tagish Lake 11 h). While in Murray, the concentration of hexamethylenetetramine (29 ± 9 parts per billion) is lower than individual amino acid concentrations (51–2834 parts per billion) in the same meteorite. It is possible that differences in the Murchison/Tagish Lake and Murray parent body conditions (e.g. temperature, water/rock ratio, etc.) led to lower abundances or higher loss rates of hexamethylenetetramine, which may partly be related to the formation of soluble organics. A mass chromatogram of the hexamethylenetetramine concentrations normalized with glycine concentrations in the same meteorite showed no obvious trend in the concentrations of hexamethylenetetramine with glycine, suggesting no obvious correlation in terms of their formation history in each meteorite. Mass chromatograms of each meteorite extract at the mass-to-charge ratio values corresponding to imidazole and its alkylsubstituted homologues (up to seven carbon chains), which are proposed as the products after the hydrothermal degradation of hexamethylenetetramine, showed that for Murchison and Murray, the presence of alkylimidazoles was strongly expected in their extracts; while, they were significantly depleted in Tagish Lake. These results do not contradict the assumption that Tagish Lake, at least the specimen used in the present study, could have experienced less extensive hydrothermal alteration than Murchison and Murray on their parent bodies.
Given the harsh extraction conditions of amino acid analyses, one possibility is that some of the hexamethylenetetramine and its derivatives can form amino acids during routine amino acid extraction and workup. In fact, acid hydrolysis of hexamethylenetetramine-containing organic mixtures yielded amino acids, and the role of hexamethylenetetramine for amino acid formation has been investigated well in recent studies. However, the argument that hexamethylenetetramine is the origin of amino acids during workup is weakened by Murray, which has a similar abundance of amino acids to Murchison, yet the hexamethylenetetramine concentration was lower by about an order of magnitude than Murchison. Moreover, sample heterogeneity between different specimens of the same meteorite, which has been often invoked for explaining different quantitative results of some molecules including their different enantiomeric distributions in the same meteorites, can also be invoked. On the other hand, it is likely that hexamethylenetetramine is formed during our laboratory workup if both ammonia and formaldehyde are present in the aqueous extract. Previous studies detected both molecules from carbonaceous meteorites after hydrothermal treatment and/or acid hydrolysis of meteorite powders at about 100°C or above, implying that both free ammonia and formaldehyde are released from their acidlabile precursors after these treatments. Although it is not clear whether such precursors can contribute to the formation of hexamethylenetetramine in aqueous solutions without acid and high-temperature treatment at room temperature, Oba et al. expect that hexamethylenetetramine formed as such does not constitute a significant fraction in the detected hexamethylenetetramine abundance. Nevertheless, there are still a number of uncertainties on the origin of the difference in hexamethylenetetramine abundance between three meteorites analysed in the present study (e.g. hexamethylenetetramine abundance when each parent body is formed by accretion).
It is reasonable that interstellar medium-derived hexamethylenetetramine would be highly deuterium-enriched. Though the typical interstellar values (i.e. a deuterium/hydrogen ration of less than 0.01) are far higher than seen in any meteoritic compound. No levels of this extreme deuteration of hexamethylenetetramine were visible. Yet, it is still possible that the hexamethylenetetramine detected has an interstellar provenance and the interstellar medium deuterium was lost to exchange with comparatively deuterium-poor parent body fluids. Oba et al. tested the deuterium/hydrogen exchange in hexamethylenetetramine upon heating with water and silicates to simulate possible variations in the deuteration level of meteoritic hexamethylenetetramine through hydrothermal processes in asteroids. When fully deuterated hexamethylenetetramine was heated with water under alkaline conditions (pH 10) at 100°C, deuterium atoms in hexamethylenetetramine were gradually replaced with hydrogen atoms in water, resulting in the formation of partly hydrogenated hexamethylenetetramine after several days. These results suggest that even if hexamethylenetetramine was enriched in deuterium upon the formation in the interstellar medium, it might get depleted in deuterium through interactions with relatively deuterium-depleted water on the parent bodies of CM chondrite meteorites (the most common type of carbonaceous chondrite).
Once hexamethylenetetramine is incorporated into planetary systems and into a meteorite parent body, it has three likely fates: (1) physicochemical desorption from the surface of asteroids into the gas phase of the solar system, (2) decomposition, and (3) preservation. It is likely that desorption of HMT from asteroids could be induced either or both by external excitation energies (e.g. cosmic rays and ultraviolet photons) and by thermal processes, although these processes have not been studied experimentally so far. Laboratory studies strongly suggest that aqueous or thermal degradation of hexamethylenetetramine on meteorite parent bodies has a potential to yield various kinds of molecules, such as formaldehyde, ammonia, amines, amino acids, and nitrogen heterocycles, many of which have been identified in carbonaceous meteorites after hydrothermal treatment at around 100°C or acid hydrolysis. Hexamethylenetetramine that survived these desorption and degradation processes might be delivered to the Earth via meteorites and possibly interplanetary dust particles.
Among the various kinds of molecules which can form via hydrothermal degradation of hexamethylenetetramine, both formaldehyde and ammonia are considered particularly important for the formation of soluble organic molecules, such as amino acids and sugars, and insoluble organic matter in meteorites through various reactions such as formose and Mannich reactions or Strecker-cyanohydrin synthesis. Although formaldehyde and ammonia are two significant components in interstellar ices, which are mainly formed by the hydrogenation of carbon, oxygen and nitrogen atoms, respectively, due to their low desorption temperatures from interstellar grains (less than 100 K), unless transformed into other (non-volatile) species by chemical reactions, both molecules are likely to be lost from grains during warming up phases toward star formation if the temperature of the grains exceeds the desorption temperature of both molecules. In contrast, since solid hexamethylenetetramine does not desorb from grains even at 330 K, it should have more opportunity to be incorporated into inner solar system bodies. Naturally, since hexamethylenetetramine is in equilibrium with formaldehyde and ammonia, it could also have been formed on meteorite parent bodies from both molecules if they are really present, which could keep the hexamethylenetetramine concentration relatively constant. However, formaldehyde and ammonia have been identified in carbonaceous meteorites upon hydrothermal treatment at around 100°C or acid hydrolysis, conversely these species may be from the decomposition of hexamethylenetetramine on the parent body or during laboratory workup. As such, it will be challenging to constrain the location of hexamethylenetetramine formation but its presence in the processed interstellar ice analogues can be a good indicator to explain its presence in meteorites. Hence, the presence of hexamethylenetetramine in carbonaceous meteorites promises its pivotal role to carry interstellar prebiotic precursors to the inner solar system, which should contribute to the chemical evolution in the primordial stage on Earth.
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