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Precambrian Research 164 (2008) 111–118 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres Review The prebiotic geochemistry of formaldehyde H. James Cleaves II ∗ Geophysical Laboratory, The Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, DC 20015, United States a r t i c l e i n f o Article history: Received 5 September 2007 Received in revised form 2 April 2008 Accepted 3 April 2008 Keywords: Formaldehyde Formose Prebiotic chemistry Origin of life Hexamethylenetetramine Cyanide Hydrogen sulfide Geochemistry a b s t r a c t Formaldehyde (HCHO), the simplest aldehyde, is an intermediate oxidation state one carbon molecule that exists transiently but prominently in the abiological carbon cycle, and is ubiquitous in the cosmos. Its potential prebiotic importance is suggested by the fact that it readily undergoes a variety of addition and redox reactions to give products of biological significance including sugars and amino acids. It is especially important with respect to the origin of an RNA or pre-RNA world, since HCHO may be a precursor to ribose and other sugars. HCHO is introduced to the environment by a number of processes including atmospheric and aqueous phase synthesis as well as extraterrestrial delivery, balanced by various destructive processes such as photolysis and redox equilibration in hydrothermal environments. While the Strecker synthesis of amino acids can occur at very low dilution, even best case scenarios for HCHO steady-state concentrations in the primitive oceans are too low for the formation of sugars to occur. Concentration mechanisms would thus be necessary. As HCHO is volatile, direct evaporation is not possible, but other geochemical mechanisms such as eutectic freezing and conversion to non-volatile derivatives by reaction with other species present in the primitive environment, followed by evaporation, could have concentrated HCHO sufficiently to allow for sugar synthesis. © 2008 Elsevier B.V. All rights reserved. Contents 1. 2. 3. 4. 5. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prebiotic sources of HCHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prebiotic sinks for HCHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prebiotic solution chemistry of HCHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Oligomerization chemistry—polyoxymethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Oligomerization chemistry—the formose reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Reactions with amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Reactions with sulfur species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Mineral interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Reaction with HCN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Concentration mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction Formaldehyde (HCHO) is a one carbon molecule intermediary along the redox continuum between CO2 and CH4 , at the same oxidation state (0) as graphite. HCHO appears to be an abundant ∗ Tel.: +1 202 478 8957; fax: +1 202 478 8464. E-mail address: [email protected]. 0301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2008.04.002 111 112 112 113 113 113 114 114 115 115 115 116 116 116 interstellar molecule and is suggested to be a major constituent of cometary ices (Schutte et al., 1993; Fomenkova et al., 1994; Biver et al., 2002). It is readily produced in prebiotic simulation experiments from a variety of gas mixtures and energy sources (Bossard et al., 1982; Schlesinger and Miller, 1983; Stribling and Miller, 1987). It has been suggested for some time that HCHO may have played an important role in the synthesis of organic molecules relevant to the origin of life. HCHO is likely a significant precursor for the prebiotic synthesis of glycine (Miller, 1953), and HCHO reacts 112 H.J. Cleaves II / Precambrian Research 164 (2008) 111–118 readily with glycine to form a number of other amino acid products (Choughuley et al., 1975) which have been detected in both meteorites and prebiotic simulation experiments (Wolman et al., 1972). It has long been known that HCHO can react to form sugars under basic conditions (Butlerow, 1861), a reaction that has potential importance for the origin of an “RNA World” (Gilbert, 1986; Gesteland and Atkins, 1993) or early nucleic acids based on alternative sugars in a potential “pre-RNA World” (Joyce et al., 1987; Eschenmoser, 2004). It has been argued based on kinetic and thermodynamic principles that HCHO may in fact be the only one carbon C-, H- and O-containing molecule capable of generating complex organic compounds for the origin of life (Weber, 2002). The chemistry of HCHO in the primitive environment would have depended on a number of factors, including the atmospheric composition, energy flux through the atmosphere, temperature and pH of the oceans, the availability and nature of minerals, dry land surfaces and microenvironments. Unfortunately there is a great deal of uncertainty in most of these considerations. Estimates of the composition of the early atmosphere range from fairly reducing (Tian et al., 2005) to neutral (Chyba and Sagan, 1992). The flux of energy was probably dominated by UV radiation, along with significantly lesser amounts of electrical energy and energy from radioactive decay (Chyba and Sagan, 1992). The early Earth’s surface temperature may have been extremely cold, resulting in a generally ice-covered planet (Bada et al., 1994; Price, 2007), extremely warm (Knauth, 2005), or perhaps with regions of both cold and hot temperature similar to today’s climate (Henderson–Sellers and Henderson–Sellers, 1988; Valley et al., 2002; Vincent et al., 2005). The oceanic composition may have been more carbonic than today (Morse and Mackenzie, 1998), slightly more saline (Morse and Mackenzie, 1998), appears to have been suffused with large amounts of ferrous iron (Holland, 1973; Walker and Brimblecombe, 1985; Isley and Abbott, 1999), and likely had a pH somewhere between 5 and 11 (Kempe and Degens, 1985; Morse and Mackenzie, 1998; Kempe and Kazmierczak, 2002), although it has been argued that ocean pH may have been maintained fairly close to the modern value (∼8) by mineral buffering (Sillén, 1967). It is debated whether there was any dry land at the time of the origin of life, as widescale crustal recycling to form continents may not yet have begun (Hofmann, 1997; Godderis and Veizer, 2000; Wilde et al., 2001), nevertheless island arcs may have been present (Barley et al., 1979). All of these considerations are important to the discussion that follows, and certainly some combinations of these are incompatible with certain proposals for the origin of life, and more specifically, HCHO’s role in it. 2. Prebiotic sources of HCHO The most important source of HCHO on the primitive Earth was probably gas phase atmospheric photochemical synthesis by the photo-reduction of CO2 with H2 O (among other mechanisms) (Reaction (1)): CO2 + H2 O ⇔ HCHO + O2 (1) Pinto et al. (1980) estimated that HCHO could be produced in yields of up to 1011 mol/year, reaching a steady-state oceanic concentration of 10−3 M in 107 years. The significance of this time interval is discussed below. Holland (1984) on the other hand estimated a steady-state oceanic concentration of HCHO of 4 × 10−4 M. It has been estimated that if all of the carbon in the Earth’s surface reservoirs (crust, atmosphere and oceans) were dissolved in oceans of the present size, a concentration of ∼1 M would be obtained (Schwartz, 1981), but this extreme is meant solely for purposes of comparison. Electric discharges acting on a variety of gas mix- tures also produce HCHO in good yield (Miller, 1953; Stribling and Miller, 1987), though there would have been considerably less energy available from electric discharges than from UV sources (Chyba and Sagan, 1992). Photochemical production from atmospheric CH4 and/or CO could also have been significant sources of HCHO (Hubbard et al., 1971; Ferris and Chen, 1975; Bar-Nun and Hartman, 1978; Schwartz, 1983; Wen et al., 1989). The production and rainout rates of HCHO are highly sensitive to energy source and atmospheric composition (Chang, 1993). HCHO could also potentially be produced under hydrothermal vent conditions (Ferris, 1994), for example by the reduction of aqueous formate, CO or CO2 , or by the oxidation of methanol or CH4 , although HCHO and methanol do not appear to be stable redox states under hydrothermal conditions (Osada et al., 2004; Seewald et al., 2006). Little HCHO is observed in modern fluids, possibly for this reason. One poorly surveyed topic is the photo-reduction of dissolved CO2 species with reducing agents such as Fe2+ in the photic zone of the primitive oceans. Fe2+ concentrations in the primitive oceans are estimated to have been as high as 10−4 M (Holland, 1973). Photoreduction of dissolved CO2 species in this manner has been claimed (Åkermark et al., 1980), and given the high UV flux, large amounts of dissolved carbonate species, and possibly transition metals in addition to water as reducing equivalents, this might have been a significant source of reduced carbon species. It has also been suggested that extraterrestrial materials such as comets may have delivered significant quantities of HCHO to the surface of the Earth (Chyba et al., 1990). Indeed depending on the composition of the atmosphere and the nature of the extraterrestrial input this could have represented the single largest source of reduced organic compounds to the surface of the Earth (Chyba and Sagan, 1992). Of course upon reaching the primitive surface, this material would have been subject to the same environmental chemistry as endogenously synthesized organic material. 3. Prebiotic sinks for HCHO HCHO decomposes thermally in the gas phase above 300 ◦ C to give CO and H2 (Bone and Smith, 1905) (Reaction (2)): HCHO(g) ⇔ CO(g) + H2 (g) (2) Between 150 ◦ C and 300 ◦ C methanol is also observed as a product (Calvert and Steacie, 1951). Relatively long wavelength photochemical destruction was also likely a significant sink for HCHO (see below) (Calvert and Steacie, 1951). Sekine (2002) found that MnO2 catalyzes the oxidation of HCHO at room temperature to CO2 under atmospheric conditions. Löb (1906) found that electric discharges acting on HCHO in the presence of water vapor give CO, CO2 , H2 and CH4 as products, which is essentially the reverse of the reaction demonstrated by Miller (1953) mentioned above. The present mean lifetime for the cycling of the entire ocean through the mid-ocean ridge hydrothermal vents is ∼107 years (Edmond et al., 1982). This cycling likely occurred much more rapidly on the primitive Earth (by perhaps a factor of 5) than it does today based on heat flux estimates (Turcotte, 1980). Small prebiotic precursor species (HCHO, HCN, NH3 , etc.) would have been redox equilibrated under hydrothermal vent conditions (Lazcano and Miller, 1994). As mentioned earlier, HCHO appears to be thermally unstable with respect to CO2 , CO, and CH4 , thus hydrothermal vents are more likely to be net destroyers of HCHO than net producers. There is reason to believe that there was a considerable flux of atmospherically synthesized and rained-out H2 O2 to the primitive oceans, particularly if the atmosphere was non-reducing (Kasting H.J. Cleaves II / Precambrian Research 164 (2008) 111–118 113 4.1. Oligomerization chemistry—polyoxymethylene Fig. 1. Cyclic HCHO oligomers, trioxane (left) and tetraoxane (right). and Brown, 1998). Reaction of HCHO with dissolved peroxide, which can also be formed photochemically, gives formate and H2 (Walker, 1964) (Reaction (3)): 2HCHO(aq) + 2OH− + H2 O2 (aq) ⇔ 2HCOO− (aq) + 2H2 O + H2 (g) (3) 4. Prebiotic solution chemistry of HCHO An excellent monograph has been written on the chemistry of HCHO (Walker, 1964). In aqueous solution HCHO is mostly present as the monohydrate, methylene glycol (Reaction (4)). Unhydrated HCHO is only present to a degree of ∼0.1% in aqueous solution. HCHO + H2 O ⇔ CH2 (OH)2 (4) One important consequence of this is that while HCHO absorbs considerable energy in the range of 250–350 nm, resulting in photolysis, the hydrate is does not absorb light at these wavelengths (Ferris and Chen, 1975), thus dissolution in water could be an important photo-protection mechanism for HCHO. Nevertheless, as discussed below, there are several major photochemical transformations of HCHO which occur in aqueous solution. A series of low molecular weight polymers, polyoxymethylene glycols (POMs) (of formula HO(CH2 O)n H) as well as the cyclic oligomers trioxane and tetraoxane (Fig. 1), form readily in neutral concentrated aqueous HCHO solutions. As the chain length of POMs grows, their solubility decreases, and their solubility is temperature dependant, being more soluble at higher temperatures. It seems unlikely that bulk ocean concentrations of HCHO and its polymers could have been high enough to form precipitates. For example, a 5 wt% (∼1.3 M) aqueous HCHO solution contains only ∼14% POM dimer, 3% trimer, and 0.5% tetramer. Polymerization equilibria would be much lower for the prebiotic oceanic concentrations of HCHO estimated above. 4.2. Oligomerization chemistry—the formose reaction HCHO can be oligomerized under basic conditions into sugars via the so-called formose reaction (Butlerow, 1861) (Fig. 2), which has been observed to occur as dilute as 0.01 M (Reid and Orgel, 1967; Schwartz and De Graaf, 1993), although there are accounts of its reaction as dilute as 10−3 M (Gabel and Ponnamperuma, 1967). As mentioned earlier, these concentrations may have been difficult to achieve in the bulk early oceans. HCHO reacts with acetaldehyde at much lower concentrations to give acrolein among other products (Cleaves, 2003). There must be limiting reaction conditions of concentration, pH and temperature under which this reaction will not occur, though these are not well surveyed. The formose reaction is generally carried out under extremely basic conditions, but reaction has been observed at near neutral pH in the presence of minerals (Cairns-Smith et al., 1972; Schwartz and De Graaf, 1993). It has been pointed out that the reaction conditions which allow oligomerization of HCHO also facilitate the products’ decomposition into tars (Reid and Orgel, 1967; Shapiro, 1988), furans and other low molecular Fig. 2. Possible mechanism for the formose reaction and observed products. 114 H.J. Cleaves II / Precambrian Research 164 (2008) 111–118 Fig. 5. Products of the reaction of H2 S with HCHO. Formthionals (top) and trithiane (bottom). Fig. 3. Branched polyols detected in photochemical formose reactions. weight degradation products (De Bruijn et al., 1986; Cooper et al., 2001). The reaction of HCHO into formose is one limiting reaction pathway for the accumulation of HCHO in primitive waters, which would depend on the rate of HCHO production and delivery to the primitive oceans (Pinto et al., 1980; Schwartz, 1983). If HCHO became sufficiently concentrated, side reactions may have established the steady-state concentration regardless of the production rate. Given the observed photochemical synthesis of pentaerythritol (Fig. 3, left hand side) (Schwartz and De Graaf, 1993), and its likely reaction mechanism, it seems possible that HCHO is converted to CH3 CHO by UV light, and thus it appears unlikely that bulk oceanic concentrations of HCHO could have been much higher than ∼10−3 M. HCHO would also have been prone to photoreaction in the primitive oceans. For example, HCHO photochemically reacts to give mostly pentaerythritol (Schwartz and De Graaf, 1993), among other non-sugar products (Fig. 3) (Shigemasa et al., 1977). 4.3. Reactions with amines Another possible limitation to oceanic HCHO concentrations is the conversion of HCHO into hexamethylenetetramine (HMT or 1, 3, 5, 7-tetraazatricyclo [3.3.1.13,7 ] decane) (Fig. 4) by reaction with ammonia (NH3 ) (Butlerov, 1860; Walker, 1964), as noted by Gabel and Ponnamperuma (1967). The concentration of NH3 in the primitive oceans would have been governed by the reduction rate of atmospheric N2 (Schwartz, 1983; Schrauzer et al., 1983; Brandes et al., 1998) and the overall redox balance of the primitive ocean–atmosphere system (Summers and Chang, 1993). NH3 is also easily photo-oxidized (Ferris and Nicodem, 1972), thus its survival would have depended on the pH of the early oceans (except under extremely basic conditions, most NH3 would end up dissolved in the oceans) as well as by the availability of reduced species such as Fe2+ (Summers and Chang, 1993; Walker and Brimblecombe, 1985). Reasonable bulk ocean concentrations of NH3 have been estimated in the range of 10−3 to 2 × 10−6 M (Bada and Miller, 1968; Summers, 1999). The Keq to form HMT is extremely high—estimated to be ∼1010 (Walker, 1964). This species may be chemically inert (Wolman et al., 1971), although there are dissenting opinions (Fox and Windsor, 1970). HMT has been shown to be produced by UV irradiation of interstellar ice analogs, along with a variety of amino acid products (Bernstein et al., 2002). The prebiotic chemistry of HMT is worthy of further investigation. It is noteworthy that HMT is non-volatile and thus could serve as a means of concentrating volatile NH3 and HCHO by evaporation. Since HMT is more stable at higher pH, changes in pH that occur as various minerals precipitate during the evaporation of seawater (Lazar et al., 1983) might then liberate these reagents in more concentrated form for further reactions. Interestingly, sulfuric acid dissociates HMT to (NH4 )2 SO4 and HCHO, and is a catalyst for POM formation (Walker, 1964). Thus, sulfate–HMT interactions during evaporation may have allowed concentration and reaction of HCHO. The reaction of fairly dilute aqueous HCHO with NH3 (∼0.003 M each) at high temperature (100–250 ◦ C) has been shown to produce amino acids, amines and amino alcohols (Aubrey et al. (submitted for publication)), and the reaction of somewhat more concentrated HCHO (0.02 M) with NH3 (0.04 M) has been shown to produce traces of amino acids at lower temperatures (40 ◦ C) (Weber, 1998). Thus for at least several simple amino acids, NH3 and HCHO may be sufficient for synthesis to occur. HCHO reacts rapidly and reversibly with urea to form methylol ureas (Walker, 1964) (Reactions (5) and (6)): HCHO + H2 NCONH2 ⇔ HOCH2 NHCONH2 (5) HCHO + HOCH2 NHCONH2 ⇔ HOCH2 NHCONHCH2 OH (6) These polymers have found widespread commercial use, and in fact the liberation of HCHO from such polymers is a serious health concern (Lemière et al., 1995). In a prebiotic context these reversible reactions could analogously serve as a method of concentrating HCHO in a non-volatile form in evaporating environments, later liberating it for subsequent reaction to form other more stable products. 4.4. Reactions with sulfur species HCHO may be lost to reduction via reaction with species such as Fe2+ and H2 S. In reducing environments, HCHO may be merely an intermediate in the redox equilibration of other more stable compounds (Seewald et al., 2006). HCHO and H2 S react rapidly to produce trithiane and other low molecular weight oligomers (Walker, 1964) (Fig. 5), which have been detected in hydrothermal vent effluent (Simoneit (1992)), as well as in hydrothermal vent simulations (Cole et al., 1994) (Reactions (7–9)). H2 S + H2 C(OH)2 ⇔ HOCH2 SH + H2 O (7) 2HOCH2 SH ⇔ HSCH2 OCH2 SH (8) HOCH2 SH + HSCH2 OCH2 SH ⇔ HSCH2 SCH2 OCH2 SH + H2 O (9) It is generally believed that the early oceans were sulfatic (SO4 2− -containing) rather than sulfidic (S2− -containing) (Walker and Brimblecombe, 1985; Grotzinger and Kasting, 1993), but high concentrations of H2 S may have been present in hydrothermal vents or other deep anoxic environments. Aqueous HCHO reacts with SO2 to give methylolsulfonic acid (Walker, 1964) (Reaction (10)): Fig. 4. Hexamethylenetetramine (HMT), a product of the condensation of HCHO with NH3 . H2 CO + H2 O + SO2 ⇔ HOCH2 SO3 H (10) H.J. Cleaves II / Precambrian Research 164 (2008) 111–118 The salts of this compound are non-volatile, but decompose in dilute acid, liberating HCHO. It is interesting to speculate that cycling of these compounds in a drying or eutectic environment might allow for the concentration of HCHO to allow for synthesis of non-volatile compounds such as sugars. It is worth noting that the Murchison meteorite contains large amounts of alkane sulfonic acids (Cooper et al., 1992), and perhaps this is one mechanism by which these compounds were formed. It has been suggested that sulfite (SO3 2− ) may have been the principal sulfur species in the primitive oceans, but the primitive sulfur cycle was likely somewhat complex (Kasting et al., 1989). 4.5. Mineral interactions Another sink for HCHO may have been mineral adsorption. There have been few studies of the adsorption of HCHO to minerals, though the ones that do exist suggest that HCHO–clay adsorption equilibria are not especially high. This is generally also true for low molecular weight POMs (Parfitt and Greenland, 1970). The adsorption equilibria may be more significant for clays such as illite and kaolinite (De and Chandra, 1978; Chandra and De, 1983). Clay adsorption may have lead to significant concentration effects which may have facilitated reactions of HCHO such as oligomerization to formose products and redox reactions to methanol, formate and CO2 . Certain minerals, in particular borates (Ricardo et al., 2004), stabilize several intermediates in the formose reaction pathway. There may be yet other minerals that alter reaction pathways (Gabel and Ponnamperuma, 1967; Reid and Orgel, 1967; Cairns-Smith et al., 1972), for example both apatite and calcite were found to be catalysts for the formose reaction (Schwartz and De Graaf, 1993). These effects have generally been observed using high concentrations of HCHO (0.01 M), it remains unknown whether they can alter the reactions of more dilute HCHO. 4.6. Reaction with HCN Aqueous HCHO reacts readily with HCN to give glycolonitrile (Henry, 1890) (Reaction (11)). HCHO + HCN ⇔ HOCH2 CN (11) Here HCHO represents the combined concentrations of HCHO and its hydrate. The Keq for the reaction is exceptionally high (Schlesinger and Miller, 1973), thus any atmospheric composition which results in the production of the two compounds essentially results in the production of glycolic acid (Arrhenius et al., 1994), after hydrolysis of the nitrile. A “best case” concentration of HCN in the primitive ocean has been estimated at 3.5 × 10−5 M (Stribling and Miller, 1987). Schwartz and Goverde (1982) found that the addition of HCHO or glycolonitrile to HCN actually accelerates the formation of HCN tetramer, diaminomaleonitrile (DAMN). Eschenmoser and co-workers found that HCHO reacts readily with DAMN to form a crystallizable product (Koch et al., 2007), a fact which noted some years earlier (Gluesenkamp, 1958). The ratio of HCN/HCHO production rates is again highly dependant on the atmospheric composition and nature of the energy source (Chang, 1993). If the concentration and reaction of HCHO on the primitive Earth were difficult because of all of the potential competing geochemical sinks, sugars may have been derived from meteoritic input. Polyols such as glycerol and ribitol have been identified in the Murchison meteorite, although actual sugars (of formula (CH2 O)n ) higher than dihydroxyacetone have not been identified (Cooper et al., 2001). This can be explained partially by the relative instability of sugars over the ∼4 billion years since the Murchison parent body 115 formed and these reactions presumably occurred: the initial synthesis to form sugars may have occurred relatively rapidly on the parent body, while only those compounds stable enough to survive until the present day are still detectable. The detection of these compounds does however present an interesting paradox for prebiotic chemistry. If the polyols and their precursors were generated by the same aqueous phase chemistry which produced the amino and hydroxy acids, as well as the purines and pyrimidines which have been detected in the Murchison meteorite (Pizzarello, 2004), apparently the inhibition of HCN chemistry by HCHO and vice versa (Schlesinger and Miller, 1973; Arrhenius et al., 1994) is not a genuine problem. The ratio of glycolic acid to glycine in Murchison suggests that NH3 concentrations were fairly high in the parent body (Peltzer et al., 1984). These quantities also suggest that the reactions which form HMT and glycolonitrile are not limiting to either purine or sugar synthesis, and that slow kinetic effects may be more important than the initial rapid equilibrium obtained. This is certainly a question worthy of further investigation. Cannizzaro reactions may also be important loss channels for HCHO. If HCHO concentrations are high enough, HCHO disproportionates significantly to form formate and methanol (Walker, 1964), which reduces the concentration of HCHO available to form sugars. These reactions are acid-, base-, and metal-catalyzed (Walker, 1964). Since the reaction reportedly shows third order kinetics, it is not clear whether it would occur significantly in dilute solution. 4.7. Concentration mechanisms A serious problem with the formose reaction as it relates to the origin of life is the concentration of formaldehyde, likely very dilute in the primitive seas, to high enough values that reactions to form sugars and higher products could occur. While it seems unlikely that this could have occurred significantly in the open oceans, it is also unlikely that HCHO could have been sufficiently concentrated on benthic clays or in evaporitic basins due to its extreme dilution (Lahav and Chang, 1976), and general low affinity for mineral surfaces in the first case, and in the second case due to its volatility. Incorporation into low molecular weight non-volatile compounds such as HMT and glycolonitrile could be a mechanism for the concentration of HCHO in evaporating environments. Eutectic freezing has been shown to be an effective method for concentrating dilute prebiotic reactants to form purines, pyrimidines, and amino acids (Sanchez et al., 1966; Levy et al., 2000; Miyakawa et al., 2002; Cleaves et al., 2006), as well as the oligomerization of HCN which is further accelerated by the addition of HCHO (Schwartz and Goverde, 1982). Whether HCHO can also be concentrated in a eutectic sufficiently to form sugars is unknown, but seems plausible, and is currently under investigation in our laboratory. One tremendous advantage of this is that sugars are generally much more stable at low temperatures (Larralde et al., 1995). It is possible that the eutectic freezing and thawing of precursor icegrain organics such as HCHO in carbonaceous chondrites may have allowed for the synthesis of sugars on these bodies. In contrast to many of the investigations which have been conducted in prebiotic chemistry, the oceanic concentrations of most precursor compounds were likely quite low (Stribling and Miller, 1987). It is necessary to consider the geochemical processes which might have concentrated these compounds sufficiently to allow them to react further. It is also worth considering what happens to dilute solutions of HCN, HCHO and aldehydes, urea, nitrate, nitrite and ferrous iron as they are evaporated in the presence of UV and visible light over mineral surfaces, or as they are frozen from dilute solution under visible and/or UV irradiation. Double layer hydroxide minerals have been shown to be catalysts for the self-condensation of HCN into non-volatile products (Boclair et al., 116 H.J. Cleaves II / Precambrian Research 164 (2008) 111–118 Fig. 6. Geochemical cycling of HCHO. HCHO is produced and destroyed in the atmosphere (1), and introduced by extraterrestrial delivery to the atmosphere and oceans (2). HCHO is also delivered to the oceans by rainout from atmospheric sources (as well as generated and destroyed photochemically in surface waters) (3) and destroyed during passage through hydrothermal vents (4). Aqueous phase reactions of HCHO may generate amino acids via the Strecker synthesis (5) as well as other non-volatile products via reaction with other dissolves species such as HCN and NH3 (6). HCHO cannot be concentrated by evaporation as it is volatile (7). Non-volatile species generated in (6) may be concentrated by evaporation and subsequently liberate HCHO in concentrations sufficient for the synthesis of sugars via the formose reaction (8). HCHO may also be concentrated in eutectics, for example in sea ice (9), allowing the formose reaction to occur. 2001) as well as catalysts for aldol reactions (Arrhenius et al., 1994). In this regard the frozen clay model proposed by Lahav and Chang (1976) also warrants careful reconsideration. The equilibrium constant for the dimerization of HCHO to give glycolaldehyde is not well known, but is of considerable interest, since while HCHO is volatile, glycolaldehyde is not, and could thus be concentrated by evaporation. A rudimentary calculation using the free energy of the aldol reaction of HCHO given by Weber (2002) suggests the equilibrium constant is ∼40, but of course this does not take into account the various kinetic factors which may limit the robustness of the reaction. This simple reaction is worthy of further investigation from the standpoint of prebiotic chemistry, especially given the apparent ubiquity of glycolaldehyde in interstellar space (Hollis et al., 2000). 5. Conclusions Sugars are important for several models for the origin of life (Gilbert, 1986; Weber, 1997, 2001). Sugars are easily made by the reaction of HCHO under certain conditions (fairly high concentration, basic to neutral pH). The attainment of such conditions of pH and concentration, is likely only possible in certain geological environments such as eutectics (occurring only in locations subject to at least occasional freezing) and in evaporative basins (after HCHO is rendered non-volatile by reaction with other aqueous species). Fig. 6 shows plausible environments where sugar synthesis could occur in the context of HCHO geochemical cycling. Given present uncertainties regarding primitive Earth conditions, the possibility that such specialized conditions existed cannot be ruled out. The synthesis of sugars in such environments from even low initial HCHO concentrations under mild conditions might be extremely facile, and warrants further investigation, as does sugar synthesis in the presence of likely congeners such as HCN, NH3 , and inorganic compounds such as borates and sulfites, and the impact of various radiation types on this chemistry. In contrast, submarine hydrothermal environments, generally characterized by low pH (with significant exceptions), low organic species concentrations, and often very high temperatures, are not highly compatible with formose chemistry, though as mentioned earlier some synthesis of amino acids might be mediated by HCHO provided sufficient concentrations. The instability of HCHO under hydrothermal conditions also argues against high concentrations of HCHO in these environments (Osada et al., 2004; Seewald et al., 2006), but the detection of trithianes in hydrothermal vent fluids may be a compelling anomaly (Simoneit, 1992), although it remains controversial whether much of the organic material detected in these systems has a truly abiological source rather than being reworked biological matter. 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