The Astrophysical Journal, 571:L177–L180, 2002 June 1 䉷 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
PATHWAYS TO GLYCINE AND OTHER AMINO ACIDS IN ULTRAVIOLET-IRRADIATED ASTROPHYSICAL ICES DETERMINED VIA QUANTUM CHEMICAL MODELING David E. Woon Molecular Research Institute, 2495 Old Middlefield Way, Mountain View, CA 94043;
[email protected] Received 2002 April 1; accepted 2002 April 16; published 2002 May 3
ABSTRACT A recent experimental study reported that glycine and other amino acids were formed when cryogenic H 2 O ice containing small amounts of CH 3 OH , NH 3 , and HCN was subjected to ultraviolet (UV) irradiation. Quantum chemical calculations were employed to evaluate the viability of various pathways to the formation of glycine, alanine, and serine in dilute H 2 O ice containing CH 3 OH and HCN. Under the experimental processing conditions of deposition and UV irradiation at 15 K followed by heating to room temperature, amino acids can form by recombining radicals produced by dehydrogenating H 2 O and CH 3 OH and subsequently hydrogenating HCN. The study indicates that isotopic substitution experiments would identify CH 3 OH as the source of the C atom in the COOH carboxylic acid group of the amino acids observed in the irradiation experiments, with the CO⫹OH reaction playing an important role. The remaining C and N atoms in glycine are predicted to originate from HCN via sequential hydrogenation to yield CH 2 NH 2. Formation pathways for alanine and serine are also discussed. Subject headings: astrochemistry — ISM: molecules — molecular processes — ultraviolet: ISM no barrier. While UV and ion irradiation may yield similar products (Gerakines, Moore, & Hudson 2000), the phenomenology of the two treatments differs in some aspects. Chemistry initiated by UV irradiation occurs by photolysis of existing chemical bonds, which depends on the wavelength of the incident radiation. Water is an interesting and relevant case in point since it is the most abundant molecule in the experiments of Bernstein et al. (2002) and in astrophysical ices (Allamandola, Bernstein, & Sandford 1997). Recent studies by Watanabe, Horii, & Kouchi (2000) on pure H 2 O ice and by Watanabe & Kouchi (2002) on H 2 O ice doped with CO are quite illuminating, particularly when combined with what is known about gas-phase H 2 O UV photolysis. Watanabe and coworkers concluded that the most important process occurring in their experiments was H i OH bond cleavage by way of an excited state. In their experiments and those of Bernstein et ˜ 1B ) R X( ˜ 1A ) and al., two processes are of interest, the A( 1 1 ˜B(1A1 ) R X( ˜ 1A1 ) electronic transitions. The excited A˜ state is not bound but dissociates directly into H and OH in their ground electronic states (Schinke 1993, pp. 10–11). There is a threshold energy for this excitation of about ∼183 nm, but absorption can occur over a continuum extending to ∼142 nm, with a peak at 165 nm. The B˜ state is bound, but it is also dissociative. Photons of higher energy (peak at ∼128 nm) are required to make this transition. Bernstein et al. used a lamp in which most of the photons were divided between 160 or 122 nm (Lya line), exciting transitions to both the A˜ and B˜ states. Photolysis introduces energy into ice in multiple ways. First, photolytic products tend to be in a “hot” state, with significant excess energy partitioned between internal rovibrational states and translational kinetic energy. Both can initiate subsequent reactions. Photolytic products, which are generally radicals, will also release energy if they are able to recombine with other radicals. In thermally processed ices (Schutte et al. 1993a, 1993b), reaction energy from very favorable processes can drive less favorable ones (Woon 1999). Therefore, in evaluating possible pathways to amino acid formation under UV irradiation, it is critical to bear in mind that much more energy is available than when no irradiation occurs. However, there are other constraints. At temperatures of 10–20 K, most radicals
1. INTRODUCTION
One of the holy grails of astrobiology is the search for amino acids in the interstellar medium (ISM). If amino acids and other biochemical precursors formed in the early evolution of the protosolar nebula and accreted into solids ranging from dust through comets, they may have provided essential feedstock for prebiotic chemistry on Earth (Chyba et al. 1990; Pendleton & Cruikshank 1994; Bernstein, Sandford, & Allamandola 1999). Short of finding primitive life elsewhere in the solar system or detecting extraterrestrial intelligent life, discovering the extent to which amino acids and other key organic molecules are distributed through the universe could contribute to understanding how widely life itself might be distributed. While there is tantalizingly tentative evidence of interstellar glycine (Snyder 1997), its presence in the ISM currently remains unconfirmed. However, a new experimental study by Bernstein et al. (2002) has reported that the amino acids glycine, alanine, and serine as well as other organic species were generated in ultraviolet (UV) irradiated ices formed at 15 K and 10⫺8 torr in their laboratory. Compositions were selected to match ices present in dense interstellar clouds: predominantly H 2 O ice with 0.5%–5% NH 3, 5%–10% CH 3 OH, and 0.5%–5% HCN. One piece of evidence that the products of these experiments are not due to contamination is that alanine and serine are racemic; their dextro- and levorotatory enantiomers are present in nearly equal quantities, while terrestrial amino acids are predominantly left-handed. It is now well known from both laboratory and theoretical studies that extensive chemistry can occur in astrophysical ices (in glacies), even at dense molecular cloud temperatures of 10–20 K. The products will reflect the initial ice composition and any energetic processing that has been applied, including the thermal history. The energetic threshold can be surprisingly low: reactions can occur in some ices at temperatures as low as ∼70 K with no additional energy (Schutte, Allamandola, & Sandford 1993a, 1993b; Woon 1999, 2001a, 2001b, 2002b). However, UV irradiation is a much different regime. As in ion irradiation with energetic particles, UV irradiation introduces significant energy into the ice. Chemical bonds are broken, generating radicals that may recombine with other radicals with L177
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were employed for all atoms. A correction to account for the influence of bulk ice on energetics was determined by means of self-consistent reaction field calculations using the isodensitysurface polarizable continuum model (IPCM; Foresman et al. 1996) at the UMP2 level using a dielectric constant of 78.5 and an isodensity cutoff of 0.0008. 3. COOH FORMATION
Fig. 1.—Schematic potential energy surfaces for the CO ⫹ OH reaction indicating stable species and transition states (TSs). Computed QCISD(T)/augcc-pVDZ energy differences in units of kcal mol1 are shown, referenced to the CO-OH reactant complex. The upper surface is the gas phase, while the lower surface includes an IPCM correction for the influence of ice.
(except H) cannot diffuse to recombine with other radicals. Also, the excess energy of photolytic products will dissipate fairly rapidly. A rigorous model of interstellar ice chemistry must take into account both microscopic chemical and physical behavior and macroscopic interactions with the radiation field and material distribution of the entire cloud. Recent work by Sorrell (1999, 2001) provides a framework for both aspects of this, with a particular emphasis on the macroscopic modeling. The present study focused on an aspect of the modeling that is less adequately treated by Sorrell, the quantitative evaluation of chemical pathways to amino acid formation in ices using quantum chemistry. After briefly outlining the methodology that was used, the formation of COOH and CH 2 NH 2 radicals in UVirradiated H 2 O/CH 3 OH/HCN ices will be described. The formation of glycine, alanine, and serine will then be discussed as well as additional predictions that should be valid if these pathways are operative. 2. METHODOLOGY
Ab initio electronic structure calculations were performed with the GAUSSIAN package of ab initio programs.1 A number of correlation methods and basis sets were used during initial benchmarking, but this Letter will focus on results computed with the most accurate method and larger basis sets. Structures of reactant complexes, transition states, intermediates, and products were optimized using single and double excitation quadratic configuration interaction (QCISD; Pople, Head-Gordon, & Raghavachari 1987). Single-point energies were then performed in which triple excitations were treated quasiperturbatively [QCISD(T); Gauss & Cremer 1988]. Zero-point energy corrections were determined via frequency calculations with unrestricted secondorder Møller-Plesset perturbation theory (UMP2; Møller & Plesset 1934). The aug-cc-pVDZ basis sets of Dunning and coworkers (Dunning 1989; Kendall, Dunning, & Harrison 1992) 1 GAUSSIAN 98, Revision A.7, M. J. Frisch et al. 1998, Gaussian, Inc., Pittsburgh, PA.
Glycine, the smallest amino acid, is NH 2 CH 2 COOH. Other a amino acids involve substitutions of one of the two H atoms on the central C atom. In alanine and serine, H is replaced with CH 3 and CH 2 OH, respectively. In order to account for the production of glycine, alanine, and serine in the work of Bernstein et al. (2002), one seeks to identify the most plausible intermediates that might arise from the given starting materials. This strategy leads to testable predictions about additional products and the origins of each heavy atom. Previous experimental work on UV-irradiated ices containing CH 3 OH (Bernstein et al. 1995) found H 2 CO, HCO, and CO among the products. Similar products are generated when H 2 O/CH 3 OH ice is irradiated with 3 keV He⫹ ions (Palumbo, Castorina, & Strazzulla 1999). This indicates that methanol can be partially or completely dehydrogenated. The recent work of Watanabe & Kouchi (2002) found that UV irradiation of H 2 O/CO ice produced significant quantities of CO 2. They concluded that this most likely arises from the attack of CO by OH produced through H 2 O photolysis. Omitting transition states, the reaction sequence is OH ⫹ CO r t-COOH r c-COOH r CO 2 ⫹ H.
(1)
COOH is a stable intermediate radical that occurs in two isomeric forms, where the H on the OH is either trans (t) or cis (c) to the carbonyl O; t-COOH must isomerize to c-COOH before the H can be expelled, but this barrier lies quite a bit below the initial barrier to addition and final barrier to elimination. Gas-phase and IPCM (solvated) reaction energetics are shown in Figure 1. Zhu et al. (2001) computed kinetic rates for reaction (1) and summarized prior theoretical and experimental work. Their emphasis was on high-temperature gasphase behavior. When H 2 O dissociates via the lower energy A˜ R X˜ transition by absorbing 160 nm UV photons, the excess energy that is available is the difference between the photon energy (7.8 eV p 178.8 kcal mol⫺1) and the H i OH bond energy (5.1 eV p 117.8 kcal mol⫺1), or about 60 kcal mol⫺1. At the QCISD(T)/aug-ccpVDZ level, the initial barrier is just 2.7 kcal mol⫺1 above the CO-OH complex, which is much less than what is available. While the barrier to eliminating the H is slightly higher (9.1 kcal mol⫺1), it is clear why CO 2 should form easily in UV-irradiated H 2 O/CO ices. In fact, a critical issue that arises when considering COOH as a precursor to amino acids is that the intermediate must be stabilized under fairly energetic conditions. In the gas phase, one expects efficient conversion of CO and OH into CO 2 and H, especially at room temperature or higher. But in ices, sufficient energy could be extracted during the lifetime of the rovibrationally excited intermediate to stabilize COOH. There is experimental evidence that this is the case. Watanabe & Kouchi (2002) observed that CO disappeared faster than CO 2 appeared and attributed the difference to stabilization of COOH. Likewise, Hudson & Moore (1999) performed an analogous study in which H 2 O ice doped with CO was irradiated with 0.8 MeV protons at 16 K. They observed CO hydrogenation products (HCO, etc.)
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as well as HCOOH, which would be formed readily by H addition to COOH. IPCM calculations suggest that the ice tends to raise both the initial and final barriers, with the latter increasing by nearly 3 kcal mol⫺1. Furthermore, the intermediate and product are comparably stable. Further studies are in progress (D. E. Woon 2002, unpublished) to determine the CO : COOH branching ratio in UV-irradiated H 2 O/CO ices using quantum chemistry and kinetic rate theory. 4. CH2NH2 FORMATION
As discussed above, photolysis of H 2 O through excitation to the A˜ state yields H and OH with about 60 kcal mol⫺1 of excess energy. Schinke (1993, p. 14) cites the work of Andresen et al. (1984), which determined that almost 90% of the excess energy of this process is partitioned in the form of translational kinetic energy. By contrast, most of the excess energy is partitioned as OH rotational energy in photolysis via the B˜ state. Since OH is likely to remain immobile in ice at 10–20 K, photolysis at 160 nm tends to introduce hot, mobile H atoms into the ice. They lose energy fairly rapidly through collisions but can potentially perform chemistry before they cool. While the diffusion constraint probably ensures that most liberated H atoms form H 2, as observed by Watanabe et al. (2000) and Sandford & Allamandola (1993), reactive interactions with HCN, CO, or other molecules would yield new species that are precursors to amino acids and other organic molecules. Computational studies of the sequential hydrogenation of HCN were performed. The results parallel a recent study (Woon 2002a) of the hydrogenation of isoelectronic CO. As in that case, odd-numbered additions possess modest barriers, while even-numbered additions are barrier-free and much more exothermic. The HCN sequence is more complex, however, since there are competitive outcomes for hydrogenating both HCN and CH 2 NH (methyleneimine). The full sequence is HCN ⫹ H r (CH 2 N, HCNH),
(2)
(CH 2 N, HCNH) ⫹ H r CH 2 NH,
(3)
CH 2 NH ⫹ H r (CH 3 NH, CH 2 NH 2 ),
(4)
(CH 3 NH, CH 2 NH 2 ) ⫹ H r CH 3 NH 2 .
(5)
and
Methylamine (CH3NH2) is the final product (as CH3OH is the final product of CO hydrogenation), but the intermediates are also of interest, particularly the two radicals produced in reaction (4). Sumathi & Nguyen (1998) performed a more thorough study of the united surface of reaction (2) than what was done here, but without considering solvation. Their barriers are somewhat lower because they used larger basis sets. The present work appears to be the first computational study of CH2NH hydrogenation. Figure 2 shows the reaction surfaces and QCISD(T)/aug-ccpVDZ energetics for both outcomes of reactions (2) and (4). In the gas phase, the barrier for hydrogenating HCN to form H2CN is about 4 kcal mol⫺1 less than the one to form HCNH. With IPCM solvation included, the H2CN formation is favored even more. The H2CN pathway is also decidedly more exothermic. While its 7.3 kcal mol⫺1 barrier is imposing at 10–20 K with
Fig. 2.—Schematic potential energy surfaces for various hydrogenation reactions indicating stable species and transition states (TSs). Computed QCISD(T)/aug-cc-pVDZ energy differences in units of kcal mol⫺1 are shown, referenced to reactant complexes. The upper surfaces are the gas phase, while the lower surfaces include an IPCM correction for the influence of ice. (a) H ⫹ HCN r H2CN; (b) H ⫹ HCN r HCNH; (c) H ⫹ CH2NH r CH3NH; and (d) H ⫹ CH2NH r CH2NH2.
only thermal energy available, it would present much less of an impediment for hot H atoms produced by photolysis. The barriers for adding H to CH2NH to form either CH3NH or CH2NH2 radicals are comparable. Both are predicted to be enhanced slightly in water ice. The respective IPCM barrier heights for forming CH3NH and CH2NH2 are predicted to be 4.6 and 6.1 kcal mol⫺1. Again, these are low enough that hydrogenation by hot H atoms is expected to be efficient. While CH3NH has a slightly lower barrier to formation, it is not a direct precursor to amino acids. Radical recombination would lead to secondary amine species. However, since CH2NH2 is more stable than CH3NH, its abundance can be enhanced by the exothermic process CH 3 NH ⫹ H r CH 2 NH 2 ⫹ H.
(6)
5. FORMATION OF GLYCINE, ALANINE, AND SERINE
The previous sections described pathways to the radicals COOH and CH2NH2. A recombination of these yields glycine directly. However, the formation of alanine and serine in the experimental irradiation of ices can be accounted for with one additional step using constituents expected to be present. When CH2NH2 and COOH combine, glycine is formed with enough
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internal energy to fragment. At the IPCM-PMP2/6-31⫹G** level, two fragmentation pathways are exothermic: ejection of NH2 to leave CH2COOH and ejection of one of the two H atoms on the central carbon atom, forming the NH2CHCOOH radical. The second process is about twice as exothermic as the first one. If NH2CHCOOH combines with CH3 or CH2OH—which are both possible products of CH3OH photolysis—alanine and serine are formed directly. While the NH2⫹CH2COOH channel is less favorable energetically, it implies that acetic acid may also be generated in UV-irradiated ices, a further prediction of the present work. A number of other species are also likely to be present if these pathways are as favorable as the calculations indicate. For example, COOH could join with CH3 to form acetic acid directly, while CH2NH2 could recombine with H or CH3 to form methylamine or ethylamine. Thus, a quantum chemical assessment of pathways is able to offer additional predictions that may be tested against existing data or new experiments. Several other predictions can be made. Bernstein et al. (2002) included NH3 in their ices, but the present calculations suggest that it is not essential to forming glycine or other amino acids. Irradiation of H2O ice doped with CH3OH and HCN should be sufficient. Likewise, CH3OH could be replaced with CO. Also, if either CH3OH or HCN were labeled with appropriate isotopes, the final disposition of the labeled atoms should be consistent with the predictions described above: the C in the carboxylic acid group in glycine should originate from CH3OH, while the other C and the N should originate from HCN. Finally, it should be emphasized that the recombination events described above require temperatures warm enough to
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allow diffusion: ∼100 K and above. If temperatures remain very cold, radicals will be limited to recombining with H or near neighbors, with the latter comparably rare. Amino acids are thus most likely to be present in interstellar ices that have experienced thermal shocks or in comets that have passed through warmer regions of the solar system. Note added in manuscript.—Mun˜oz Caro et al. (2002) identified 16 amino acids produced by UV irradiating a 2 : 1 : 1 : 1 : 1 mixture of H2O, CH3OH, NH3, CO, and CO2 at 12 K. This is a much higher fraction of N- and C-bearing species than studied by Bernstein et al. (2002). Space does not permit a detailed analysis, but the NH2 attack of bonds (such as CO carbonyl bonds) can lead to amino acid precursors in the absence of HCN. However, these pathways rely on having comparably large concentrations of NH3 and C-bearing species and are not expected to be very important in dilute ices such as Bernstein et al. studied. The support of the NASA Exobiology Program for this research through NASA/Ames Research Center grant NAG 21396 is gratefully acknowledged. The author thanks E. L. O. Bakes (NASA/Ames) for helpful conversations and M. P. Bernstein (NASA/Ames) for both helpful conversations and prepublication access to results from his amino acid experiments. Finally, the author thanks A. Derecskei (Millennium Chemicals) and K. A. Peterson (Pacific Northwest National Laboratory) for their willingness to listen to and evaluate ideas that were developed for this work.
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