Orig Life Evol Biosph DOI 10.1007/s11084-015-9420-y ORIGINS 2014
Phosphorus: a Case for Mineral-Organic Reactions in Prebiotic Chemistry Matthew Pasek & Barry Herschy & Terence P. Kee
Received: 30 October 2014 / Accepted: 11 January 2015 # Springer Science+Business Media Dordrecht 2015
Abstract The ubiquity of phosphorus (P) in modern biochemistry suggests that P may have participated in prebiotic chemistry prior to the emergence of life. Of the major biogenic elements, phosphorus alone lacks a substantial volatile phase and its ultimate source therefore had to have been a mineral. However, as most native P minerals are chemically un-reactive within the temperature-pressure-pH regimes of contemporary life, it begs the question as to whether the most primitive early living systems on earth had access to a more chemically reactive P-mineral inventory. The meteoritic mineral schreibersite has been proposed as an important source of reactive P on the early earth. The chemistry of schreibersite as a P source is summarized and reviewed here. Recent work has also shown that reduced oxidation state P compounds were present on the early earth; these compounds lend credence to the relevance of schreibersite as a prebiotic mineral. Ultimately, schreibersite will oxidize to phosphate, but several high-energy P intermediates may have provided the reactive material necessary for incorporating P into prebiotic molecules. Keywords Phosphorus . Prebiotic . Schreibersite . Surface chemistry . Radical reactions . Phosphorylation
Introduction Phosphorus (P) is a critical element in modern biochemical systems, where it serves to store metabolic energy as ATP, forms the backbone of genetic material such as RNA and DNA, and separates cells from the environment as phospholipids. The demonstration of a prebiotically
Paper presented at ORIGINS 2014, Nara Japan, July 6–11 2014. M. Pasek (*) School of Geosciences, University of South Florida, 4202 E Fowler Ave, Tampa, FL 33620, USA e-mail: [email protected] B. Herschy Department of Genetics, Evolution and Environment, University College London, Gower Street, London WC1E 6BT, UK T. P. Kee School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
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plausible route to forming phosphorylated biomolecules, or ancestral versions of these molecules, has been confounded by the poor solubility and low reactivity of phosphate minerals under early earth conditions (e.g., Keefe and Miller 1995). Of the major biogenic elements (e.g., CHONPS), phosphorus alone lacks a significant volatile phase. Ultimately, the phosphorus that formed the first phosphorylated biomolecules had to come from a mineral source. The variety of phosphate minerals likely present on the early earth was quite small relative to the large variety present under modern earth conditions (Hazen 2013). The most likely phosphate minerals present on the early earth would have included calcium phosphate minerals such as apatite and whitlockite, and the rare earth element phosphate mineral monazite, as well as a few even rarer phosphate minerals (Hazen 2013). The mineral struvite, MgNH4PO4×6H2O, has shown the greatest promise as a plausible, prebiotic phosphate mineral (Handschuh and Orgel 1973), though this mineral was not likely abundant on the early earth (Gull and Pasek 2013). The formation of organophosphates from these minerals is not spontaneous: the equilibrium constant K is less than 0.001 for the formation of phosphorylated sugars using orthophosphate; other organic compounds are even less favorable. The most plausible route to forming organophosphates from phosphate minerals typically invokes reaction of these minerals with hot, acidic water, then heating of dissolved phosphate to form polyphosphates (Yamagata et al. 1991). Polyphosphates, the simplest of which is diphosphate or pyrophosphate, could then phosphorylate organic compounds in higher yields (e.g., Pasek and Kee 2011). An alternative to a phosphate mineral source on the early earth is a meteoritic phosphide source—the mineral schreibersite—as proposed by Pasek and Lauretta (2005) and Bryant and Kee (2006). Recent results have shown that schreibersite could have influenced early earth P chemistry (Pasek et al. 2013), and that schreibersite is capable of spontaneously phosphorylating organic compounds, with phosphorylation of glycerol demonstrated thus far. In this paper we review the process by which the mineral schreibersite reacts with water, provide new estimates on the flux of meteoritic phosphorus to the surface of the earth, place constraints on the ultimate fate of schreibersite as it reacts over geologic time, and provide a brief summary on the history of the investigation of schreibersite as a potential prebiotic P source.
Review of the Reactions of Schreibersite with Water Schreibersite is a phosphide mineral common to many meteorites, but is found on the earth only in exceedingly rare environments (e.g., Essene and Fisher 1986). Its formula is (Fe,Ni)3P, and it forms a solid solution with the mineral nickelphosphide (Ni,Fe)3P. Schreibersite was first described by Berzelius (1832), and is one of oldest Bmeteorite-only^ minerals characterized by early mineralogists (Pasek 2014). Alloys containing Fe-Ni are amongst the earliest minerals to have condensed from the solar nebula (Grossman 1972; Kelly and Larimer 1977) and both schreibersite and perryite [(Fe,Ni)~8(Si,P)~3] are important minor phases therein (Lehner et al. 2010). Indeed, the presence of nickeliferous (>5 % Ni) schreibersite can be a diagnostic feature of a meteorite. Schreibersite forms as a result of the siderophilic (metal-loving) nature of the element phosphorus at high temperatures under reducing conditions. Though generally recognized as a lithophile (rock-loving element), on a cosmic scale phosphorus will tend to bind with iron before it oxidizes and reacts with calcium to form phosphate minerals. During planetary differentiation, or the separation of a planet into a lighter silicate mantle and a denser metal core, phosphorus will preferentially enter the core. In fact, about 97 % of the earth’s phosphorus is within the core, based on chondritic and mantle abundances of P (McDonough and Sun 1995).
Phosphorus: Mineral-Organic Reactions in Prebiotic Chemistry
The oxidation state of phosphorus in schreibersite is not obvious to a traditional student of chemistry (Fig. 1). An inorganic chemist who deals with synthetic phosphides such as AlP or Ca3P2 typically assumes an oxidation state for P of −3 in these materials. This oxidation state is consistent with the primary product of reaction of synthetic phosphides with water: the gas phosphine (PH3). However, the oxidation state of phosphorus in schreibersite appears to be closer to −1, based on electron binding energy curves of the P and iron atoms in the metallic mineral (Fig. 2, see also Bryant et al. 2013). The oxidation of schreibersite is exergonic under most conditions, and certainly within the modern atmosphere. The phosphorus within schreibersite is an efficient scavenger of oxidizing agents, and reacts with air, water, CO2, and most other oxygen-bearing molecules. Characterizations of the surface of schreibersite demonstrate that the mineral is coated with phosphorus oxides at thicknesses of 80 nm or more (Pirim et al. 2014). This coating serves to protect the schreibersite from further oxidation in air, and in many meteorite samples schreibersite is one of the last minerals to accumulate rust when stored in air. The interesting chemistry occurs when schreibersite is placed within water (Fig. 1). Schreibersite reacts with water to oxidize the phosphorus and the iron to form P oxyanions and ferrous iron. There is a stoichiometric release of H2 gas during this process (Pasek and Lauretta 2005), carrying away extra electrons during this net oxidation. Bryant and Kee (2006) also demonstrated that the oxygen reacting with the phosphorus came from the water: when schreibersite was reacted with water labeled with the 18O isotope, the isotopomer ratio indicated that three of the 18O- oxygen atoms attached to the P oxyanions could be traced to water, the remaining oxygen to adventitious presence of dioxygen, perhaps bound to the surface (e.g., Pirim et al. 2014). The major products of schreibersite corrosion are phosphite (HPO32−), hypophosphate (P2O64−), phosphate (PO43−), and pyrophosphate (P2O74−). The oxidation state of P in these anions is thus +3, +4, and +5 respectively (Pasek and Lauretta 2005; Bryant and Kee 2006; Pasek et al. 2007). In addition to these primary ions, which are generated in almost every schreibersite corrosion experiment, the other anions that may be present (Pasek and Lauretta 2005; Bryant and Kee 2006; Pech et al. 2011) depending on reaction conditions include the dimer of phosphite pyrophosphite (H2P2O52−), the trimer of phosphate triphosphate (P3O104−),
Fig. 1 Schreibersite corrosion schematic. Schreibersite, a picture of which is provided as the inset (with surrounding material kamacite, an iron-nickel mineral alloy) corrodes to phosphite radicals and iron oxides in water, releasing H2 gas. These phosphite radicals terminate to form a variety of P oxyanions. These P oxyanions may react with organics via phospho-aldol reactions to form organophosphonates or organophosphinates, or by condensation reactions to form organophosphates
M. Pasek et al.
Fig. 2 Oxidation state of iron in schreibersite as determined by XPS. An increase in binding energy corresponds to an increase in oxidation state a XPS analyses of both Fe2P & Fe3P surfaces showing core 2P3/2 electron binding energy region of iron. The oxidation state of iron in the iron phosphide correlates to close to zero to slightly oxidizing (Descostes et al. 2000). The P oxidation state is −1 based on its internal binding energy of 129.5 eV (Pelavin et al. 1970; Bryant et al. 2013). b Fe-2p3/2 electron XPS line scan analysis traversing matrixschreibersite-matrix regions of a sectioned sample of Sikhote-Alin. The increase in binding energy of iron corresponds to Increasing P-composition and locates the inclusion region that was sampled for XPS analysis
a very reduced form of P as hypophosphite (H2PO2−) and the cyclic trimer of phosphate trimetaphosphate (P3O93−). Note that the production of pyrophosphite during schreibersite corrosion (e.g., Pasek and Lauretta 2005) has been difficult to replicate, despite its utility in prebiotic P chemistry (Kee et al. 2013), but it is the known to be the principal product from dehydration of aqueous phosphate solutions at pH’s below 5, when heated to dryness at temperatures above 60 °C. Intriguingly the production of these oxyanions proceeds by release of radicals from the schreibersite, specifically PO32− radicals (Pasek et al. 2007). These phosphite radicals were detected by electron paramagnetic resonance spectroscopy (EPR), and recombine to give three products (Schäfer and Asmus 1980) (Schema 1): PO3− and PO33− both react with OH− and H+ respectively to give HPO42− and HPO32−. The yield of 18 % hypophosphate relative to 82 % phosphate and phosphite is consistent with P NMR speciation, though this was not the path considered by Pasek et al. (2007). A radical reaction is also implicated in the production of pyrophosphate, triphosphate and trimetaphosphate, which occurs as the reduced P compounds phosphite or hypophosphite are oxidized (Pasek et al. 2008) in a Fenton reactor, which consists of adding H2O2 to a solution of Fe2+ to generate OH and OOH radicals. Similar condensed P-oxyanions are produced when phosphate and hypophosphite solutions are exposed to microwave plasmas (Pasek et al. 2008). Schema 1 The reaction of two phosphite radicals results in the recombination product, hypophosphate, or the disproportionation products, phosphite and phosphate
Phosphorus: Mineral-Organic Reactions in Prebiotic Chemistry
The formation of organophosphate compounds using schreibersite has only been reported with low yield (e.g., 1 % by Pasek et al. 2007, and 4 % in Pasek et al. 2013). These reactions most likely occur by dehydration reactions on the surface where there is lower water activity, and yields are enhanced when an organic such as urea is added, perhaps following the urea chemistry of Orgel (Österberg et al. 1973). Yields of organophosphates can be increased significantly using a few additional steps (Gull et al., in prep.). Although the formation of organophosphates has only recently been demonstrated using schreibersite, the more general formation of organophosphorus compounds using schreibersite is more reliable and has occurred since the first experiments of Pasek and Lauretta (2005) and Bryant and Kee (2006). In these reactions, phosphite or hypophosphite reacts with simple aldehydes to generate phospho-aldol addition products (Fig. 3). However, this can occur even if no organic has been added to the solution (see Fig. 2 of Pasek et al. 2007). These organophosphorus compounds must have formed by introducing carbon from one of four sources. 1) The solutions of water are contaminated or have been incompletely cleaned. This is unlikely as experiments are washed in acid, and formation of organophosphorus compounds occurs even in new glassware, and does not occur even when other organics are left over. 2) The carbon comes from trace carbon dissolved in the synthetic Fe3P powder. Pirim et al. (2014) demonstrated carbon contaminated the surface of both natural and artificial schreibersite samples, though the origin of this carbon is unknown. 3) The carbon comes from the plastic coating of the stir bar. Since the Fe3P is an abrasive powder, the stir bar could be abraded itself and might be the ultimate source of carbon for this reaction. However,
Fig. 3 31P NMR spectrum resulting from reaction of acetaldehyde and Fe3P in water. The top of the spectrum has been truncated due to the height of the phosphite (+3.5 ppm) peak Decoupled spectrum is in black, coupled is green. The main organic product of this reaction is the phospho-aldol reaction product between H3CCHO and H2PO2−, at about +30 ppm. The yield of this compound was 12 % based on 31P NMR integration, though quantitative NMR was not performed with this sample. The other major peak is hypophosphite (+7 ppm)
M. Pasek et al.
since the stir bars are coated with PTFE (Teflon), no aldehydes should be present. 4) Atmospheric carbon dioxide is somehow being fixed by reaction with the water and Fe3P. This is by far the most interesting path, and could be plausible with the volume of CO2 trapped in the 250 mL round bottom flasks used. This area remains open to more research. One P species conspicuously absent from the Fe3P reactions is the gas phosphine, PH3. However, it has been shown that small amounts of PH3 gas can be generated during the acidic corrosion of Fe3P at low (
Phosphorus: a Case for Mineral-Organic Reactions in Prebiotic Chemistry Matthew Pasek & Barry Herschy & Terence P. Kee
Received: 30 October 2014 / Accepted: 11 January 2015 # Springer Science+Business Media Dordrecht 2015
Abstract The ubiquity of phosphorus (P) in modern biochemistry suggests that P may have participated in prebiotic chemistry prior to the emergence of life. Of the major biogenic elements, phosphorus alone lacks a substantial volatile phase and its ultimate source therefore had to have been a mineral. However, as most native P minerals are chemically un-reactive within the temperature-pressure-pH regimes of contemporary life, it begs the question as to whether the most primitive early living systems on earth had access to a more chemically reactive P-mineral inventory. The meteoritic mineral schreibersite has been proposed as an important source of reactive P on the early earth. The chemistry of schreibersite as a P source is summarized and reviewed here. Recent work has also shown that reduced oxidation state P compounds were present on the early earth; these compounds lend credence to the relevance of schreibersite as a prebiotic mineral. Ultimately, schreibersite will oxidize to phosphate, but several high-energy P intermediates may have provided the reactive material necessary for incorporating P into prebiotic molecules. Keywords Phosphorus . Prebiotic . Schreibersite . Surface chemistry . Radical reactions . Phosphorylation
Introduction Phosphorus (P) is a critical element in modern biochemical systems, where it serves to store metabolic energy as ATP, forms the backbone of genetic material such as RNA and DNA, and separates cells from the environment as phospholipids. The demonstration of a prebiotically
Paper presented at ORIGINS 2014, Nara Japan, July 6–11 2014. M. Pasek (*) School of Geosciences, University of South Florida, 4202 E Fowler Ave, Tampa, FL 33620, USA e-mail: [email protected] B. Herschy Department of Genetics, Evolution and Environment, University College London, Gower Street, London WC1E 6BT, UK T. P. Kee School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
M. Pasek et al.
plausible route to forming phosphorylated biomolecules, or ancestral versions of these molecules, has been confounded by the poor solubility and low reactivity of phosphate minerals under early earth conditions (e.g., Keefe and Miller 1995). Of the major biogenic elements (e.g., CHONPS), phosphorus alone lacks a significant volatile phase. Ultimately, the phosphorus that formed the first phosphorylated biomolecules had to come from a mineral source. The variety of phosphate minerals likely present on the early earth was quite small relative to the large variety present under modern earth conditions (Hazen 2013). The most likely phosphate minerals present on the early earth would have included calcium phosphate minerals such as apatite and whitlockite, and the rare earth element phosphate mineral monazite, as well as a few even rarer phosphate minerals (Hazen 2013). The mineral struvite, MgNH4PO4×6H2O, has shown the greatest promise as a plausible, prebiotic phosphate mineral (Handschuh and Orgel 1973), though this mineral was not likely abundant on the early earth (Gull and Pasek 2013). The formation of organophosphates from these minerals is not spontaneous: the equilibrium constant K is less than 0.001 for the formation of phosphorylated sugars using orthophosphate; other organic compounds are even less favorable. The most plausible route to forming organophosphates from phosphate minerals typically invokes reaction of these minerals with hot, acidic water, then heating of dissolved phosphate to form polyphosphates (Yamagata et al. 1991). Polyphosphates, the simplest of which is diphosphate or pyrophosphate, could then phosphorylate organic compounds in higher yields (e.g., Pasek and Kee 2011). An alternative to a phosphate mineral source on the early earth is a meteoritic phosphide source—the mineral schreibersite—as proposed by Pasek and Lauretta (2005) and Bryant and Kee (2006). Recent results have shown that schreibersite could have influenced early earth P chemistry (Pasek et al. 2013), and that schreibersite is capable of spontaneously phosphorylating organic compounds, with phosphorylation of glycerol demonstrated thus far. In this paper we review the process by which the mineral schreibersite reacts with water, provide new estimates on the flux of meteoritic phosphorus to the surface of the earth, place constraints on the ultimate fate of schreibersite as it reacts over geologic time, and provide a brief summary on the history of the investigation of schreibersite as a potential prebiotic P source.
Review of the Reactions of Schreibersite with Water Schreibersite is a phosphide mineral common to many meteorites, but is found on the earth only in exceedingly rare environments (e.g., Essene and Fisher 1986). Its formula is (Fe,Ni)3P, and it forms a solid solution with the mineral nickelphosphide (Ni,Fe)3P. Schreibersite was first described by Berzelius (1832), and is one of oldest Bmeteorite-only^ minerals characterized by early mineralogists (Pasek 2014). Alloys containing Fe-Ni are amongst the earliest minerals to have condensed from the solar nebula (Grossman 1972; Kelly and Larimer 1977) and both schreibersite and perryite [(Fe,Ni)~8(Si,P)~3] are important minor phases therein (Lehner et al. 2010). Indeed, the presence of nickeliferous (>5 % Ni) schreibersite can be a diagnostic feature of a meteorite. Schreibersite forms as a result of the siderophilic (metal-loving) nature of the element phosphorus at high temperatures under reducing conditions. Though generally recognized as a lithophile (rock-loving element), on a cosmic scale phosphorus will tend to bind with iron before it oxidizes and reacts with calcium to form phosphate minerals. During planetary differentiation, or the separation of a planet into a lighter silicate mantle and a denser metal core, phosphorus will preferentially enter the core. In fact, about 97 % of the earth’s phosphorus is within the core, based on chondritic and mantle abundances of P (McDonough and Sun 1995).
Phosphorus: Mineral-Organic Reactions in Prebiotic Chemistry
The oxidation state of phosphorus in schreibersite is not obvious to a traditional student of chemistry (Fig. 1). An inorganic chemist who deals with synthetic phosphides such as AlP or Ca3P2 typically assumes an oxidation state for P of −3 in these materials. This oxidation state is consistent with the primary product of reaction of synthetic phosphides with water: the gas phosphine (PH3). However, the oxidation state of phosphorus in schreibersite appears to be closer to −1, based on electron binding energy curves of the P and iron atoms in the metallic mineral (Fig. 2, see also Bryant et al. 2013). The oxidation of schreibersite is exergonic under most conditions, and certainly within the modern atmosphere. The phosphorus within schreibersite is an efficient scavenger of oxidizing agents, and reacts with air, water, CO2, and most other oxygen-bearing molecules. Characterizations of the surface of schreibersite demonstrate that the mineral is coated with phosphorus oxides at thicknesses of 80 nm or more (Pirim et al. 2014). This coating serves to protect the schreibersite from further oxidation in air, and in many meteorite samples schreibersite is one of the last minerals to accumulate rust when stored in air. The interesting chemistry occurs when schreibersite is placed within water (Fig. 1). Schreibersite reacts with water to oxidize the phosphorus and the iron to form P oxyanions and ferrous iron. There is a stoichiometric release of H2 gas during this process (Pasek and Lauretta 2005), carrying away extra electrons during this net oxidation. Bryant and Kee (2006) also demonstrated that the oxygen reacting with the phosphorus came from the water: when schreibersite was reacted with water labeled with the 18O isotope, the isotopomer ratio indicated that three of the 18O- oxygen atoms attached to the P oxyanions could be traced to water, the remaining oxygen to adventitious presence of dioxygen, perhaps bound to the surface (e.g., Pirim et al. 2014). The major products of schreibersite corrosion are phosphite (HPO32−), hypophosphate (P2O64−), phosphate (PO43−), and pyrophosphate (P2O74−). The oxidation state of P in these anions is thus +3, +4, and +5 respectively (Pasek and Lauretta 2005; Bryant and Kee 2006; Pasek et al. 2007). In addition to these primary ions, which are generated in almost every schreibersite corrosion experiment, the other anions that may be present (Pasek and Lauretta 2005; Bryant and Kee 2006; Pech et al. 2011) depending on reaction conditions include the dimer of phosphite pyrophosphite (H2P2O52−), the trimer of phosphate triphosphate (P3O104−),
Fig. 1 Schreibersite corrosion schematic. Schreibersite, a picture of which is provided as the inset (with surrounding material kamacite, an iron-nickel mineral alloy) corrodes to phosphite radicals and iron oxides in water, releasing H2 gas. These phosphite radicals terminate to form a variety of P oxyanions. These P oxyanions may react with organics via phospho-aldol reactions to form organophosphonates or organophosphinates, or by condensation reactions to form organophosphates
M. Pasek et al.
Fig. 2 Oxidation state of iron in schreibersite as determined by XPS. An increase in binding energy corresponds to an increase in oxidation state a XPS analyses of both Fe2P & Fe3P surfaces showing core 2P3/2 electron binding energy region of iron. The oxidation state of iron in the iron phosphide correlates to close to zero to slightly oxidizing (Descostes et al. 2000). The P oxidation state is −1 based on its internal binding energy of 129.5 eV (Pelavin et al. 1970; Bryant et al. 2013). b Fe-2p3/2 electron XPS line scan analysis traversing matrixschreibersite-matrix regions of a sectioned sample of Sikhote-Alin. The increase in binding energy of iron corresponds to Increasing P-composition and locates the inclusion region that was sampled for XPS analysis
a very reduced form of P as hypophosphite (H2PO2−) and the cyclic trimer of phosphate trimetaphosphate (P3O93−). Note that the production of pyrophosphite during schreibersite corrosion (e.g., Pasek and Lauretta 2005) has been difficult to replicate, despite its utility in prebiotic P chemistry (Kee et al. 2013), but it is the known to be the principal product from dehydration of aqueous phosphate solutions at pH’s below 5, when heated to dryness at temperatures above 60 °C. Intriguingly the production of these oxyanions proceeds by release of radicals from the schreibersite, specifically PO32− radicals (Pasek et al. 2007). These phosphite radicals were detected by electron paramagnetic resonance spectroscopy (EPR), and recombine to give three products (Schäfer and Asmus 1980) (Schema 1): PO3− and PO33− both react with OH− and H+ respectively to give HPO42− and HPO32−. The yield of 18 % hypophosphate relative to 82 % phosphate and phosphite is consistent with P NMR speciation, though this was not the path considered by Pasek et al. (2007). A radical reaction is also implicated in the production of pyrophosphate, triphosphate and trimetaphosphate, which occurs as the reduced P compounds phosphite or hypophosphite are oxidized (Pasek et al. 2008) in a Fenton reactor, which consists of adding H2O2 to a solution of Fe2+ to generate OH and OOH radicals. Similar condensed P-oxyanions are produced when phosphate and hypophosphite solutions are exposed to microwave plasmas (Pasek et al. 2008). Schema 1 The reaction of two phosphite radicals results in the recombination product, hypophosphate, or the disproportionation products, phosphite and phosphate
Phosphorus: Mineral-Organic Reactions in Prebiotic Chemistry
The formation of organophosphate compounds using schreibersite has only been reported with low yield (e.g., 1 % by Pasek et al. 2007, and 4 % in Pasek et al. 2013). These reactions most likely occur by dehydration reactions on the surface where there is lower water activity, and yields are enhanced when an organic such as urea is added, perhaps following the urea chemistry of Orgel (Österberg et al. 1973). Yields of organophosphates can be increased significantly using a few additional steps (Gull et al., in prep.). Although the formation of organophosphates has only recently been demonstrated using schreibersite, the more general formation of organophosphorus compounds using schreibersite is more reliable and has occurred since the first experiments of Pasek and Lauretta (2005) and Bryant and Kee (2006). In these reactions, phosphite or hypophosphite reacts with simple aldehydes to generate phospho-aldol addition products (Fig. 3). However, this can occur even if no organic has been added to the solution (see Fig. 2 of Pasek et al. 2007). These organophosphorus compounds must have formed by introducing carbon from one of four sources. 1) The solutions of water are contaminated or have been incompletely cleaned. This is unlikely as experiments are washed in acid, and formation of organophosphorus compounds occurs even in new glassware, and does not occur even when other organics are left over. 2) The carbon comes from trace carbon dissolved in the synthetic Fe3P powder. Pirim et al. (2014) demonstrated carbon contaminated the surface of both natural and artificial schreibersite samples, though the origin of this carbon is unknown. 3) The carbon comes from the plastic coating of the stir bar. Since the Fe3P is an abrasive powder, the stir bar could be abraded itself and might be the ultimate source of carbon for this reaction. However,
Fig. 3 31P NMR spectrum resulting from reaction of acetaldehyde and Fe3P in water. The top of the spectrum has been truncated due to the height of the phosphite (+3.5 ppm) peak Decoupled spectrum is in black, coupled is green. The main organic product of this reaction is the phospho-aldol reaction product between H3CCHO and H2PO2−, at about +30 ppm. The yield of this compound was 12 % based on 31P NMR integration, though quantitative NMR was not performed with this sample. The other major peak is hypophosphite (+7 ppm)
M. Pasek et al.
since the stir bars are coated with PTFE (Teflon), no aldehydes should be present. 4) Atmospheric carbon dioxide is somehow being fixed by reaction with the water and Fe3P. This is by far the most interesting path, and could be plausible with the volume of CO2 trapped in the 250 mL round bottom flasks used. This area remains open to more research. One P species conspicuously absent from the Fe3P reactions is the gas phosphine, PH3. However, it has been shown that small amounts of PH3 gas can be generated during the acidic corrosion of Fe3P at low (
