J Mol Evol DOI 10.1007/s00239-014-9619-y
LETTER TO THE EDITOR
The Role of Fluoride in Montmorillonite-Catalyzed RNA Synthesis Michael F. Aldersley • Prakash C. Joshi
Received: 21 March 2014 / Accepted: 3 April 2014 Springer Science+Business Media New York 2014
Abstract The montmorillonite-catalyzed reactions of the 50 -phosphorimidazolide of adenosine in the presence of fluoride were investigated to complete our study on the effect of salts on this type of reaction. Both anions and cations have been found to influence the oligomerization reactions of the activated nucleotides, being used here as a model system for pre-biotic RNA synthesis. However, in total contrast to the behavior of the activated nucleotides in the presence of montmorillonite and other salts, alkali metal fluorides did not yield any detectable oligomerization products except in very dilute (\0.005 M) solutions of fluoride. Instead, 50 -phosphorofluoridates were formed. Their identity was confirmed by a combination of HPLC, mass spectrometry, synthesis, and NMR. Keywords Montmorillonite Activated nucleotides Fluoride ion Phosphorofluoridates
Introduction The dual properties of RNA as an enzyme catalyst (Ribozyme and its ability to store genetic information) suggest that early life was based upon RNA; DNA and protein evolved later from it (Joyce and Orgel 2006). We have demonstrated in our model system the synthesis of up to 50-mers of RNA oligomers by Na?-montmorillonite-catalyzed reaction of the 50 -end-activated mononucleotides (Ferris 2006; Joshi et al. 2009). The Na?-montmorillonite
M. F. Aldersley (&) P. C. Joshi New York Center for Astrobiology and the Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, NY 12180, USA e-mail: [email protected]
not only catalyzes the prebiotic synthesis of RNA but also facilitates homochiral selection (Joshi et al. 2011). The oligomerization of the activated nucleotides (e.g. Fig. 1) can be achieved by the use of a clay mineral catalyst, montmorillonite, which occurs on Earth. The general processes occurring in the oligomer synthesis from ImpN are illustrated by Scheme 1. These reactions were found to be dependent on the nature of mineral salts present. While Na?-montmorillonite (pH 7) with ImpA produced only dimers in water, the addition of sodium chloride (1 M) enhanced the chain length of oligomers to 10-mers as detected by HPLC. Magnesium chloride also produced a similar effect, but the combination of sodium chloride and magnesium chloride did not produce any difference in the oligomer’s chain length. The effect of monovalent cations in RNA synthesis was of the following order: Li? [ Na? [ K?. A similar effect was observed with the monovalent anions, enhancing catalysis in the following order: Cl- [ Br- [ I-. Furthermore, a 45 % reduction in the yield of cyclic dimer was observed upon increasing the sodium chloride concentration from 0.1 to 2.0 M (Joshi and Aldersley 2013). Inhibition of cyclic dimer formation is essential for increasing the yield of linear dimers as well as the overall chain length. The results of this study showed that the presence of salts is essential in prebiotic RNA synthesis catalyzed by clay minerals. The maximum yield of oligomers occurred at the [NaCl] similar to that on the prebiotic Earth. Hoping to further improve our understanding of the catalytic properties of this montmorillonite-salt system, and for the sake of completeness (Joshi and Aldersley 2013), we began to explore alkali metal fluorides, and •
We focused on sodium fluoride covering a range of concentrations of several orders of magnitude with 1 M sodium chloride always present.
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J Mol Evol
N
O N
N
O
P O O HO
NH2
N
OH
N
N
Fig. 1 Model activated nucleotide ImpA used in this work
ImpN + clay →
(pN)n + c-dimers + NppN + NMP
Scheme 1 The general reaction of an activated nucleotide in the presence of montmorillonite
• •
We used ImpA as a model although all other ImpNs behave similarly. In this way, we attempted to define the role of fluoride ion, if any, in pre-biotic clay-mediated RNA synthesis.
Methods General methodology has already been extensively reported (Joshi et al. 2009; Aldersley et al. 2011). Reactions in the Absence of Montmorillonite 15 mM solutions of ImpA were prepared in sodium fluoride/sodium chloride solution ([NaF] from 1 M to 1 9 10-5 M, [NaCl] = 1 M) and the pH measured. The reaction mixtures were allowed to stand for 3–8 days at 24 C subjected to periodic monitoring by HPLC using a reverse-phase column and gradient elution (Buffer A: 0.2 % aqueous formic acid; Buffer B: 30 % acetonitrile in water with 0.2 % formic acid, 1 %/min) at a flow rate of 1 mL/min. After completion of the reaction in 1 M sodium fluoride/1 M sodium chloride, the product mixture comprised the corresponding phosphorofluoridate, AMPF, together with a very small amount (\1 %) of the AMP hydrolysis product. The materials were separated where necessary using a semi-preparative reverse-phase Alltima C-18, 5 l (10 9 300 mm) column (Alltech, Grace Davison) under isocratic conditions (88 % of 0.2 % aqueous formic acid and 12 % of 30 % acetonitrile in water with 0.2 % formic acid) at a flow rate of 3 mL/min. Reactions in the Presence of Montmorillonite Analogous experiments were carried out in the presence of Na?-Montmorillonite: 10 mg of the clay mineral was dispersed in 200 lL of a solution of sodium fluoride/sodium chloride ([NaF] from 1 M to 1 9 10-5 M, [NaCl] = 1 M). After 3 days, the reaction mixtures were centrifuged and
123
the supernatant removed. The clay was extracted (200 lL 1 M sodium chloride in 30 % acetonitrile, 2 9 1 h, overnight, 1 9 2 h). The four extracts and supernatants were combined, and the whole mixture was diluted to 1 mL for analysis by ion exchange chromatography using a DNAPac column (Dionex) and gradient elution (Buffer A: 2 mM Tris, pH 8; Buffer B: 2 mM Tris, with 0.4 M NaClO4, pH 8, 1 mL/min and a gradient of 1 %/min for 25 min).
Results Effect of Sodium Fluoride upon ImpA in the absence of Montmorillonite In general, the presence of fluoride and the absence of the clay led to a simpler reaction in which the only major product was the 50 -phosphorofluoridate: for example, AMPF, together with much less of the hydrolysis product, for example, AMP (Scheme 2). The yield of AMPF from ImpA in the presence of both 1 M sodium chloride and 1 M sodium fluoride is almost quantitative compared with 91 % in the presence of only 1 M sodium fluoride (Aldersley et al. 2014). Progressively, more hydrolysis product was formed with the increasing amount of dilute sodium fluoride in the 1 M sodium chloride solution. For all the activated nucleotides that have been studied, the identities of the NMPF were confirmed by HPLC, MS, synthesis, and NMR (Aldersley et al. 2014). Yields of the 50 -phosphorofluoridate varied with the nature of the alkali metal with Li? \ Na? \ K?, the opposite of the order for promoting oligomerization (Joshi and Aldersley, 2013). As the concentration of fluoride ion is changed, an interesting variation of hydrolysis versus conversion to the 50 -phosphorofluoridate occurs which is shown for the sodium fluoride/ImpA case in Fig. 2. These results represented a series of controls for the reaction of fluoride ion across the concentration range with 15 mM ImpA in the absence of montmorillonite at pH 8.7. Effect of Sodium Fluoride upon ImpA in the presence of Montmorillonite There was no oligomer formation with 15 mM ImpA until the [NaF] in the reaction mixtures was less than approximately 0.0002 M. This behavior is shown in Fig. 3. Under these conditions, we also noted the loss of the phosphate ImpN + MF
→
NMPF and NMP
Scheme 2 The general reaction of an activated nucleotide with an alkali metal fluoride
J Mol Evol
the cleavage of phosphate by other salts such as those of cesium (Joshi and Aldersley 2013).
100 90 80
%Yield
70
An Estimate of the Prebiotic Fluoride Ion Concentration
60 50 40 30 20 10 0 0.00001
0.0001
0.001
0.01
0.1
1
[NaF], M
Fig. 2 Product Analysis from 15 mM ImpA as influenced by [NaF](aq.). The blue decreasing curve and open diamond is the yield of hydrolysis product AMP. The red increasing curve and open diamond is the yield of AMPF (Color figure online)
9 8
oligomer length
7 6 5 4 3 2 1 0 0.00001
0.0001
0.001
0.01
0.1
1
[NaF], M
Fig. 3 Oligomer length from 15 mM ImpA as influenced by [NaF](aq) and Montmorillonite. The blue line is the control with no montmorillonite or fluoride being present. The brown line and open diamond is the result of added fluoride and montmorillonite. Generally with montmorillonite in the presence of 1 M NaCl, octamers, or longer, are formed (black line) (Color figure online)
group with a yield of approximately 50 % adenosine for [NaF] greater than 0.0002 M, and this occurred across the range of higher concentrations of fluoride ion. The presence of strong Lewis acid centers of fluoro-aluminate character within the clay may be responsible for the phosphate cleavage, and this is a topic worthy of further research. In the presence of 1 M sodium chloride and Na?montmorillonite, the oligomer length is generally 8 or higher as determined by ion exchange HPLC analysis (Joshi and Aldersley 2013) (solid black line in Fig. 3). Across the range of fluoride ion concentrations studied, the pH range of the reaction mixtures is in all cases between 8.1 and 8.5, measured both before and after the reactions. This range of pH would, under our standard conditions with 1 M sodium chloride, lead to oligomerization (Aldersley et al. 2011). We have previously noted
Unlike chloride ion, the concentration of fluoride ion in the environment is limited by the extreme insolubility of the common fluoride containing minerals. The minerals fluorite, CaF2, and fluorapatite, Ca5(PO4)3F, will be discussed, with Ksp values of 3 9 10-11 (Garand and Mucci 2004; Aylward and Findlay 1975) and, between 1.3 9 10-60 and 7.4 9 10-60 (Jaynes et al. 1999) respectively. These data provide equilibrium concentrations of fluoride ion of 3.1 9 10-4 M and, from 2.9 9 10-7 to 3.5 9 10-7 M, with fluoride ions that are in equilibrium with the minerals, respectively. Fluorite is by far the most important fluoridecontaining phase in the Earth’s crust and is thought to control the fluoride ion concentrations in seawater and other natural aqueous solutions (Nordstrom and Jenne 1977; Elrashidi and Lindsay 1986). It is recognized that the soluble alkali metal fluorides and other alkali earth metal fluorides are found in nature, but such minerals are rare in comparison with the two considered above (Clark 1993). It is clear that the inhibitory effect upon a clay-catalyzed RNA synthesis by fluoride ion would be absent if the only source of fluoride ion came from these poorly soluble minerals since the equilibrium concentrations of fluoride suggested here are so low, although, interestingly, the [F-] generated by fluorite is close to the threshold for oligomerization determined in this work. Furthermore, a current estimate of the [F-] in the sea is approximately 1.3 ppm (8.95 9 10-5 M), (Environmental Protection Division, British Columbia, Canada) which is too low to inhibit the imidazolide chemistry that we have explored. The nature of these insoluble fluorine-containing minerals may well have maintained a similar low fluoride ion concentration throughout geological time since, for example, the value of Ksp for fluorite changes little with varying concentrations of magnesium or sodium chlorides (Garand and Mucci 2004).
Conclusion It has been demonstrated that the reaction of sodium fluoride alone with an activated nucleotide such as the 50 phosphorimidazolide, ImpA, provides the corresponding 50 -phosphorofluoridate, AMPF. The lack of any oligomer formation with fluorides ([F-] [ 0.002 M) in the presence of montmorillonite is in total contrast with the behaviors of almost all other salts. However, we suggest that the presence of prebiotic fluoride ion is not incompatible with
123
J Mol Evol
RNA synthesis via an activated nucleotide (whatever the pre-biotic activation may have been) because of the extremely low fluoride ion concentrations considered to be present in the pre-biotic environment. Acknowledgments The authors are grateful to Professor Douglas Whittet, Director, NY Center for Astrobiology, and Professor Curt Breneman, Chair of the Department of Chemistry and Chemical Biology, for their support and interest in this research. This research was supported by NASA Astrobiology Institute Grant NNA09DA80A.
References Aldersley MF, Joshi PC, Price JD, Ferris JP (2011) The role of montmorillonite in its catalysis of RNA synthesis. Appl Clay Sci 54:1–14 Aldersley MF, Joshi PC, Schwartz HM, Kirby AJ (2014) The reaction of activated RNA species with aqueous fluoride ion: a convenient synthesis of nucleotide 50 -phosphorofluoridates and a note on the mechanism. Tetrahedron Lett 55:1464–1466 Aylward GH, Findlay TJV (1975) SI chemical data. Wiley, Sydney Clark AM (1993) Hey’s Mineral Index, 3rd edn. Chapman and Hall, London Elrashidi MA, Lindsay WL (1986) Chemical equilibria of fluorine in soils: a theoretical development. Soil Sci 141:275–280
123
Environmental Protection Division, British Columbia, Canada http:// www.env.gov.bc.ca/wat/wq/BCguidelines/fluoride/fluoridetoo01.html. Accessed 14 Apr 2014 Ferris JP (2006) Montmorillonite-catalysed formation of RNA oligomers: the possible role of catalysis in the origin of life. Philos Trans R Soc B Biol Sci 361:1777–1786 Garand A, Mucci A (2004) The solubility of fluorite as a function of ionic strength and solution composition at 25C and 1 atm total pressure. Mar Chem 91:27–35 Jaynes WF, Moore PA Jr, Miller DM (1999) Solubility and ion activity products of calcium phosphate minerals. J Environ Qual 28:530–536 Joshi PC, Aldersley MF (2013) Significance of mineral salts in prebiotic RNA synthesis catalyzed by montmorillonite. J Mol Evol 76:371–379 Joshi PC, Aldersley MF, Delano JW, Ferris JP (2009) Mechanism of montmorillonite catalysis in the formation of RNA oligomers. J Am Chem Soc 131:13369–13374 Joshi PC, Aldersley MF, Ferris JP (2011) Progress in demonstrating total homochiral selection in montmorillonite-catalyzed RNA synthesis. Biochem Biophys Res Commun 413:594–598 Joyce GF, Orgel LE (2006) The RNA World (3rd Edition) In: Gesteland RF, Cech TR, Atkins JF (eds) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York Nordstrom DK, Jenne AJ (1977) Fluorite solubility equilibria in selected geothermal waters. Geochim Cosmochim Acta 41:175–188
LETTER TO THE EDITOR
The Role of Fluoride in Montmorillonite-Catalyzed RNA Synthesis Michael F. Aldersley • Prakash C. Joshi
Received: 21 March 2014 / Accepted: 3 April 2014 Springer Science+Business Media New York 2014
Abstract The montmorillonite-catalyzed reactions of the 50 -phosphorimidazolide of adenosine in the presence of fluoride were investigated to complete our study on the effect of salts on this type of reaction. Both anions and cations have been found to influence the oligomerization reactions of the activated nucleotides, being used here as a model system for pre-biotic RNA synthesis. However, in total contrast to the behavior of the activated nucleotides in the presence of montmorillonite and other salts, alkali metal fluorides did not yield any detectable oligomerization products except in very dilute (\0.005 M) solutions of fluoride. Instead, 50 -phosphorofluoridates were formed. Their identity was confirmed by a combination of HPLC, mass spectrometry, synthesis, and NMR. Keywords Montmorillonite Activated nucleotides Fluoride ion Phosphorofluoridates
Introduction The dual properties of RNA as an enzyme catalyst (Ribozyme and its ability to store genetic information) suggest that early life was based upon RNA; DNA and protein evolved later from it (Joyce and Orgel 2006). We have demonstrated in our model system the synthesis of up to 50-mers of RNA oligomers by Na?-montmorillonite-catalyzed reaction of the 50 -end-activated mononucleotides (Ferris 2006; Joshi et al. 2009). The Na?-montmorillonite
M. F. Aldersley (&) P. C. Joshi New York Center for Astrobiology and the Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, Troy, NY 12180, USA e-mail: [email protected]
not only catalyzes the prebiotic synthesis of RNA but also facilitates homochiral selection (Joshi et al. 2011). The oligomerization of the activated nucleotides (e.g. Fig. 1) can be achieved by the use of a clay mineral catalyst, montmorillonite, which occurs on Earth. The general processes occurring in the oligomer synthesis from ImpN are illustrated by Scheme 1. These reactions were found to be dependent on the nature of mineral salts present. While Na?-montmorillonite (pH 7) with ImpA produced only dimers in water, the addition of sodium chloride (1 M) enhanced the chain length of oligomers to 10-mers as detected by HPLC. Magnesium chloride also produced a similar effect, but the combination of sodium chloride and magnesium chloride did not produce any difference in the oligomer’s chain length. The effect of monovalent cations in RNA synthesis was of the following order: Li? [ Na? [ K?. A similar effect was observed with the monovalent anions, enhancing catalysis in the following order: Cl- [ Br- [ I-. Furthermore, a 45 % reduction in the yield of cyclic dimer was observed upon increasing the sodium chloride concentration from 0.1 to 2.0 M (Joshi and Aldersley 2013). Inhibition of cyclic dimer formation is essential for increasing the yield of linear dimers as well as the overall chain length. The results of this study showed that the presence of salts is essential in prebiotic RNA synthesis catalyzed by clay minerals. The maximum yield of oligomers occurred at the [NaCl] similar to that on the prebiotic Earth. Hoping to further improve our understanding of the catalytic properties of this montmorillonite-salt system, and for the sake of completeness (Joshi and Aldersley 2013), we began to explore alkali metal fluorides, and •
We focused on sodium fluoride covering a range of concentrations of several orders of magnitude with 1 M sodium chloride always present.
123
J Mol Evol
N
O N
N
O
P O O HO
NH2
N
OH
N
N
Fig. 1 Model activated nucleotide ImpA used in this work
ImpN + clay →
(pN)n + c-dimers + NppN + NMP
Scheme 1 The general reaction of an activated nucleotide in the presence of montmorillonite
• •
We used ImpA as a model although all other ImpNs behave similarly. In this way, we attempted to define the role of fluoride ion, if any, in pre-biotic clay-mediated RNA synthesis.
Methods General methodology has already been extensively reported (Joshi et al. 2009; Aldersley et al. 2011). Reactions in the Absence of Montmorillonite 15 mM solutions of ImpA were prepared in sodium fluoride/sodium chloride solution ([NaF] from 1 M to 1 9 10-5 M, [NaCl] = 1 M) and the pH measured. The reaction mixtures were allowed to stand for 3–8 days at 24 C subjected to periodic monitoring by HPLC using a reverse-phase column and gradient elution (Buffer A: 0.2 % aqueous formic acid; Buffer B: 30 % acetonitrile in water with 0.2 % formic acid, 1 %/min) at a flow rate of 1 mL/min. After completion of the reaction in 1 M sodium fluoride/1 M sodium chloride, the product mixture comprised the corresponding phosphorofluoridate, AMPF, together with a very small amount (\1 %) of the AMP hydrolysis product. The materials were separated where necessary using a semi-preparative reverse-phase Alltima C-18, 5 l (10 9 300 mm) column (Alltech, Grace Davison) under isocratic conditions (88 % of 0.2 % aqueous formic acid and 12 % of 30 % acetonitrile in water with 0.2 % formic acid) at a flow rate of 3 mL/min. Reactions in the Presence of Montmorillonite Analogous experiments were carried out in the presence of Na?-Montmorillonite: 10 mg of the clay mineral was dispersed in 200 lL of a solution of sodium fluoride/sodium chloride ([NaF] from 1 M to 1 9 10-5 M, [NaCl] = 1 M). After 3 days, the reaction mixtures were centrifuged and
123
the supernatant removed. The clay was extracted (200 lL 1 M sodium chloride in 30 % acetonitrile, 2 9 1 h, overnight, 1 9 2 h). The four extracts and supernatants were combined, and the whole mixture was diluted to 1 mL for analysis by ion exchange chromatography using a DNAPac column (Dionex) and gradient elution (Buffer A: 2 mM Tris, pH 8; Buffer B: 2 mM Tris, with 0.4 M NaClO4, pH 8, 1 mL/min and a gradient of 1 %/min for 25 min).
Results Effect of Sodium Fluoride upon ImpA in the absence of Montmorillonite In general, the presence of fluoride and the absence of the clay led to a simpler reaction in which the only major product was the 50 -phosphorofluoridate: for example, AMPF, together with much less of the hydrolysis product, for example, AMP (Scheme 2). The yield of AMPF from ImpA in the presence of both 1 M sodium chloride and 1 M sodium fluoride is almost quantitative compared with 91 % in the presence of only 1 M sodium fluoride (Aldersley et al. 2014). Progressively, more hydrolysis product was formed with the increasing amount of dilute sodium fluoride in the 1 M sodium chloride solution. For all the activated nucleotides that have been studied, the identities of the NMPF were confirmed by HPLC, MS, synthesis, and NMR (Aldersley et al. 2014). Yields of the 50 -phosphorofluoridate varied with the nature of the alkali metal with Li? \ Na? \ K?, the opposite of the order for promoting oligomerization (Joshi and Aldersley, 2013). As the concentration of fluoride ion is changed, an interesting variation of hydrolysis versus conversion to the 50 -phosphorofluoridate occurs which is shown for the sodium fluoride/ImpA case in Fig. 2. These results represented a series of controls for the reaction of fluoride ion across the concentration range with 15 mM ImpA in the absence of montmorillonite at pH 8.7. Effect of Sodium Fluoride upon ImpA in the presence of Montmorillonite There was no oligomer formation with 15 mM ImpA until the [NaF] in the reaction mixtures was less than approximately 0.0002 M. This behavior is shown in Fig. 3. Under these conditions, we also noted the loss of the phosphate ImpN + MF
→
NMPF and NMP
Scheme 2 The general reaction of an activated nucleotide with an alkali metal fluoride
J Mol Evol
the cleavage of phosphate by other salts such as those of cesium (Joshi and Aldersley 2013).
100 90 80
%Yield
70
An Estimate of the Prebiotic Fluoride Ion Concentration
60 50 40 30 20 10 0 0.00001
0.0001
0.001
0.01
0.1
1
[NaF], M
Fig. 2 Product Analysis from 15 mM ImpA as influenced by [NaF](aq.). The blue decreasing curve and open diamond is the yield of hydrolysis product AMP. The red increasing curve and open diamond is the yield of AMPF (Color figure online)
9 8
oligomer length
7 6 5 4 3 2 1 0 0.00001
0.0001
0.001
0.01
0.1
1
[NaF], M
Fig. 3 Oligomer length from 15 mM ImpA as influenced by [NaF](aq) and Montmorillonite. The blue line is the control with no montmorillonite or fluoride being present. The brown line and open diamond is the result of added fluoride and montmorillonite. Generally with montmorillonite in the presence of 1 M NaCl, octamers, or longer, are formed (black line) (Color figure online)
group with a yield of approximately 50 % adenosine for [NaF] greater than 0.0002 M, and this occurred across the range of higher concentrations of fluoride ion. The presence of strong Lewis acid centers of fluoro-aluminate character within the clay may be responsible for the phosphate cleavage, and this is a topic worthy of further research. In the presence of 1 M sodium chloride and Na?montmorillonite, the oligomer length is generally 8 or higher as determined by ion exchange HPLC analysis (Joshi and Aldersley 2013) (solid black line in Fig. 3). Across the range of fluoride ion concentrations studied, the pH range of the reaction mixtures is in all cases between 8.1 and 8.5, measured both before and after the reactions. This range of pH would, under our standard conditions with 1 M sodium chloride, lead to oligomerization (Aldersley et al. 2011). We have previously noted
Unlike chloride ion, the concentration of fluoride ion in the environment is limited by the extreme insolubility of the common fluoride containing minerals. The minerals fluorite, CaF2, and fluorapatite, Ca5(PO4)3F, will be discussed, with Ksp values of 3 9 10-11 (Garand and Mucci 2004; Aylward and Findlay 1975) and, between 1.3 9 10-60 and 7.4 9 10-60 (Jaynes et al. 1999) respectively. These data provide equilibrium concentrations of fluoride ion of 3.1 9 10-4 M and, from 2.9 9 10-7 to 3.5 9 10-7 M, with fluoride ions that are in equilibrium with the minerals, respectively. Fluorite is by far the most important fluoridecontaining phase in the Earth’s crust and is thought to control the fluoride ion concentrations in seawater and other natural aqueous solutions (Nordstrom and Jenne 1977; Elrashidi and Lindsay 1986). It is recognized that the soluble alkali metal fluorides and other alkali earth metal fluorides are found in nature, but such minerals are rare in comparison with the two considered above (Clark 1993). It is clear that the inhibitory effect upon a clay-catalyzed RNA synthesis by fluoride ion would be absent if the only source of fluoride ion came from these poorly soluble minerals since the equilibrium concentrations of fluoride suggested here are so low, although, interestingly, the [F-] generated by fluorite is close to the threshold for oligomerization determined in this work. Furthermore, a current estimate of the [F-] in the sea is approximately 1.3 ppm (8.95 9 10-5 M), (Environmental Protection Division, British Columbia, Canada) which is too low to inhibit the imidazolide chemistry that we have explored. The nature of these insoluble fluorine-containing minerals may well have maintained a similar low fluoride ion concentration throughout geological time since, for example, the value of Ksp for fluorite changes little with varying concentrations of magnesium or sodium chlorides (Garand and Mucci 2004).
Conclusion It has been demonstrated that the reaction of sodium fluoride alone with an activated nucleotide such as the 50 phosphorimidazolide, ImpA, provides the corresponding 50 -phosphorofluoridate, AMPF. The lack of any oligomer formation with fluorides ([F-] [ 0.002 M) in the presence of montmorillonite is in total contrast with the behaviors of almost all other salts. However, we suggest that the presence of prebiotic fluoride ion is not incompatible with
123
J Mol Evol
RNA synthesis via an activated nucleotide (whatever the pre-biotic activation may have been) because of the extremely low fluoride ion concentrations considered to be present in the pre-biotic environment. Acknowledgments The authors are grateful to Professor Douglas Whittet, Director, NY Center for Astrobiology, and Professor Curt Breneman, Chair of the Department of Chemistry and Chemical Biology, for their support and interest in this research. This research was supported by NASA Astrobiology Institute Grant NNA09DA80A.
References Aldersley MF, Joshi PC, Price JD, Ferris JP (2011) The role of montmorillonite in its catalysis of RNA synthesis. Appl Clay Sci 54:1–14 Aldersley MF, Joshi PC, Schwartz HM, Kirby AJ (2014) The reaction of activated RNA species with aqueous fluoride ion: a convenient synthesis of nucleotide 50 -phosphorofluoridates and a note on the mechanism. Tetrahedron Lett 55:1464–1466 Aylward GH, Findlay TJV (1975) SI chemical data. Wiley, Sydney Clark AM (1993) Hey’s Mineral Index, 3rd edn. Chapman and Hall, London Elrashidi MA, Lindsay WL (1986) Chemical equilibria of fluorine in soils: a theoretical development. Soil Sci 141:275–280
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Environmental Protection Division, British Columbia, Canada http:// www.env.gov.bc.ca/wat/wq/BCguidelines/fluoride/fluoridetoo01.html. Accessed 14 Apr 2014 Ferris JP (2006) Montmorillonite-catalysed formation of RNA oligomers: the possible role of catalysis in the origin of life. Philos Trans R Soc B Biol Sci 361:1777–1786 Garand A, Mucci A (2004) The solubility of fluorite as a function of ionic strength and solution composition at 25C and 1 atm total pressure. Mar Chem 91:27–35 Jaynes WF, Moore PA Jr, Miller DM (1999) Solubility and ion activity products of calcium phosphate minerals. J Environ Qual 28:530–536 Joshi PC, Aldersley MF (2013) Significance of mineral salts in prebiotic RNA synthesis catalyzed by montmorillonite. J Mol Evol 76:371–379 Joshi PC, Aldersley MF, Delano JW, Ferris JP (2009) Mechanism of montmorillonite catalysis in the formation of RNA oligomers. J Am Chem Soc 131:13369–13374 Joshi PC, Aldersley MF, Ferris JP (2011) Progress in demonstrating total homochiral selection in montmorillonite-catalyzed RNA synthesis. Biochem Biophys Res Commun 413:594–598 Joyce GF, Orgel LE (2006) The RNA World (3rd Edition) In: Gesteland RF, Cech TR, Atkins JF (eds) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York Nordstrom DK, Jenne AJ (1977) Fluorite solubility equilibria in selected geothermal waters. Geochim Cosmochim Acta 41:175–188
