PREBIOTIC SYNTHESIS OF METHIONINE AND OTHER SULFUR-CONTAINING ORGANIC COMPOUNDS ON THE PRIMITIVE EARTH: A CONTEMPORARY REASSESSMENT BASED ON AN UNPUBLISHED 1958 STANLEY MILLER EXPERIMENT Eric T. ParkerL*, H. James Cleaves2, Michael P. Callahan, Jason P. Dworkin 3, Daniel P. Glavin, Antonia Lazcanoa, and Jeffrey L. Badal Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093, ; A5. 'Geophysical Laboratory, Carnegie Institution of Washington, 5251. Broad Branch Rd. NW, "Washington, DC 20015, USA; 3'LTASA Goddard Space flight Center, Solar System Exploration Division, Greenbelt, MD 20771, USA; 4Facultad de Ciencias, UNAM, Apdo. Postal 74-447 Cd. Universitaria, 04510 Mexico D. F., Mexico Address correspondence to: Jeffrey L. Bada, Scripps Institution of Oceanography, University of California at San Diego, 8615 Kennel Way, La Jolla, CA 92093-0212, USA; phone: 858 534- 4258; fax: 858 534-2995; email; jb d < cscl.c i * Present address, School of Earth & Atmospheric Sciences, Georgia Institute of Technology,. Atlanta, GA 34332, USA ABSTRACT Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CU4), hydrogen sulfide (H2S), ammonia (NH3), and carbon dioxide (CO2). Racemic methionine was farmed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H2S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery. KEYWORDS: prebiotic chemistry, methionine, amino acids, sulfur INTRODUCTION Even though the presence of sulfur-containing compounds in proteins had been known since the mid-19`h century, it was only with the laborious work of John Mueller in the early 1920s that one of the components was identified as an amino acid other than cysteine. Using 5- 68 kg of casein, Mueller successfully isolated 100-200 g of an amino acid that he assigned the empirical formula C51111SNO2 (Mueller 1923a. Mueller 1923b), Using Mueller's procedure, Barger and Coyne (1928) also isolated the new amino acid from casein and showed that it was identical to the amino acid synthesized by the Strecker reaction (HCN + N143 aldehyde) when -I- 3-methylthiopropanal (HSCH2CII2CH0) was used as the starting aldehyde. They determined its structure as the _-methylthiol of _-amino-n-butyric acid (2-amino-4-methylthio-butyric acid, CH;SCH2CH2Cll(NH2)COO1-1) and after conferring with Mueller, named the amino acid methionine. Following methi.onine's discovery and chemical characterization, the study of its biochemical role together with that of cysteine and eystine (Lewis et al. 1936) soon lead to the recognition of the important structural role of these sulfur amino acids in proteins. The metabolic importance of the sulfur amino acids was also elucidated, as well as that of other sulfur-bearing organic compounds like coenzyme A and iron-sulfur clusters. Cysteine and honnocysteine were found to play a key role in transulfuration and methyl transfer reactions in degradative and biosynthetic pathways. The recognition of the significance of sulfur in various aspects of contemporary biochemistry soon raised the issue of the presence of methionine, cysteine and other sulfur-containing organic molecules on the primitive Earth prior to the emergence of life. There have been several attempts to synthesize sulfur amino acids from a variety of model reducing prebiotic atmospheres and different energy sources including spark discharges (Heyns et al. 1957), electron beams (Choughuley and Lemmon 1966) and UV light (Khare and Sagan. 1971; Sagan and Khare 1971; Steinman et al. 1968). In all of these experiments methionine was either not reported as a product or was only tentatively identified (Van Trump and Miller 1972). A detailed investigation of the prebiotic synthesis of methionine was carried out by Van Trump and Miller (1972) who used an electric discharge acting on a simulated primitive Earth atmosphere containing methane (CH4), .molecular nitrogen (N2), ammonia (NH3), water (H20), and hydrogen sulfide (1-125) or methane thiol (CH3SH). The finding of acrolein (propenal, CH2=CH-CHO) as a product of the discharge and the demonstration of its likely- involvement in the abiotic formation of methionine led to the suggestion that acrolein had played a central role as a precursor in the prebiotic synthesis of a number of amino acids that included methionine, glutamic acid, homocysteine (HSCH2CH2C1WH2COOH), homoserine (HOCH2CH2CHNH2COOH) and _._-diaminobutyric acid (Van Trump and Miller 1972). The late Stanley L. Miller performed a number of electric discharge experiments in the 1950s and saved portions of many of these as dried residues (Johnson et al. 2008). One particuiarly interesting experiment used a CH4, H2S, NH3, and CO2 gas mixture and was performed while he was at Columbia University in 1958. For unknown reasons, the results of the experiment were never analyzed or published by Miller. 'rhe discovery of several boxes containing vials of dried residues from this experiment led us to reanalyze the products of this unpublished experiment using modern analytical methods. As discussed below, the formation of methionine from a model prebiotic atmosphere containing 142S suggests that its synthesis is robust under the reducing conditions that may have existed in the Earth's early atmosphere, either globally, or in localized environments around volcanic eruptions that were accompanied 2 by intense lightning (Johnson et a. 2008; Tian et al. 2005; Urey 1952; Walker and Brimblecobe 1985). EXPERIMENTAL PROCEDURES Identification of Vials and Experimental Description Miller's archived samples were found stored in labeled four-dram vials. They were catalogued and identified by consulting Miller's original laboratory notebooks, which are kept in the Mandeville Special Collections in the Geisel Library at the University of California, San Diego (Stanley L. Miller collection, Laboratory Notebook 2, page 114, Serial number 655, MSS642, Box 25, Mandeville Collections, Geisel Library). The samples chosen for analysis came from a collection consisting of several vials containing dried residues prepared by Miller from his aforementioned 1958 experiment. In this experiment he used the classic two-chambered apparatus configuration that he originally tested in 1953 (Mi1hT 1953; iV apparatus was filled with 300 rmL HO and a mixture of CH4 (258 mom Hg), CO2 (87 mm Hg), H2S (100 rum Hg) and NH3 (250 mm Hg). According to Miller's 1953 laboratory notebooks, a few minutes after the experiment was initiated on March 24, 1958, a yellowing of the solution was observed, possibly from the formation of sulfur-bearing organic compounds or the polymerization of hydrogen cyanide (HCN). A day after the start of the experiment, Miller reported "a large amount of [elemental] sulphur had deposited in the 5 liter flask. Shook up the flask to get the sulphur away from the electrode". No major changes were subsequently observed the day after, and on March 27, 1958 the sparking and boiling were stopped and the products were placed in a freezer. A few days later, on March 30, a pressure of 854 mm Hg was registered, with a pH of approximately 8, with "little NH3, ITS (or CO2) present" (S. L. Miller, 1958, Laboratory Notebook 2, page 116, Serial number 655, MSS642, Box 25, Mandeville Collections, Geisel Library). The increase in pressure at the end of the experiment was not addressed by Miller but may have been due to the production of carbon monoxide (CO) and molecular hydrogen (H2). The experiment was terminated three days later, and the products were placed in a freezer. On June 17, 1958 he passed the solution through filter paper with suction. The solution had a yellowy-red color, "somewhat like cytochrorne C" (S. L. Miller, 1958, Laboratory Notebook 2, page 114, Serial number 655, MSS642, Box 25, Mandeville Collections, Geisel Library). The solution from the experiment was separated into various fractions by ion chromatography (Miller 1955), evaporated, and saved. A portion of these sample fractions were saved and these were studied here. When Miller moved from Columbia University to the University of California, San Diego in 1960, he took the vials described above with him, together with the products of many other experiments he had conducted earlier while at the Univ=ersity of Chicago (Johnson et al. 2008). These were stored in a cardboard box until we rediscovered them a few months before his death on May 20, 2007. Chemicals and Reagents All glassware and sample handling tools were rinsed with Millipore water (18.2 M_, <1.0 ppb total organic carbon), "Tapped in aluminum foil, and then heated in air at 500°C overnight. All of the chemicals used in this study were purchased from Sigma-Aldrich or fisher Scientific. 3 Stock amino acid solutions (_10-3 M) were prepared by mixing individual amino acid crystals (97-99% purity) with doubly distilled (dd) H-,O. The reagent o-phthaldialdehyde../ -acetyl-L - cysteine (OPA./,NAC) was used as a chemical tag for the fluorescence detection and enantiomeric separation of primary amines. The derivatization solution was prepared by dissolving 4 mg OPA in 300 L methanol (Fisher Optima), and then adding 250 _L. 0.4 M sodium borate buffer (pH 9.4), 435 µL 1:120, and 15 _L of 1 M NAC. The ammonium formate buffer used in the time of flight-mass spectrometry (ToF-MS) analyses described below was prepared by NH40H titration of a 50 mM formic acid solution to pH 8. A 1 pM phenolphthalein solution in acetonitrile with 0.1% formic acid was used for mass calibration of the ToF-MS via an independent electrospray emitter (Glavin and Dworkin 2009). High Performance Liquid Chromatography with UV Fluorescent Detection (HPLC-UV) This method was used to pre-screen the various samples to provide an indication of the relative abundances in order to optimize the more detailed analyses done with combined HPLC- ToF-MS described below. The residues in the various vials were first re-suspended in 1.5 mL ddH20 and subjected to vortex stirring and sonication prior to being brought to dryness using a vacuum centrifuge set at 40°C. The samples were then resuspended into 1 mL aliquots of ddll?O and diluted from initial stock concentrations according to optimal fluorescent signal response. in Amino acids and primary amines were separated and detected using a 5 250 mm x 4.6 mm C-18 reverse phase HPLC column (Phenornenex) coupled with a Shimadzu RF-535 fluorescence detector (_^,=340 nm, ,1=450 nm). Buffer flow rate was 1 mL/min with gradients optimized for separation of amino acid enantiomers (Zhao and Bada 1995). Buffers were Optima grade Methanol (A) and 0.05 M sodium acetate with 8% methanol (B). Samples were prepared for analysis by mixing 5 µL sample aliquots with 10 }iL, of 0.4 M, pH 9.4 sodium borate prior to 1 minute derivatization with 5 _L: OPA'NAC. Reactions were quenched with 0.05 M sodium acetate buffer (pH 5.5) to a final volume of 500 _L and immediately analyzed. Concentrations of peaks were determined based o comparison with standard peak areas of known. concentrations. HPLC-FD and Time of Flight-Mass Spectrometry (LC-FDIToF-MS) A fraction of each residue was prepared and similarly derivatized for analysis by LC- FDiToF-MS as described elsewhere (Johnson et al. 2008). In addition to using retention times to identify fluorescent peaks in the LC-FD/ToF-MS chromatograms, we also determined compound identities by the presence of the appropriate monoisotopic mass at the correct retention time. RESULTS Typical LC-FD/ToF-MS chromatograms and mass spectra detailing the detection of the various sulfur-bearing organic compounds in Miller's original 1958 sample fractions are shown in Figure 1. A summary of the yields of these sulfur-containing compounds relative to glycine is shown in Figure 2 (a more extensive manuscript describing the entire suite of amino acids and amines detected in this experiment is in preparation). Chiral amino acids were racemic within the precision of the measurements. We were not able to calculate actual yields for the various amino acids because there was no record of how much of the water from the experiment was saved. However, Van Trump and Miller (1972) gave the yield of glycine from a similar experiment 4 (based on carbon added as methane) as 0.068 %. In addition to methionine and glutamic acid (detected here but not listed), which were reported by Van Trump and Miller (1972), we have also identified the non-proteinogenic sulfur- containing amino acid S-etylcysteine (CH3SCH2CH(NH2)COOH) and have tentatively identified the non-proteinogenic sulfur-containing amino acid ethionine (2-amino-4- etbylthiobutyric acid (CH3CH2SCH2CI-12CH(NH2)C00H)), the lover and higher homologues of methionine, respectively. Several of the molecules listed in Figure 2 are likely decomposition. products of cysteine, homocysteine, and methionine, including cysteamine (HSCH2CH2NH2), ho ocysteic acid (H 03SCH2CH2CH(NTH2)CO2H)), methionine sulfone (CI-I,,S02CH2CH2CH(Nfi2)C00fl) and methionine s u I f o x i d e (CH3SOCH2CH2CH^^H2)COOH), among others. It is possible that cysteine and homocysteine were also present in the analyzed samples, but the OPAINAC derivatization method does not provide high sensitivity for eysteine or hoinoeysteine detection and thus their presence could not be established with certainty. This may be due to cyclization of compounds containing highly nucleop ilic functional groups (such as amine or sulfhydryl groups) in 1, 2 or 1, 3 positions, either eliminating the fluorescent tag (OPAtNAC also does not effectiv=ely tag 2, 3-diamino propionie acid, 2,4-diamino but yric acid or 2, 3-diamino succinic acid, but does tag ornithine and lysine), but could also be due to internal fluorescence quenching of doubly-labeled compounds. We also attempted to detect cysteine by GC-MS and DART-ToF-MS analysis. Cysteine was undetectable by GC-MS, and only traces of a compound with c y steine's mass were detectable by DART-ToF-MS analysis, suggesting that either any cysteine produced in this experiment decomposed during storage, possibly due to oxidative coupling to produce cystine. However, the were also unable to detect cystine by LC_MS analysis, and cystine would presumably not suffer from the evelization issues mentioned above, suggesting that any cysteine produced was rapidly degraded during storage into products besides eysteine and cystine. DISCUSSION It is likely that H2S, liberated from volcanic gases, hydrothermal vents, and other sites of furnarole activity, was present in the atmosphere of the primitive Earth (Urey 1952; Walker and Brimblecombe I985; Kasting et al. 1989; Domagal-Goldman et al. 2008). This possibility is supported by models of thermal outgassing of volatiles based on ordinary chondritic material (Schaefer and Fegley 2007). As has been pointed out (Sagan and Khare (1971); Miller and Orgel (1974); Raulin and Toupance (1977)), H2S can act as a long wavelength UV photon acceptor for the energetic activation of other molecules such as methane. Thus, it could have played a central role as a sulfur donor in the abiotic synthesis of thio-amino acids and other sulfur-bearing compounds. Van Trump and Miller (1972) demonstrated that methionine is synthesized b y the action of an electric discharge on a simulated primitive Earth atmosphere containing CH 4, N2, NH3, H2O, and H2S or CH;SH at yields of —3x10° relative to glycine. This is very similar to the ratio we determined (Figure 2). They suggested acrolein as an intermediate in the synthesis of methionine. As shown here, analysis of the samples from experiments performed by Miller in 1958, 14 years before those he conducted in collaboration with Van Trump, demonstrate that methionine and other sulfur-bearing compounds, including S-metylcysteine, ethionine, homocysteic acid, methionine sulfone, methionine sulfoxide, and cysteamine, can be synthesized 5 in good vields from a spark discharge acting on a CH4, NH3, CO2, and H2S gas mixture. The results presented here also expand the list of sulfur amino compounds that may have been formed prebiotically and are the first report of the synthesis of the non-proteinogenic amino acid S-methyleysteine. Additionally, a peals consistent with ethionine, but coeluting with a contaminant leads us to the tentative report of the synthesis of ethionine in a prebiotie simulation experiment. The abiotic formation of methionine by a Strecker synthesis involving 3- rnethylthiopropanal, KCN and NH4CI has been reported (Barger and Coyne 1925). Van Trump and Miller (1972) suggested that 3-methylthiopropanal could be produced from a reducing atmosphere containing hydrogen sulfide by the addition of methane thiol to acrolein even under dilute conditions (Figure 3). Acrolein is a byproduct of the decomposition of methionine (Lieberman et al. 1965) and can also be produced in significant amounts from very dilute solutions of formaldehyde and acetaldehyde under neutral to basic conditions (Cleaves 2003). It is significant to note that the two predominant amino acids produced in electric discharge experiments are glycine and alanine, the Strecker synthesis products of formaldehyde and acetaldehyde, respectively (Miller 1955). As suggested by Van Trump and Miller (1972), acrolein may have also played a key role as a precursor in the formation of glutamic acid, homocysteine, homoserine and _,_-diaminobutyric acid. It has been suggested that the reaction of ammonium thiocyanate, thiourea, and thiacetamide (all of which are produced from electric discharges acting on NH 3, CH4, H2O, and HZS gas mixtures (Heyns et al. 1957)) with formaldehyde can lead to the production of glycine, cysteine, and cystine (Herrera 1942; Perezgasga et al. 2003). It has also been shown that 142S, together with pyrite and other metal sulfides, can partake in surface-mediated reactions that provide electrons for the reduction of organic compounds under simulated volcanic conditions (Huber and Wachsterhauser 2010; and references therein). However, organic sulfur-containing amino acids and amines, such as homocysteic acid, eysteamine, taurine (HO3SCH2CH2NH2) (Choughuley and Lernmon 1966), cysteine (Sagan and Khare 1971; Khare and Sagan 1971) and methionine, seem to be produced more readily from model H2S-containing primitive atmospheres than from pyrite/metal sulfide reactions (Huber and Wachsterhauser 2010). Two alternative pathways can be suggested for the production of cysteine from glycine under possible prebiotic conditions (Figure 4). As suggested by 'Veber and Miller (1981), S- methyleysteine could have formed under primitive conditions by the Michael addition of CH3SH to dehydroalanine (Figure 4). We could not confirm the formation of dehydroalanine because it is very reactive and thus if present its levels would be below our detection limits, which are in the low femtomole range. The notion that methionine is a product of the addition of CH3SH to acrolein (Van Trump and Miller 1972) is supported by the tentative detection of ethionme (Figure 3), which could have been formed in part by the addition of ethane thiol (CH3CH2SH) to acrolein. Cysteamine has also been produced in a model reducing atmosphere with electron beams, albeit in low yields (Choughuley and Lennon 1966). Several of the compounds we have detected are known decomposition products of cysteine and methionine. Cysteamine, the simplest aminothiol, is produced by the degradation of cysteine (Figure 4), and methionine sulfone and methionine sulfoxide are produced by the oxidation of methionine (Lieberman et al. 1965). We did not detect taurine, which would be the product of the decarboxylation of eysteic acid or the oxidation of cy steamine (Figure 4). perhaps due to their relative instabilities, neither indigenous cysteine nor methionine has so far been conclusively detected in carbonaceous chondrites (Pizzarello and Shock 2010). 6 The presence of homocysteic acid in the samples we have analyzed could be explained by the Strecker degradation of methionine (Schonberg and Moubacher 1952). The Strecker degradation of methionine proceeds via the catalytic decarboxy lation and deamination with a carbonyl compound or an inorganic catalyst to produce 3-methylmercaptopropanal (Schonberg and Moubacher 1952), which we did not attempt to detect. However, the Strecker degradation of methionine is also known to produce homocystine, among other compounds (Lieberman et al. 1965). As long as free oxygen was absent in the primitive atmosphere and oceans, methionine could have persisted for significant periods of geologic time (Van Trump and Miller 1972). However, as oxygen began to accumulate in the early atmosphere (Ku p 2008), oxidation by metal ions, peroxides, etc. would have likely been important in regulating the concentration of methionine and cysteine present in the primitive oceans and other water bodies (Weber and Miller 1981). Methioni e decomposes readily in the presence of oxygen and produces methionine sulfoxide, methionine sulfone, and various sulfides and thiols (Lieberman et al. 1965). It is thus possible that the compounds detected here represent both products synthesized due to the action of electric discharges on an atmosphere of CH 4, H2S, NH_, and CO2 and the various Strecker and oxidative decomposition products of methionine and eysteine formed during the storage of the extracts. Even though these samples were not preserved in anoxic conditions, the manner in which they were preserved (dry=, roam temperature, —50 years) implies that prebiotic methionine may not have been stable once oxygen began to accumulate in the early: atmosphere. CONCLUSIONS Our findings confirm and extend previous work by Van Trump and Miller (1972) on the prebiotic synthesis of methionine and other sulfur-bearing organic compounds, which could have been formed under primitive Earth conditions. Why Miller never analyzed these 1958 experimental sample fractions is un novm, however, the results presented here indicate that in addition to abiotic synthetic processes, degradation of organic compounds of biochemical significance on the primordial Earth could have played a significant role in diversifying the inventory of molecules not readily fanned from other endogenous abiotic reactions, or derived from extraterrestrial delivery. Our results may have evolutionary implications in the understanding of the earliest metabolic pathways. It has been hypothesized that eysteamine, which is a chemical precursor of the pantetheine moiety of coenzyme A. was formed in the primitive oceans from ethylene sulfide and ammonia or from ethylene imine and hydrogen sulfide (Keefe et al. 1995). However, our results suggest that cysteamine could have also formed readily from electric discharges. The recently discovered enzymatic conversion of ey steate into sulfopyruvate in the biosynthesis of coenzyme M (2-mercaptoethanesulfonic acid, f1SCH 2CH2SO3H) in Methanosareina acetivorans (Graham et al. 2009) supports the idea that products of cysteine degradation and other sulfur- bearing organic compounds of prebiotic origin ma y have been involved in early biological processes. The selection of the two thin-amino acids present in proteins is likely the outcome of a combination of their availability coupled with their functional utility (Cleaves 2010; Weber and Miller 1981). The possibility that cysteine could be understood as the evolutionary replacement 7 of an ancestral sulfhydryl-containing coenzyme has been raised (White 1982). However, it is possible that eysteine was first incorporated into proteins because of its ability to form RNA- recognizing zinc-fingers, to bind to Pe; S clusters and to dimerize and covalently link to form disulfide bonds that play a key role in maintaining functional three-dimensionally folded protein structures. In addition to its role as a building block in proteins, methionine is the immediate precursor of S-adenosyimethionine (SAM), the major methyl-group donor in transmethylation reactions in contemporary biochemistry. It has been proposed that methyl group transfer from SAM to amines may be vestigial of prebiotic methylation reactions involving formaldehyde (Waddell et al. 2000). However, the possibility that ribonucleotide-like coenzy=mes are remnants of an ancestral stage in which ribozyes played a more conspicuous role in metabolism (Orgel and Sulston 1971; White 19. 7su6g)gests that methionine may have been first incorporated into biological systems because of its involvement in methyltransferase activities that evolved in a primordial RI A-dependent world. In other words, it is possible that methionine was initially incorporated into the RNA world as a cofactor. ACKNOWLEDGEMENTS We are grateful to the librarians of the Mandeville Special Collections in the Geisel Library at the University of California, San Diego campus. Support from a UC Mexus- CONACYT fellowship to A.L. and the NASA Astrobiology Institute and Goddard Center for Astrobiology for J.P.D. and D.P.G. are gratefully acknowledged. H.J.C. and M.P.C. were supported by the NASA Post-Doctoral Program (NPP). 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