Introduction and background

One of the important goals of astrobiology, as outlined in the NASA's Astrobiology Roadmap, is to determine the history of the prebiotic chemical ingredients that are important for habitable environments and which could sustain living systems (Des Marias et al. Reference Des Marais, Nuth, Alamandola, Boss, Farmer, Hoehler, Jakosky, Meadows, Pohorille, Runnegar and Spormann2008). Myriads of organic compounds have been found on the carbonaceous chondrites meteorites, such as Murchison (Cronin & Chang Reference Cronin, Chang, Greenberg, Mendoza-Gomez and Pirronello1993; Cronin Reference Cronin and Brack1998; Cronin et al. Reference Cronin, Cooper and Pizzarello1995; Kerridge Reference Kerridge1999; Sephton & Gilmour Reference Sephton, Gilmour, Gilmour and Keoberl2000; Cooper et al. Reference Cooper, Kimmich, Belisle, Sarinana, Brabham and Garrel2001; Sephton Reference Sephton2002; Shaw Reference Shaw2006; Schmitt-Kopplin et al. Reference Schmitt-Kopplin, Gabelica, Gougeon, Fekete, Kanawati, Harir, Gebefuegi, Eckel and Hertkorn2010). Tens of thousands of different molecular compositions have been revealed on Murchison by the application of ultra-high resolution analytical methods that combine state-of-the-art mass spectrometry, liquid chromatography and nuclear magnetic resonance (Schmitt-Kopplin et al. Reference Schmitt-Kopplin, Gabelica, Gougeon, Fekete, Kanawati, Harir, Gebefuegi, Eckel and Hertkorn2010). Meteorites are obtained from meteoroids, and the latter from asteroids (Kerridge Reference Kerridge1999; Shaw Reference Shaw2006). Meteorite is a solid body of extraterrestrial material that survives passage through the Earth's atmosphere and reaches the ground. This definition can be expanded to include Martian and Lunar meteorites (Rubin & Grossman Reference Rubin and Grossman2010). Meteors are objects entering the Earth's atmosphere that burn up completely during the passage through the upper atmosphere. They are also known as ‘shooting stars’ (Shaw Reference Shaw2006). Meteoroids are natural solid objects (typically <10 m across), which are moving in the interplanetary space, before they enter the Earth's (or other planetary) atmosphere (Shaw Reference Shaw2006; Rubin & Grossman Reference Rubin and Grossman2010). Thus, meteors and meteorites are derived from meteoroids. Asteroids, also known as ‘minor planets’, are small solar-system bodies (typically tens of metres to almost 1000 km across) that orbit around the Sun, mainly between the orbits of Mars and Jupiter. They are classified by their characteristic composition/spectra to carbon-rich (C-type), stony (S-type) and metallic (M-type) (Chapman Reference Chapman, Beatty, Petersen and Chaikin1999; Seeds Reference Seeds1994; Asteroid 2011).

Chemistry on asteroids is considered important for the early chemical evolution (Lurquin Reference Lurquin2003; Glavin et al. Reference Glavin, Aubrey, Callahan, Dworkin, Elsila, Parker, Bada, Jenniskens and Shaddad2010; Abramov & Mojzsis Reference Abramov and Mojzsis2011). Water on asteroids was available only during the periods of aqueous alteration (Kerridge Reference Kerridge1999; Bland et al. Reference Bland, Jackson, Coker, Cohen, Webber, Lee, Duffy, Chater, Ardakani, McPhail, McComb and Benedix2009). However, most organic materials are not water soluble. Thus, even though water became periodically available, it is not clear how this would help the water-insoluble materials to react. Some of the problems of the organic synthesis on asteroids or meteoroids can be solved by borrowing the knowledge from a new and emerging field of green chemistry (Anastas & Warner Reference Anastas and Warner2000; Lancaster Reference Lancaster2002; Lankey & Anastas Reference Lankey and Anastas2002; Doxsee & Hutchinson Reference Doxsee and Hutchinson2004), which is environmentally friendly. Since most organic reactions require an organic solvent, which may be toxic, running the reactions without solvents is considered beneficial to the environment. Various chemical procedures that do not utilize organic solvents have been developed by the green chemists. In this paper, we propose that many reactions on meteorites could occur as solventless (Tanaka & Toda Reference Tanaka and Toda2000; Cave et al. Reference Cave, Raston and Scott2001; Raston Reference Raston2004; Tanaka Reference Tanaka2009) or solid-state reactions (Thomas Reference Thomas1979; Toda et al. Reference Toda, Takumi and Akehi1990; Toda Reference Toda1995; Rothenberg et al. Reference Rothenberg, Downie, Raston and Scott2001; Kaupp Reference Kaupp2005). We have previously explored some of these reactions in the astrobiological context (Kolb Reference Kolb2010a, Reference Kolb, Hoover, Levin, Rozanov and Davisb). This paper is an expanded and revised version of a preliminary paper that was published in a conference proceeding (Kolb Reference Kolb, Hoover, Levin, Rozanov and Davis2010b).

Solventless and solid-state reactions

Chemists have recently explored the so-called solventless reactions (Tanaka & Toda Reference Tanaka and Toda2000; Cave et al. Reference Cave, Raston and Scott2001; Raston Reference Raston2004; Tanaka Reference Tanaka2009), which are green by virtue of not needing any solvents at all. For example, as the solid reaction components are mixed together, the melting point of the mixture becomes lower than the melting points of its individual components. In some cases, the entire mixture may melt upon mixing. The reactions could then occur in a viscous liquid state.

The principle is that of the ‘mixed melting point’, which is the lowering of the melting point of a pure component, when another component is added to it (Cave et al. Reference Cave, Raston and Scott2001; Raston Reference Raston2004). This is due to the incorporation of the impurities into the crystalline lattice of the compound, which breaks up the regular crystalline pattern. When more impurities are present, the melting point becomes even lower. Yet another scenario is to grind together the solid reactants, without melting them (Thomas Reference Thomas1979; Toda et al. Reference Toda, Takumi and Akehi1990; Toda Reference Toda1995; Rothenberg et al. Reference Rothenberg, Downie, Raston and Scott2001; Kaupp Reference Kaupp2005). Such a ground mixture may become reactive. Sometimes mild heating can be employed to speed up the reactions. These are solid–solid reactions. Another way is to use a liquid and a solid component, and grind them together. The suspension may then become reactive. Sometimes a gas–solid reaction is successful. All these scenarios could be fruitful on asteroids. Our main focus is on the solventless and the solid–solid reactions. Some asteroids, such as the C-type (Chapman Reference Chapman, Beatty, Petersen and Chaikin1999; Seeds Reference Seeds1994; Asteroid 2011) are rich in the organic compounds, judging also by the analysis of Murchison and similar carbonaceous chondrites (Cronin & Chang Reference Cronin, Chang, Greenberg, Mendoza-Gomez and Pirronello1993; Cronin Reference Cronin and Brack1998; Cronin et al. Reference Cronin, Cooper and Pizzarello1995; Kerridge Reference Kerridge1999; Sephton & Gilmour Reference Sephton, Gilmour, Gilmour and Keoberl2000; Cooper et al. Reference Cooper, Kimmich, Belisle, Sarinana, Brabham and Garrel2001; Sephton Reference Sephton2002; Sephton et al. Reference Sephton, Wright, Gilmour, de Leeuw, Grady and Pillinger2002; Shaw Reference Shaw2006; Schmitt-Kopplin et al. Reference Schmitt-Kopplin, Gabelica, Gougeon, Fekete, Kanawati, Harir, Gebefuegi, Eckel and Hertkorn2010). Some of these compounds may be reactive towards each other, but some others may not be. However, the unreactive compounds could lower the melting points of the reactive ones, enabling the reactions to occur in the melted state. We have shown this in the example of the solid-state Diels–Alder reaction between anthracene-9-methanol and N-methylmaleimide to which we have added an unreactive third component, naphthalene (Fuller & Kolb Reference Fuller and Kolb2011). The temperature estimates on the asteroids include a range from 25 to 100°C, which is consistent with aqueous alterations (Kerridge Reference Kerridge1999) and is friendly to preservation of the organic compounds. The heat is provided by the radioactive processes or impacts (Kerridge Reference Kerridge1999). This temperature range is fortuitously also a typical range for many standard organic reactions that are performed in the laboratory.

Representative examples of astrobiologically relevant solventless and solid-state reactions

A substantial body of the literature illustrates that many organic reactions occur as solventless (Tanaka & Toda Reference Tanaka and Toda2000; Cave et al. Reference Cave, Raston and Scott2001; Raston Reference Raston2004; Tanaka Reference Tanaka2009) or in the solid state (Thomas Reference Thomas1979; Toda et al. Reference Toda, Takumi and Akehi1990; Toda Reference Toda1995; Rothenberg et al. Reference Rothenberg, Downie, Raston and Scott2001; Kaupp Reference Kaupp2005). They often occur rapidly and with remarkable efficiency (Toda Reference Toda1995; Kaupp Reference Kaupp2005). Examples include all major types of organic reactions, such as oxidations, reductions, eliminations, substitutions, condensations, cyclizations, rearrangements, formation of carbon–carbon, carbon–oxygen and carbon–nitrogen bonds, among many others. Detailed experimental procedures for solvent-free reactions are compiled in a recent book (Tanaka Reference Tanaka2009). Many such procedures are either completely prebiotic, or could be made prebiotic rather easily. However, the real velocity of the reactions should be tested in planetary chambers under interplanetary conditions. This fortunate fact was not addressed in the book, which emphasis is a general organic synthesis. Especially interesting is the formation of ethers in the solid state, from the solid mixture of the alcohols and acid catalysts (Toda et al. Reference Toda, Takumi and Akehi1990). Such a synthesis could be a prebiotically feasible way to produce various ethers. Such ethers could be used as membranes for the primitive organisms. While most contemporary organisms use lipids that are esters (Voet et al. Reference Voet, Voet and Pratt2006), some archaea's membranes are ethers (De Rosa et al. Reference De Rosa, Gambacorta and Gliozzi1986; Kates Reference Kates, Kates, Kushner and Matheson1993). Other interesting examples include syntheses of large molecules that are related to porphyrins. These and other examples are presented below, with the chemical equations and essential conditions. They illustrate a great potential for prebiotic chemistry and should be investigated further in this context. All the examples come from Tanaka (Tanaka Reference Tanaka2009), who cites all the primary references. Additional references are cited if they provide more details about the procedures and to show the prebiotic significance that was not addressed by Tanaka.

Oxidation of alcohols to aldehydes

We show an example of oxidation reaction in which alcohols are oxidized to aldehydes in a liquid/gas or a solid/gas reaction. We are not arguing here for the prebiotic role of the NO₂ gas (Fig. 1). Instead, the prebiotic significance of this reaction is that the available equivalent prebiotic gaseous oxidizing agents could have worked in an analogous way. Aldehydes are an important group of reactive molecules which can lead to various complex prebiotic molecules. For example, the Maillard reaction, which occurs between the sugar aldehydes and the amino acids, results in a myriad of complex molecules (Kolb et al. Reference Kolb, Bajagic, Zhu, Cody, Hoover, Levin, Rozanov and Gladstone2005, Reference Kolb, Bajagic, Liesch, Philip, Cody, Hoover, Levin and Rozanov2006; Kolb & Bajagic Reference Kolb and Bajagic2006a, Reference Kolb, Bajagic, Yingst, Brandt, Borg, Dutch, Gustafson, Rudd and Roethelb). The astrobiological importance of the Maillard reaction is very high since the starting materials are definitively prebiotic. Sugars can be formed by prebiotic means (Kim et al. Reference Kim, Ricardo, Illangkoon, Kim, Carrigan, Frye and Benner2011; Ricardo et al. Reference Ricardo, Carrigan, Olcott and Benner2004; Lambert et al. Reference Lambert, Lu, Singer and Kolb2004, Reference Lambert, Gurusamy-Thangavelu and Ma2010). The sugar-related molecules have been found on meteorites (Cooper et al. Reference Cooper, Kimmich, Belisle, Sarinana, Brabham and Garrel2001). The amino acids are the ubiquitous prebiotic molecules (Miller Reference Miller1953, Reference Miller1955; Fitz et al. Reference Fitz, Reiner and Rode2007), which are abundant on the meteorites (e.g. Cronin et al. Reference Cronin, Cooper and Pizzarello1995; Sephton Reference Sephton2002).

Fig. 1. Oxidation with NO₂ gas (Tanaka Reference Tanaka2009, p. 31).

Carbon–carbon bond formation by the oxidative coupling of phenols

Prebiotic and thus astrobiological significance of phenols was established (Hayatsu et al. Reference Hayatsu, Winans, Scott, McBeth, Moore and Studier1980; Tian et al. Reference Tian, Yuan, Mu, He and Feng2007; Kolb & Liesch Reference Kolb, Liesch, Hoover, Levin, Rozanov and Davis2008). A synthesis of phenol by treatment of sodium bicarbonate with water and iron as a catalyst under hydrothermal conditions was reported (Tian et al. Reference Tian, Yuan, Mu, He and Feng2007) and is shown in Fig. 2.

Fig. 2. Prebiotic synthesis of phenol (Tian Reference Tian, Yuan, Mu, He and Feng2007).

This synthesis has placed phenol on the map of the compounds that were feasible on the prebiotic Earth, or other planets in which hydrothermal conditions exist or have existed.

An example of the oxidative coupling of phenols, which results in the formation of a carbon–carbon bond, is shown in Fig. 3. It can occur as a solid-state reaction. The molecule obtained, so-called ‘binol’ ([1,1′-binaphthalene]-2,2′-diol), is not planar due to the steric hindrance. Instead, it is twisted and thus asymmetric, which makes it chiral (Carey & Sundberg Reference Carey and Sundberg2007). This molecule is obtained in the laboratory as a racemic mixture of two enantiomers, which can be separated, for example by the chromatography with a chiral stationary phase. We have checked the procedure for the formation of binol in our laboratory. It works really well, as described. Under the prebiotic conditions enantiomeric enrichment can be achieved perhaps under the influence of the available chiral molecules or minerals. Thus, the prebiotic synthesis of binol is important since it has a good potential for the development of enantiomeric selectivity on the prebiotic Earth (Fig. 3).

Fig. 3. Oxidative coupling of a phenol (a naphthol) to a binol (Tanaka Reference Tanaka2009, p. 33).

Formation of ethers under the solid-state conditions

Formation of ethers by condensation of alcohols is not favourable in the aqueous solution. However, this reaction works well in the solid state with the solid p-toluene sulfonic acid (TsOH) as a catalyst (Toda et al. Reference Toda, Takumi and Akehi1990). One such example is shown in Fig. 4. We have reproduced this experiment in our laboratory. It works really well, especially if anhydrous TsOH is used or is created in situ by a mild heating of the commonly available TsOH hydrate in the reaction mixture.

Such solid-state synthesis could be prebiotically feasible. The use of TsOH as a catalyst can probably be substituted by various acidic clays (Fig. 4).

Fig. 4. Solvent-free etherification reaction (Tanaka Reference Tanaka2009, p. 276).

Ethers may have been used as primitive membranes in the original organisms. Even today some archaea use ethers as membranes (De Rosa et al. Reference De Rosa, Gambacorta and Gliozzi1986; Kates Reference Kates, Kates, Kushner and Matheson1993). However, most contemporary organisms use lipids, which are esters, as their membranes (Voet et al. Reference Voet, Voet and Pratt2006).

Formation of esters in the solid state

Esterification reaction between alcohols and carboxylic acids is reversible. It produces the desired ester and water. The equilibrium of the reaction needs to be shifted towards the ester product. Thus water should be removed. In the organic laboratory, water can be removed by using concentrated H₂SO₄ as a catalyst and a dehydrating agent, or by use of the contraptions such as Dean–Stark trap, which remove water (e.g. Harwood et al. Reference Harwood, Moody and Percy1999). Neither procedure is prebiotic, since it involves intervention. However, the solvent-free esterification is shown in Fig. 5 appears prebiotically feasible. Perchlorates, which catalyse this reaction, are found on Earth, especially under arid conditions (Duncan et al. Reference Duncan, Morrison and Vavricka2005; Navarro-Gonzáles et al. Reference Navarro-Gonzáles, Rainey, Molina, Bagaley, Hollen, de la Rosa, Small, Quinn, Grunthaner, Cáceres, Gomez-Silva and McKay2003), and also on Mars (Hecht et al. Reference Hecht, Kounaves, Quinn, West, Young, Ming, Catling, Clark, Boynton, Hoffman, DeFlores, Gospodinova, Kapit and Smith2009).

Fig. 5. Solvent-free esterification (Tanaka Reference Tanaka2009, p. 288).

Another prebiotically feasible solid-state esterification is that of cholesterol. The ester thus obtained, shown below, could perhaps have served as a primitive lipid membrane (Fig. 6).

Fig. 6. Cholesterol solid–solid esterification (Tanaka Reference Tanaka2009, p. 431).

Aldol condensations

Aldol-type condensation reactions, which form carbon–carbon bonds, play an important role in chemistry (e.g. Laue & Plagens Reference Laue and Plagens2005) and biology, e.g. in the citric-acid cycle (e.g. Bettelheim et al. Reference Bettelheim, Brown and March2004). Aldol reactions under solventless, solid–solid and solid–liquid conditions, with various catalysts, have been reviewed by Tanaka (Reference Tanaka2009). An example in which basic alumina is used as a catalyst, under solvent-free conditions, follows (Fig. 7).

Fig. 7. Aldol condensation with basic alumina (Tanaka Reference Tanaka2009, p. 39).

Another example of aldol reaction, this time with NaOH, is performed under solventless conditions. One component is a liquid and the other is a solid. They are made reactive by grinding (Fig. 8).

Fig. 8. Liquid–solid aldol reaction (Tanaka Reference Tanaka2009, p. 40).

The solventless aldol reaction in which two solids melt upon grinding is shown in Fig. 9.

Fig. 9. Aldol reaction in a melted state (Tanaka Reference Tanaka2009, p. 45).

Solventless synthesis of corrole

A rather spectacular synthesis follows, in which a corrole, a porphyrin analogue, is formed. The reaction occurs under solventless conditions. Corrole could serve as a primitive enzyme, since it has a metal complexing site in the centre of the molecule (Fig. 10).

Fig. 10. Solventless condensation of pyrrole and aldehyde to a corrole (Tanaka Reference Tanaka2009, p. 43).

Synthesis of a macromolecule with a cavity

It is often difficult to get a high yield of well-defined macromolecular materials in the aqueous solution. The example below illustrates success of the solid-state chemistry. The large cyclic molecule that is formed could perhaps serve as a primitive enzyme on the account of its cavity. The TsOH could perhaps be substituted with some acidic clay (Fig. 11).

Fig. 11. Resorcinol/aldehyde solvent-free cyclocondensation to a macromolecule with a cavity (Tanaka Reference Tanaka2009, pp. 50–51).