Formation of Amino Acids on the Sonolysis of Aqueous Solutions Containing Acetic Acid, Methane, Or Carbon Dioxide, in the Presen

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Formation of Amino Acids on the Sonolysis of Aqueous Solutions Containing Acetic Acid, Methane, or Carbon Dioxide, in the Presence of Nitrogen Gas Leena Dharmarathne and Franz Grieser*

Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia

ABSTRACT: The sonolysis of aqueous solutions containing acetic acid, methane, or carbon dioxide in the presence of nitrogen gas was found to produce a number of diﬀerent amino acids at a rate of ∼1 to 100 nM/min, using ultrasound at an operating power of 70 W and 355 kHz. Gas-phase elementary reactions are suggested, and discussed, to account for the formation of the complex biomolecules from the low molar mass solutes used. On the basis of the results, a new hypothesis is presented to explain the formation of amino acids under primitive atmospheric conditions and how their formation may be linked to the eventual abiotic genesis of life on Earth.

■ INTRODUCTION hydrocarbons, including ethane, propane, ethylene, acetylene, and even butadiene.6 There are many examples in the literature showing that the Perhaps more noteworthy, from a synthetic standpoint, the ultrasonication of aqueous solutions, in the presence of air or irradiation of aqueous solutions of di-n-butyl sulfide using 800 argon, containing organic additives leads to the decomposition kHz ultrasound, in an argon atmosphere, yielded di-n-butyl of the organic solute and, in some cases after prolonged sulfoxide, in addition to a large range of lower molar mass sonication, to the complete mineralization of the additive. The products.7 It has also been reported that amino acids could be chemical processes involved in these systems are primarily formed, through oxime precursors, by sonication of aqueous driven by the oxidative reaction of OH radicals, produced from solutions of aliphatic acids, for example, succinic acid. the sonochemical homolysis of water molecules, with the Sonication was first conducted in a nitrogen atmosphere organic solutes in solution and, progressively over longer followed by sonication in a hydrogen atmosphere.8 However, in sonication times, with their degradation products. Conse- the Ph.D. thesis work of Staas,9 he states this result could not quently, there have been many studies that have examined the be reproduced, although Margulis10 has reported that, whereas use of ultrasonic irradiation of contaminated aqueous solutions the oximes could not be identified, amino acids were produced. 1−5 as an advanced oxidation process (AOP). Margulis10 and Él’piner11 both reported that on the sonolysis of Whereas the emphasis in sonochemical AOP studies has acetic acid, under a nitrogen atmosphere, glycine was formed as been on the degradation of the starting compounds, it has been well as other unidentified amino acids. noticed that products with a higher molar mass than the The formation of amino acids from hydrocarbons and starting material can also be generated. This is particularly the nitrogen is a reaction for “fixing” nitrogen, and the latter has case when the starting compound has an aromatic moiety as been of some interest in the formation of ammonia, as well as − part of its chemical structure. These higher molar mass nitrous and nitric acids.12 14 To increase our understanding of products, although chemically stable, appear as intermediates nitrogen fixation using sonochemistry, particularly with respect during the sonication process and are themselves degraded at to producing biologically significant molecules, we studied the longer sonication times of the system. following three systems: However, largely neglected has been the deliberate study of 1. acetic acid/N2 in an argon atmosphere the formation of complex organic molecules from lower molar 2. methane/N in the presence and absence of an argon mass starting molecules by sonicating aqueous solutions in the 2 presence of solubilized gases. This is despite the fact that it has atmosphere long been known that quite interesting chemistry can be 3. CO2/N2 in the presence and absence of an argon achieved with certain starting materials in combination with atmosphere selected saturating gases, that is, argon, nitrogen, air, etc. For example, it has been found that the sonication of aqueous Received: December 3, 2015 solutions containing methane and argon can yield a range of Published: December 22, 2015

The first of these systems was chosen largely to see if the diagram of the overall sonication arrangement used has been apparently conflicting results of the earlier studies mentioned shown in a previous study.22 above could be resolved. The second two systems were of To check on the consistency of the acoustic power delivered particular interest to examine the hypothesis put forward by to the sample solutions over the duration of the study, periodic 15 Ben-Amots and Anbar, without any experimental support, measurements were made of the H2O2 formation rate in water that cavitation chemistry may have been responsible for saturated with air, argon, or nitrogen over a 10 min period in producing complex organic molecules in prebiotic times. The the reaction vial. The method used has been described chemical composition on Earth prior to ca. 4 billion years ago elsewhere.23 The formation rates were found to be constant contained no complex organic molecules, and yet, biological at 12 ± 1 μM/min (air), 1.6 ± 0.3 μM/min (nitrogen), and 18 and other organic compounds were formed over time, by still ± 1 μM/min (argon). unresolved pathways, that eventually led to life. The second two Sample Analysis and Calibrations. Electrospray ioniza- systems above were chosen as representatives of the many tion mass spectrometry (ESI-MS) was used to quantitatively possible combinations of gases (apart from argon) that may measure the identified products of the sonicated solutions. The have been present at significant levels in the Earth’s primordial instrument used was an Agilent 6520 Q-TOF Mass − atmosphere.16 21 Spectrometer coupled to an Agilent 2100 Series LC System. Samples of either 1 or 100 μL volumes were mixed with 50% ■ EXPERIMENTAL DETAILS methanol/50% Milli-Q water (mobile phase) with a flow rate of Chemicals and Solution Preparation. All solutions were 0.3 mL/min, and pumped into the MS. The capillary voltage of prepared using Milli-Q water. Amino acid standard solutions the MS was set at +100 V. Under these conditions the amino fi were prepared using Sigma-Aldrich products of 98% and 99% acids identi ed were the protonated forms of the parent purity. Alanine and ethylglycine (2-aminobutyric acid) were DL compounds. Some analyses were made using a capillary voltage − fi forms, and the rest of the amino acids used were L forms. set at 100 V. This was done to con rm that the species being (Comparing the mass spectrometer (MS), mass-to-charge ratio detected were indeed the protonated forms of the parent (m/z) position, and signal intensity of the DL-alanine with β- compounds. For example, at +100 V the protonated form of + − alanine standard solutions revealed no difference between the glycine [gly+H] appeared at m/z 76.039, whereas at 100 V mode of operation of the MS, the deprotonated form [gly-H]− forms.) The gases N2,H2, argon, methane, and CO2 used in the experiments were all Coregas (99% purity). All sample appeared at m/z 74.025. solutions were prepared just prior to conducting the sonication In the ESI-MS sample analysis runs, the routine followed was fi experiments, or MS analysis, and the standard reference to rst record a background Milli-Q water mass spectrum (in solutions were freshly made if older than three weeks. the m/z range from 21 to 500), followed by the sonicated Sonication Procedure. The experimental setup used has samples in order of their increasing sonication time, followed fi been described in detail elsewhere.22 Briefly, samples were by standard solutions of increasing concentration (usually ve sonicated using an ELAC Nautik USW-51−052 ultrasonic to six concentrations), and last Milli-Q water. The standard transducer (reactor) operating at 70 W and 355 kHz. The solutions provided the calibration needed to quantitatively fi volume of the liquid samples that were sonicated was 10 mL, convert the identi ed m/z signal counts to a known amino acid contained in 15 mL resealable cylindrical SUPELCO glass vials. concentration. This routine was followed to ensure calibration The vials were all sealed with screw caps fitted with gastight against standards always took into account any possible day-to- silicone-polytetrafluoroethylene septa. The liquid samples were day instrument sensitivity variations. In practice the instrument sparged with argon (15 min) using inlet (directly into the was very stable over the period of the study. liquid) and outlet (from the headspace) needles through the The criteria used to identify an ESI-MS m/z signal from a septa. After the liquid was sparged with argon (and in some sonicated solution with an amino acid were: experiments N2), selected amounts (in milliliter quantities) of (1) the sonicated sample signal had to match the amino acid methane, CO2,N2, and H2 were injected through the septum standard m/z value to an overlap of ±0.004 (There was a (with a calibrated syringe) into the headspace (5 mL) of the small amount of random fluctuation in the m/z signal sealed sample vial. On the basis of the volumes injected into the positions on processing the instrument signals, which headspace, and under equilibrium conditions, the concentration limited the accuracy in the m/z value); of N2 in the water phase was 0 to 0.6 mM, for H2 it was 0 to (2) the sonicated sample m/z signal had to increase in 0.15 mM, for methane it was 0 to 0.5 mM, and for CO it was 0 2 intensity with increasing sonication time of the sample; to 10 mM. Following the injection of methane, CO ,N, and 2 2 (3) the identified m/z signal counts had to remain zero or H2 into the sealed vial, the sample was shaken and left to equilibrate for 15 min before being placed into the water “bath” constant (a background signal) on sonication of the of the reactor, and sonication started. Each sample was sample in the absence of a carbon (e.g., CH3COOH, continuously sonicated for a chosen sonication time, removed CH4, etc.) or a nitrogen (N2) source. from the bath, and stored for later analysis. The sample vial was As all the signals of interest were weak in intensity and positioned at the center of the water bath with the liquid level comparable to the intensity of the background “noise” inside the vial set at the same height as the bath water level. spectrum recorded of the control solutions used, the above The water bath was a double-walled glass collar, the base of listed criteria were used to ensure a reliable identification of the which was directly mounted, via a flange fitting, around the amino acids of interest. There were many more signals stainless steel plate (base diameter of 5.4 cm) of the transducer. recorded that fitted criteria 2 and 3, but remained unidentified The bath volume was 250 mL and filled with 200 mL of water. due to the finite number of amino acid standards employed in Thermostated water was circulated between the inner and outer the present study. walls of the glass collar, and the water in the bath was Results. 1. Acetic acid/N2 in an Argon Atmosphere maintained in the range from 22 to 30 °C during sonication. A System. In Figure 1 (left) is shown a typical growth in the ESI-

192 DOI: 10.1021/acs.jpca.5b11858 J. Phys. Chem. A 2016, 120, 191−199 The Journal of Physical Chemistry A Article

+ ﬀ Figure 1. (left) ESI-MS alanine signals ([ala+H] )atdi erent sonication times of a 1 mM acetic acid: 1 mL of N2 system, in an argon atmosphere (Ar). (right) ESI-MS signals from standard solutions of alanine. The selected MS spectra show all the signals observed in the m/z range chosen.

MS m/z signal at 90.055 corresponding to the protonated form of alanine ([ala+H]+), based on comparing signals from alanine standard solutions (right). The system was 1 mM acetic acid with 1 mL N2 in an argon atmosphere. Similar data were also obtained for ethylglycine (([ethygly+H]+ = m/z 104.066) and glycine (([gly+H]+ = m/z 76.039). In control experiments, where sonicated samples did not contain either acetic acid or N2, no signal growth at m/z 90.055, 104.066, or 76.039 was observed, as was the case on sonicating pure water, all sparged with argon. No serious attempt was made to optimize the production of the amino acids produced, as our main objective was to explore the feasibility of the sonochemical production of these compounds. However, from several preliminary trials undertaken, 1 mM acetic acid appeared to be a favorable level to use, Figure 3. Rate of formation of glycine, alanine, and ethylglycine in (a) fi 1 mM acetic acid: 1 mL of N (Ar) and in (b) 1 mM acetic acid: 1 mL and this level was xed as both N2 and H2 amounts were added. 2 The sonochemical rate of formation of the three amino acids of N2: 1 mL H2 (Ar), systems. identified are shown in Figure 2 for the system 1 mM acetic ff acid: 1 mL N2 in an argon atmosphere. The e ect of adding H2 of formation for different amino acids for acetic acid/N (Ar) as a gas component was also examined as this gas was also 2 and acetic acid/N2/H2 (Ar). (The gas in brackets indicates the added in the earlier studies. In Figure 3 are displayed the rates gas used in the initial sparging of the solution.) From the results of Figure 3 it can be seen that the presence of hydrogen has only a minor effect, if any, considering the experimental variation involved on the rates of formation of the identified amino acids. These outcomes support the work of Sokol’skaya and Él’piner,8 and also Margulis,10 mentioned in the Introduction. As a consequence of these results we turned our attention to the system, methane/N2. 2. Methane/N2 in the Presence and Absence of an Argon Atmosphere System. In Figure 4 (left) is shown a typical growth in the EMI-MS m/z signal at 104.066 corresponding to the protonated form of ethylglycine ([ethgly+H]+) based on comparing signals from ethylglycine standard solutions (right). The system was 1 mL of methane with 1 mL of N2 in an argon-sparged water sample. For this system several gas variations were studied. An example of the formation of glycine, alanine, and ethylglycine, Figure 2. Concentrations of glycine, alanine, and ethylglycine on sonicating a methane/N2/H2 (Ar) mixture is shown in produced as a function of sonication time for the system 1 mM Figure 5, and the variation in the rates of formation as a acetic acid: 1 mL of N2, in an argon atmosphere. function of methane and N2 is shown in Figure 6.

193 DOI: 10.1021/acs.jpca.5b11858 J. Phys. Chem. A 2016, 120, 191−199 The Journal of Physical Chemistry A Article

+ ﬀ Figure 4. (left) ESI-MS ethylglycine signal (as [ethylgly+H] )atdierent sonication times for a 1 mL of methane/1 mL of N2 in an argon atmosphere (Ar) system. (right) ESI-MS signals from standard solutions of ethylglycine. The selected MS spectra show all the signals observed in the m/z range chosen.

Figure 6. Rate of formation of glycine, alanine, and ethylgylcine as a − function of added gas composition for the system X mL of CH4/(2 ≤ ≤ Figure 5. Concentrations of glycine, alanine, and ethylglycine X)mLofN2/1 mL of H2 (Ar) [0 X 2]. The percent methane produced as a function of sonication time for a 1 mL of CH4/1 mL refers to the percentage of methane in the mixture of N2 and methane fi of N2/1 mL of H2 (Ar) system. at a xed H2 level. At 0% methane the system consists of 2 mL of N2/1 mL of H2 (Ar), and at 100% methane the system consists of 2 mL of CH4/1 mL of H2 (Ar). The rate shown for alanine at 50% methane is On the basis of these results several other gas compositions the average of three separate experiments on separate occasions, and were studied, and other amino acids were sought. A summary the error bar shown encompasses the separate results. On the basis of of these systems and experimental results is given in Table 1.It other multiple experiments this error (±20−25%) is typical of all rate is likely that other amino acids were produced in addition to results shown. those shown in Table 1 as there were other m/z signals − observed that fitted criteria 2 and 3, but these were not systems.12 14 Furthermore, we measured the pH of the matched with chosen standards. In the systems 1 mL of CH4 solutions after sonication in the systems: 1 mL of CH4/1 mL (N2), 2 mL of CH4 (N2), and 1 mL of CH4/1 mL of N2 (Ar), of N2 (Ar) pH after 180 min of sonication, pH = 3.9; 1 mL of adenine (the protonated form at m/z = 136.066) was also CH4 (N2) pH after 180 min of sonication, pH = 4.1. (The identified with the rates of formation of 1 × 10−4, 1.5 × 10−4, initial pH of the water before sonication, but after sparging with × −5 μ ∼ and 5 10 M/min, respectively. either Ar or N2, was 7.) The drop in pH on sonication can be We also found that in the system 1 mL of CH4/1 mL of N2/ attributed to the formation of nitric and nitrous acids in these 1mLofH2 (Ar), acetic acid was produced. We note that systems. 24 Henglein detected formaldehyde in CH4 (Ar) systems. These 3. CO2/N2 in the Presence and Absence of an Argon products were not examined further. One other product we Atmosphere System. This system was studied to see if an identified was the nitrate ion, and this is consistent with what alternative carbon source to methane was still able to form others have reported in studies on the sonolysis of N2/H2 amino acids in a sonicated system. The reason for choosing this

194 DOI: 10.1021/acs.jpca.5b11858 J. Phys. Chem. A 2016, 120, 191−199 The Journal of Physical Chemistry A Article

a Table 1. Sonolysis Rates of Formation of Amino Acids in Aqueous Solutions in CH4/N2 Systems gas [gly+H]+ [ala+H]+ [ethgly+H]+ [ser+H]+ [pro+H]+ [val+H]+ [thre+H]+ [leu+H]+ system compositionb m/z = 76.039 m/z = 90.055 m/z = 104.066 m/z = 106.049 m/z = 116.071 m/z = 118.086 m/z = 120.065 m/z = 132.101

CH4/N2 0CH4/2 N2 00 0 (Ar) (Ar)

0.5 CH4/1.5 0.026 0.009 0.003 N2 (Ar)

1CH4/1 N2 0.008 0.040 0.012 0.001 0.002 0.003 0.052 0.001 (Ar)

1.5 CH4/0.5 0.009 0.012 0.005 N2 (Ar)

2CH4/2 N2 00 0 (Ar)

CH4 0CH4 (N2)00000000 (N2)

0.5 CH4 0.053 0.015 0.006 (N2)

1CH4 (N2) 0.110 0.014 0.011 0.003 0.005 0.004 0.121 0.002

1.5 CH4 0.050 0.007 0.008 (N2)

2CH4 (N2) 0.023 0.007 0.006 0.003 0.017 0.004 0.127 0.004

CH4/ 1CH4/1 N2/ 0.020 0.088 0.015 0.215 0.005 0.024 0.134 0.001 N2/H2 0H2 (Ar) (Ar) c 1CH4/1 N2/ 0.030 0.054 0.016 0.115 0.002 0.007 0.180 0.5 H2 (Ar) c 1CH4/1 0.002 0.008 0.069 0.002 0.011 0.068 0.001 N2:/1 H2 (Ar)

1CH4/1 N2/ 0.005 0.034 0.008 0.130 0.002 0.014 0.042 0.001 2H2 (Ar) aThe rates are in micromolar per minute. bGas quantities are in milliliters. cResults were not sufficiently accurate to obtain a reliable rate; note m/z values are only accurate to ±0.004. system will become clear later in the Discussion Section. Figure and N2. The pH measured on sonolysis of 1 mL of CO2/1 mL ff 7 shows the results obtained for di erent gas mixtures of CO2 of N2/1 mL of H2 (Ar) after 180 min of sonication was 3.6. Also, in the system 0.5 mL of CO2/1.5 mL of N2 (Ar), a signal corresponding to the protonated form of adenine was detected at a rate of 7 × 10−4 μM/min. For the system 0.5 mL CO2 in a N2 atmosphere a more extensive examination of the amino acids formed was undertaken, and the rates of formation of the identified amino acids are given in Table 2.

■ DISCUSSION As indicated earlier, the detection of the three amino acids in the acetic acid system in our study supports the work by Sokol’skaya and Él’piner,8 and Margulis.10 The most likely reason Staas9 could not conﬁrm these two earlier studies probably lies with the detection sensitivity of his analysis methods. As we have found in our work, the rate of formation Figure 7. Rate of formation of glycine, alanine, and ethylgylcine as a of the amino acids is a factor of 100 to 500 times lower than function of added gas composition for the systems (a) 0 mL of CO2/2 typical sonolysis products in water, that is, H2O2,H2, under mL of N2 (Ar), (b) 0.5 mL of CO2/1.5 mL of N2 (Ar), (c) 1 mL of comparable experimental operating conditions. CO2/1 mL of N2 (Ar), (d) 1.5 mL of CO2/0.5 mL of N2 (Ar), (e) 2 The reaction pathways involved in the formation of the three mL of CO2/0 mL of N2 (Ar). amino acids in the acetic acid systems are diﬃcult to deduce due to the complexity of the likely reactions occurring inside imploding cavitation bubbles. However, plausible elementary

a Table 2. Sonolysis Rates of Formation of Amino Acids in Aqueous Solutions Sparged with N2 and 0.5 mL of CO2 Added gas [gly+H]+ [ala+H]+ [ethgly+H]+ [ser+H]+ [pro+H]+ [val+H]+ [thre+H]+ [leu+H]+ system composition m/z = 76.039 m/z = 90.055 m/z = 104.066 m/z = 106.049 m/z = 116.071 m/z = 1118.086 m/z = 120.065 m/z = 132.101 b b CO2 0.5 mL CO2 0.0054 0.0026 0.0009 0.0028 0.0094 0.0003 (N2) (N2) aThe rates are in micromolar per minute. bResults were not suﬃciently accurate to obtain a reliable rate.

195 DOI: 10.1021/acs.jpca.5b11858 J. Phys. Chem. A 2016, 120, 191−199 The Journal of Physical Chemistry A Article reactions that may be considered for the formation of glycine system as to some extent this has been investigated in previous are studies.6,24 )))) In comparing the data on the rates of formation of the HO2 ⎯→⎯⎯ · OH + ·H (1) glycine, alanine, and ethyl glycine in the CH4/N2/H2 (Ar) system with the rates measured in the acetic acid system, it can )))) ̇ be seen that they are comparable. This is perhaps not surprising N2N2 ⎯→⎯⎯ · · (2) as similar reactions are involved in both systems, and the final CH3222 COOH+· OH( · H) → H O(H ) +· CH COOH products are formed through complex secondary reactions (3) under similar temperature and pressure conditions. Some likely ̇ ̇ elementary reactions that my occur in the methane system as ·NHNH·+· →· (4) has been considered by Henglein24 are ̇ ·NH+· H →· NH2 (5) )))) CH4 ⎯→⎯⎯ · CH3 + · H (11a) ·+·NH22 CH COOH → H 22 NCH COOH (glycine) (6) CH432+· OH → CH ·+ H O (11b) All these elementary reactions would occur within the imploding cavitation bubbles. The designation )))) on the CH432+· H → CH ·+ H (11c) reaction arrow is to indicate ultrasound is applied to initiate the process. Reaction 1 is a clearly identified sonolysis reaction in Two possible reactions that may be involved in the water.25 Reaction 2, although not directly proven, must occur in incorporation of oxygen in the product molecule are some form as ammonia is a known product of sonolysis of ··+O:C:C → O (11d) water in the presence of nitrogen gas.12,14 It has been suggested that N atoms are not produced by direct thermal fragmentation ·+→OH :C: CO +· H (11e) as shown in reaction 2 but by the reaction14 And following reaction 11e ̇ ··+ON2 →· NON +·· (7) ·+OH CO → CO2 +· H (11f) The source of oxygen atoms coming from the reaction CO2 +· H →· COOH (11g) 2·⇌··+OH O H2O (8) Other reactions, as already suggested in reaction sets 9 and The formation of alanine and ethylglycine most likely 10, can then account for the formation of amino acids in this involves the fragmentation of acetic acid within cavitation system. Although some of these reactions above are bubbles. Acetic acid is sufficiently volatile to be able to enter an endothermic the extreme temperature reached inside an expanding bubble, and so in addition to reaction 3 the imploding cavitation bubble provides the energy to enhance degradation of acetic acid can be expected as per reaction 9a: the comparatively low rate constants at ambient temperatures. )))) − − CH COOH⎯→⎯⎯ · CH + · COOH Also, most radical radical and radical molecule reactions 3 3 (9a) require a three-body collision to proceed, and the very high

·→··+·CH32 CH H (9b) pressures (several hundred atmospheres) inside a collapsing bubble facilitate these types of reactions. ̇ ··→·CH2 CH·+· H (9c) Support for the above family of reactions and species is available from pyrolysis studies on hydrocarbons.28,29 Also, the ̇ ̇ (9d) * 30 ·+·COOH CH·→· CHCOOH observation that C2 is formed in the sonolysis of hydrocarbons, for example, pentane, benzene, in water, as ·+·CH CHCOOḢ → CH CHCOOḢ 6,24 33 (9e) well as product analysis of methane -doped water, supports the existence of a pyrolysis-like environment inside an ·+ ̇ → 31 NH23 CHC HCOOH CH 3 CH(NH 2 )COOH imploding cavitation bubble. (alanine) (9f) The more detailed examination of other amino acid products formed in this system is in accord with the general observation ·+··→·CH32 CH CH 32 CH (10a) that amino acids can be formed under a variety of gas and reaction initiating conditions,20,21 for example, electrical ̇ ̇ 32 CH32 CH·+· CHCOOH→ CH 32 CH CHCOOH (10b) discharge, shock heating, UV photolysis, etc. (A summary of the amino acids produced by various methods is presented in ̇ ·+NH232 CH CH CHCOOH Table 3.) The gas components used in this system were originally suggested to be present in Earth’ssecondary → CH CH CH(NH )COOH (ethylglycine) (10c) 32 2 atmosphere during the period where life originated on Earth, Considering the above reaction pathways it is revealing that a ca. 4 Ga ago. This composition has been reconsidered and number of secondary reactions are involved in the formation of revised over time, and it is now proposed that the primitive the stable end products. Keep in mind that the radical atmosphere was composed of CO2 and mainly N2, system concentrations within cavitation bubbles are vey high26,27 (1 to 3.18,19,33 This is the reason system 3 was chosen for study. The 10 mM) making the rate of radical−radical reactions results obtained indicate that switching the carbon source to − comparable to radical molecule reactions. The product yields CO2 is not an impediment to producing amino acids by involved will no doubt be dependent on the amounts of the sonolysis. That is, gas composition, albeit in a limited way, is diﬀerent starting additives in the system, as is typical of such not a crucial factor in the formation of biologically signiﬁcant sonolysis reactions. We did not explore this aspect of the molecules by cavitation chemistry. This is essentially the same

196 DOI: 10.1021/acs.jpca.5b11858 J. Phys. Chem. A 2016, 120, 191−199 The Journal of Physical Chemistry A Article

Table 3. Abiotic Amino Acid Formation Processes, and clays, and other silicates have been recognized. Once the Amino Acids Detected (√), from Primitive Atmosphere adsorption of the primary amino acids has occurred onto the a Gas Compositions right catalytic surface, for example, montmorillonite, oligopep- 39 d e tides may be formed. In the same way nucleotides may also electric shock heating photolysis 39−41 amino acid sonolysis dischargeb tubec (950 °C) (UV) be formed and as well oligonucleotides. The nucleobases required in this product formation may not necessarily be glycine √√√√√ formed directly in a cavitation bubble, as seems to be possible alanine √√√√√ for adenine,42 but by reaction of cyanide precursors.41,43,44 We ethylglycine √√ √ have not investigated cyanide formation in our cavitation serine √√ √ chemistry work; however, electrical discharge experiments with proline √√ √ prebiotic gases show that hydrogen cyanide is readily valine √√√√√ produced.44,45 Likewise, we have not been able to identify the threonine √√ √ formation of any sugars in our study, but we note that sugar leucine √√√√ √√√ √ precursors, such as formaldehyde, have been formed in other probably 24 a cavitation chemistry, as they have in electrical discharge Carbon sources: CH4 or CO2. Nitrogen sources: NH3 or N2.H2 may 20 b experiments. Finally, via the surface polymerization of or may not be an added component. References 16, 17, 52, and 53. 46 c d e unsaturated hydrocarbons (which are formed through Reference 32. Reference 54. Reference 21, page 92−includes additives other than just those in the Table title. cavitation chemistry as mentioned in the Introduction), lipids could be formed and through self-assembly produce mono- layers or bilayers; the latter a primitive membrane. These lipids conclusion reached in other studies using different reaction may not necessarily be phospholipids but simpler amphiphilic initiating methods.20,21 entities.47 The elementary reactions that may be involved in amino acid In the reaction of forming a nucleotide, phosphate is formation in the CO2/N2 system possible include the following required, and the source of this has been an issue in proposals reactions occurring in the cavitation bubble on collapse. for the formation of RNA and this model for the beginning of 48 )))) life. Most phosphate is incorporated in rocks, for example, CO2 ⎯→⎯⎯ CO + · O· (12a) apatite, and highly immobile in that state especially as the seawater in the first oceans had a pH believed to be ∼8. CO→+·· :C: O (12b) Because of this it has been proposed that the phosphorus CO2 +·→· H COOH (11g) source to make the original RNA had come from P4O10,a volatile form of phosphorus that can lead to phosphate :C:+·→· H CḢ · (12c) formation. On the basis of the results of the cavitation The other reactions to follow will be very similar to those experiments we can now propose that through the formation of described in the reaction sets 9, 10, and 11. There are certainly nitric acid it is possible that localized pools of water at the other reactions that may be invoked, and the above really just seashore, perhaps in small shallow rock-pools, would have a pH lower than 8 and, we suggest, sufficiently low to dissolve illustrate plausible pathways that lead to amino acid formation. 49 The most significant outcome of the present study is that it phosphate bearing minerals to release the ion for the underpins the hypothesis put forward by Ben-Amots and conversion of a nucleoside to a nucleotide to take place. Anbar15 that cavitation chemistry could have been responsible The commonly proposed hypotheses considered for for creating the first complex organic molecules on Earth and primordial amino acid formation, and the route to abiogenesis, hence have been involved in the abiogenesis of life. The are, atmospheric lightening for amino acid formation and likelihood that ultrasound, or any sound for that matter, was biomolecule precursors, meteorite supply of amino acids and fl responsible for initiating cavitation chemistry can be comfort- perhaps primitive life, and hydrothermal vents at the sea oor ably dismissed; however, hydrodynamic cavitation produced for formation of amino acids and precursors to biomolecules. from waves crashing onto rocks at the seashore is a plausible The reaction scheme proposed in our above discussion has candidate for consideration.34 In a similar fashion to acoustic several advantageous features over these main models. The cavitation, hydrodynamic cavitation produces “hot spots” and features as we see them are − cavitation chemistry.35 37 1. The formation of amino acids and other complex If the premise is accepted that complex organic molecules are biologically important molecules, by hydrodynamic produced by cavitation events on seashore rocks exposed to cavitation at the seashore, provides a reliable and wave action, it then is also reasonable to expect that some of continuous source of these materials in a relatively the compounds will be adsorbed onto the rocks under certain fi 38 localized and con ned environment, namely, the coast- conditions. For example, during tidal variations where line. evaporation from water pools would lead to the concentration 2. The coastal rocks provide catalytic surfaces, and the of solutes and hence favor their adsorption onto a surface. This source of phosphate, for making both peptides and aspect has been recognized to be a significant condition if low level of biomolecules are produced, which from our studies nucleotides. (The coastal environment also provides the seems to be the case.38 This latter step leads to several condition of cyclical wet and dry periods, considered to interesting outcomes considering the results of studies that be necessary for some of the catalytic reactions have been performed with respect to reactions of biomolecules mentioned to progress.) on catalytic surfaces. 3. The coastal rocks provide the substrate surface for lipid In studies of oligomer formation of peptides, nucleic acid formation and for the beginning of self-assembly bases, and hydrocarbons, the catalytic qualities of minerals, structures. These self-assembly objects are formed in

197 DOI: 10.1021/acs.jpca.5b11858 J. Phys. Chem. A 2016, 120, 191−199 The Journal of Physical Chemistry A Article the same environment as the other bio-oligomers, and it ■ ACKNOWLEDGMENTS is reasonable to expect that the latter would be We are very appreciative of Mrs. S. S. Volaric for her assistance incorporated into the self-assembly structures. with the MS operation and analysis training and her thoughtful It is reasonable to envisage the surface self-assembly suggestions over the period of the experiments. We also thank structures further evolving into vesicle structures that act as our colleagues, R. O’Hair, R. Tabor, and T. W. Healy, for their hosts to macromolecules and possible precursors to unicellular useful comments and helpful suggestions over the course of this organisms. Figure 8 summarizes the above scenario. This highly work. This work has been supported by an Australian Research Council grant to F.G. ■ REFERENCES (1) Serpone, N.; Terzian, R.; Colarusso, P.; Minero, C.; Pelizzetti, E.; Hidaka, H. Sonochemical Oxidation of Phenol and Three of its Intermediate Products in Aqueous Media. Res. Chem. Intermed. 1993, 18, 183−202. (2) Berlan, J.; Trabelsi, F.; Delmas, H.; Wilhelm, A. M.; Petrignani, J. F. Oxidative Degradation of Phenol in Aqueous Media Using Ultrasound. Ultrason. Sonochem. 1994, 1, S97−S102. (3) Singla, R.; Ashokkumar, M.; Grieser, F. The Mechanism of the Sonochemical Degradation of Benzoic Acid in Aqueous Solution. Res. Chem. Intermed. 2004, 30, 723−733. (4) He, Y.; Ashokkumar, M.; Grieser, F. Kinetics and Mechanism for the Sonophotocatalytic Degradation of p-Chlorobenzoic acid. J. Phys. Chem. A 2011, 115, 6582−6588. (5) Peller, J.; Wiest, O.; Kamat, P. V. Synergy of Combining Sonolysis and Photocatalysis in the Degradation and Mineralization of Chlorinated Aromatic Compounds. Environ. Sci. Technol. 2003, 37, − Figure 8. A highly simplified diagram depicting cavitation bubbles at 1926 1932. the seashore producing complex organic molecules from prebiotic (6) Hart, E. J.; Fischer, C.-H.; Henglein, A. Sonolysis of atmospheric gases, and plausible subsequent pathways for these Hydrocarbons in Aqueous Solution. Int. J. Radiat. Phys. Chem. 1990, − biomolecules leading to single lipid-wall proto-cells. 36, 511 516. (7) Reifsneider, S. B.; Spurlock, L. A. Chemistry of Ultrasound. II. Irradiative Behaviour of Aliphatic Aldehydes and Carboxylic Acids in an Aqueous Medium. J. Am. Chem. Soc. 1973, 95, 299−305. ’ ́’ fi (8) Sokol skaya, A. V.; El piner, I. E. Fixation of Molecular Nitrogen simpli ed reaction schematic does not preclude other possible by the Action of Ultrasonic Waves with the Formation of Biologically reactions being involved in the formation of biomacromolecules Important Substances. Akust. Zhur. 1960, 6, 263−264. and indeed intermediate processes involving cyanide or other (9) Staas, W. H. Chemical Effects of Ultrasound on Selected Amino 50 active chemicals, for example, lactic acid, that may well occur Acids. Ph.D. thesis, Brown University, USA, 1974. in tandem with, or even instead of, the pathways depicted. The (10) Margulis, M. A. Sonochemistry and Cavitation; Gordon and most important aspect of the scheme is that it draws together a Breach Publishers: Australia, 1995. ́ continuous and reliable production of amino acids, and other (11) El’piner, I. E. Ultrasound: Physical, Chemical and Biological biomolecules, with subsequent tenable chemical pathways that Effects; Consultants Bureau: New York, 1964. 51 (12) Supeno; Kruus, P. Fixation of Nitrogen with Cavitation. can lead to proto-cell like entities. In its entirety this scheme − depicts a plausible route to the formation of life on Earth and Ultrason. Sonochem. 2002, 9,53 59. (13) Supeno; Kruus, P. Sonochemical Formation of Nitrite and has several features in its favor compared with other schemes Nitrate in Water. Ultrason. Sonochem. 2000, 7, 109−113. suggested to date. The possible reaction steps needed to (14) Hart, E. J.; Fischer, C.-H.; Henglein, A. Isotopic Exchange in the produced “live” cells from the proto-cells considered above can 14,14 15,15 Sonolysis of Aqueous Solutions Containing N2 and N2. J. Phys. only be described as breathtaking, and to say much remains to Chem. 1986, 90, 5989−5991. be understood and resolved is clearly an immense understate- (15) Ben-Amots, N.; Anbar, M. Sonochemistry on Primordial Earth- ment.47 Its Potential Role in Prebiotic Molecular Evolution. Ultrason. Sonochem. 2007, 14, 672−675. ■ CONCLUSIONS (16) Miller, S. L. A Production of Amino Acids Under Possible − This study has revealed that amino acids can be synthesized Primative Earth Conditions. Science 1953, 117, 528 529. (17) Miller, S. L. The Atmosphere of the Primative Earth and the from the sonolysis of water in the presence of the gases − ’ Prebiotic Synthesis of Amino Acids. Origins Life 1974, 5, 139 151. suggested to have been present in the Earth s atmosphere prior (18) Cleaves, H. J.; Chalmers, J. H.; Lazcano, A.; Miller, S. L.; Bada, J. to life on Earth. The work supports the hypothesis that L. A Reassessment of Prebiotic Organic Synthesis in Neutral Planetry cavitation chemistry at the seashore may have been the source Atmospheres. Origins Life Evol. Biospheres 2008, 38, 105−115. of biologically important molecules that subsequently led to life (19) Kasting, J. F. Earth’s Early Atmosphere. Science 1993, 259, 920− on Earth. 926. (20) Kenyon, D. H.; Steinman, G. Biochemical Predestination; ■ AUTHOR INFORMATION McGraw-Hill Inc.: New York, 1969. Corresponding Author (21) Fox, S. W.; Dose, K. Molecular Evolution and the Origin of Life; * W. H. Freeman and Company: San Francisco, 1972. E-mail: [email protected]. (22) Dharmarathne, L.; Ashokkumar, M.; Grieser, F. Photocatalytic Notes Generation of Hydrogen using Sonoluminescence and Sonochemilu- The authors declare no competing financial interest. minescence. J. Phys. Chem. C 2012, 116, 1056−1060.

198 DOI: 10.1021/acs.jpca.5b11858 J. Phys. Chem. A 2016, 120, 191−199 The Journal of Physical Chemistry A Article

(23) Dharmarathne, L.; Ashokkumar, M.; Grieser, F. Reaction of (46) Spoto, G.; Bordiga, S.; Ricchiardi, G.; Scarano, D.; Zecchina, A.; Ferricyanide and Methyl Viologen with Free Radicals Produced by Borello, E. J. IR Study of Ethene and Propene Oligomerization on H- Ultrasound in Aqueous Solutions. J. Phys. Chem. A 2012, 116, 7775− ZSM-5: Hydrogen-bonded Precursor Formation, Initiation and 7782. Propagation Mechanisms and Structure of the Entrapped Oligomers. (24) Henglein, A. Sonolysis of Carbon Dioxide, Nitrous Oxide and J. Chem. Soc., Faraday Trans. 1994, 90, 2827−2835. Methane in Aqueous Solution. Z. Naturforsch., B: J. Chem. Sci. 1985, (47) Ruiz-Mirazo, K.; Briones, C.; de la Escosura, A. Prebiotic 40b, 100−107. Systems Chemistry: New Perspectives for the Origins of Life. Chem. (25) Makino, K.; Mossoba, M. M.; Riesz, P. Effects of Ultrasound on Rev. 2014, 114, 285−366. Aqueous Solutions. Formation of Hydroxyl Radicals and Hydrogen (48) Schwartz, A. W. Phosphorus in Prebiotic Chemistry. Philos. − Atoms. J. Phys. Chem. 1983, 87, 1369−1377. Trans. R. Soc., B 2006, 361, 1743 1749. (26) von Sonntag, C.; Mark, G.; Tauber, A.; Schuchmann, H. − P. (49) Guidry, M. W.; MacKenzie, F. T. Experimental Study of Igneous OH Radical Formation and Dosimetry in the Sonolysis of Aqueous and Sedimentary Apatite Dissolution: Control of pH, Distance from − Equilibrium and Temperature on Dissolution Rates. Geochim. Solutions. Adv. Sonochem. 1999, 5, 109 145. − (27) Gutierrez,́ M.; Henglein, A.; Ibanez, F. Radical Scavenging in the Cosmochim. Acta 2003, 67, 2949 2963. − − − (50) Forsythe, J. G.; Yu, S.-S.; Mamajanov, I.; Grover, M. A.; Sonolysis of Aqueous Solutions of I ,Br, and N3 . J. Phys. Chem. ́ 1991, 95, 6044−6047. Krishnamurthy, R.; Fernandez, F. M.; Hud, N. V. Ester-Mediated Amide Bond Formation Driven by Wet−Dry Cycles: A Possible Path (28) Kruse, T.; Roth, P. Kinetics of C2 Reactions during High- Temperature Pyrolysis of Acetylene. J. Phys. Chem. A 1997, 101, to Polypeptides on the Prebiotic Earth. Angew. Chem., Int. Ed. 2015, 54, 9871−9875. 2138−2146. (51) Hanczyc, M. M.; Mansy, S. S.; Szostak, J. W. Mineral Surface (29) Dors, M.; Nowakowska, H.; Jasinski, M.; Mizeraczyk, J. Directed Membrane Assembly. Origins Life Evol. Biospheres 2007, 37, Chemical Kinetics of Methane Pyrolysis in Microwave Plasma at 67−82. Atmospheric Pressure. Plasma Chem. Plasma Process. 2014, 34, 313− (52) Johnson, A. P.; Cleaves, H. J.; Dworkin, J. P.; Glavin, D. P.; 326. Lazcano, A.; Bada, J. L. The Miller Volcanic Spark Discharge (30) Didenko, Y. T.; McNamara, W. B., III; Suslick, K. S. Hot Spot Experiment. Science 2008, 322, 404. Conditions during Cavitation in Water. J. Am. Chem. Soc. 1999, 121, − (53) Oro, J. Synthesis of Organic Compounds by Electric Discharges. 5817 5818. Nature 1963, 197, 862−867. (31) Hart, E. J.; Fischer, C.-H.; Henglein, A. Isotopic Exchange in the (54) Harada, K.; Fox, S. W. Thermal Synthesis of Natural Amino Sonolysis of Aqueous Solutions Containing D2 and CH4. J. Phys. Chem. Acids from a Postulated Primitive Terrestrial Atmosphere. Nature − 1987, 91, 4166 4169. 1964, 201, 335−336. (32) Bar-Nun, A.; Bar-Nun, N.; Bauer, S. H.; Sagan, C. Shock Synthesis of Amino Acids in Simulated Primitive Environments. Science 1970, 168, 470−472. (33) Trainer, M. G. Atmospheric Prebiotic Chemistry and Organic Hazes. Curr. Org. Chem. 2013, 17, 1710−1723. (34) Panizza, M. Environmental Geomorphology; Elsevier: New York, 1996. (35) Suslick, K. S.; Mdleleni, M. M.; Ries, J. T. Chemistry Induced by Hydrodynamic Cavitation. J. Am. Chem. Soc. 1997, 119, 9303−9304. (36)Braeutigam,P.;Wu,Z.-L.;Stark,A.;Ondruschka,B. Degradation of BTEX in Aqueous Solution by Hydrodynamic Cavitation. Chem. Eng. Technol. 2009, 32, 745−753. (37) Rooze, J.; Andre,́ M.; van der Gulik, G.-J. S.; Fernandez-Rivas,́ D.; Gardeniers, G. E.; Rebrov, E. V.; Schouten, J. C.; Keurentjes, J. T. F. Hydrodynamic Cavitation in Micro Channels with Channel Sizes of 100 and 750 Micrometers. Microfluid. Nanofluid. 2012, 12, 499−508. (38) Cleaves, H. J., II; Scott, A. M.; Hill, F. C.; Leszczynski, J.; Sahai, N.; Hazen, R. Mineral−organic Interfacial Processes: Potential Roles in the Origins of Life. Chem. Soc. Rev. 2012, 41, 5502−5525. (39) Ferris, J. P.; Hill, A. R., Jr; Liu, R.; Orgel, L. E. Synthesis of Long Prebiotic Oligomers on Mineral Surfaces. Nature 1996, 381,59−61. (40) Ferris, J. P.; Ertem, G. Montmorillite Catalysis of RNA Oligomer Formation. A Model for the Prebiotic Formation of RNA. J. Am. Chem. Soc. 1993, 115, 12270−12275. (41) Ferris, J. P. Montmorillonite-catalysed Formation of RNA Oligomers: the Possible Role of Catalysis in the Origins of Life. Philos. Trans. R. Soc., B 2006, 361, 1777−1786. (42) We note that adenine has also been produced by other methods. Ponnamperuma, C.; Lemmon, R. M.; Mariner, R.; Calvin, M. Formation of Adenine by Electron Irradiation of Methane, Ammonia and Water. Proc. Natl. Acad. Sci. U. S. A. 1963, 49, 737−740. (43) Sanchez, R.; Ferris, J. P.; Orgel, L. E. Conditions for Purine Synthesis: Did Prebiotic Synthesis Occur at Low Temperatures? Science 1966, 153,72−3. (44) Orgel, L. E.; Lohrmann, R. Prebiotic Chemistry and Nucleic Acid Replication. Acc. Chem. Res. 1974, 7, 368−377. (45) Miller, S. L. Production of some Organic Compounds under Possible Primitive Earth Conditions. J. Am. Chem. Soc. 1955, 77, 2351−2361.

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