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Abstract

The enantiomeric excess (ee) of l-form amino acids found in the Murchison meteorite poses some issues about the cosmic origin of their chirality. Circular dichroism (CD) spectra of amino acids in the far-ultraviolet (FUV) at around 6.8 eV (182 nm) indicate that the circularly polarized light can induce ee through photochemical reactions. Here, we resort to ab initio calculations to extract the CD spectra up to the vacuum-ultraviolet (VUV) region (∼11 eV), and we propose a novel equation to compute the ee applicable to a wider range of light frequency than what is available to date. This allows us to show that the strength of the induced ee (|ee|) in the 10 eV VUV region is comparable to the one in the 6.8 eV FUV region. This feature is common for some key amino acids (alanine, 2-aminobutyric acid, and valine). In space, intense Lyman-α (Lyα) light of 10.2 eV is emitted from star forming regions. This study provides a theoretical basis that Lyα emitter from an early starburst in the Milky Way plays a crucial role in initiating the ee of amino acids.

1. Introduction

Amino acids are the basic building blocks of proteins, featuring two optical isomers, l- and d-forms, termed enantiomers. Most of the life-forms on Earth make use of the l-forms, whereas the d-forms are occasionally harmful, even though l- and d-amino acids are produced in equal amounts as racemic state by chemical syntheses in vitro. From this scenario, it can be inferred that the dominance of l-amino acids (hereafter referred to as “homochirality”) must date back to the early stage of chemical evolution. The enantiomeric excess (ee) ratio is given by %ee = (C _L − C _D)/(C _L + C _D) × 100, where C _L and C _D are the amounts of each enantiomer. %ee is zero in a racemic mixture, and %ee becomes equal to 100 in homochiral all-l world. It has been shown that even if %ee is initially as low as a few percent, it can be amplified by the separation of l- and d-amino acids through polymerizations and recrystallizations. Even so, the astrophysical and molecular mechanisms responsible for the breaking of the chiral symmetry of amino acids remain an open issue (Bonner, 1991; Meierhenrich, 2008).

The detection of amino acids in the Murchison meteorite that has fallen in Australia in 1969 and the discovery of an amount of ee of l-amino acids above 10% provided a basis for speculating on a cosmic origin of the homochirality (Cronin, 1997; Engel and Macko, 1997). Since the isotopic ratios of the meteoritic amino acids differ from the terrestrial ones and the abiotic species in this meteorite display positive ees, these are believed to be of extraterrestrial origin. More recently, available data of extraterrestrial materials and asteroids have been enriched by direct sample return to Earth by space missions such as the Hayabusa2. In Ryugu particles, at least 23 amino acids including complex α- and β-amino acids were detected, providing further support to the abundancy of organic matter formed on asteroids (Nakamura et al., 2022). Enrichment of l-amino acids in Ryugu particles has not been detected; however, irradiation of circularly polarized light (CPL) is the factor that is supposed to be at the origin of the amino acid asymmetry (Rubenstein et al., 1983).

Indeed, in laboratory experiments, the CPL irradiation induces the asymmetric photosynthesis and decomposition of amino acids leading to nonzero ee (Bailey, 2000; Munoz Caro et al., 2002; Nuevo et al., 2007). From photolysis experiments and circular dichroism (CD) measurement of l-amino acids in the solid state, the far-ultraviolet (FUV) region between 170 and 200 nm (7.3–6.2 eV) has been proposed to be the most effective to induce the homochirality in amino acids because of analogously large and positive magnitude of their CD spectra (Norden, 1977; Meierhenrich et al., 2010; Meinert et al., 2012). In the gas phase, the CD active region is expected to be in a wider region of FUV ranging from 130 to 280 nm (9.5–4.4 eV) (Meinert et al., 2022). On the other hand, the CD spectrum at higher energy seems to be more limited only for l-Ala (Tanaka et al., 2010a). In fact, the CD spectra of other amino acids, including 2-aminobutyric acid (Aba) and valine (Val), were observed in a photon energy range lower than 10 eV (Kaneko et al., 2009; Tanaka et al., 2010b). For energies above 10 eV, the CD features of amino acids have not been characterized experimentally to date.

In space, various light sources exist and are known to be responsible for different types of radiation field; however, the Kuhn–Condon zero-sum rule entails a cancelation of the CD contributions by summing over the entire wavelength range (Kuhn, 1930; Condon, 1937). Therefore, a nonzero ee cannot be originated by the irradiation of polychromatic CPL. This implies that a specific wavelength of CPL that gives the same sign of CD is required to promote the arising of ee of amino acids. In an early phase of the galactic evolution, the strongest emission in the pan-galactic light is Lyman-α (Lyα) line, an emission peaked at 10.2 eV due to a relaxation from the first excited state to the ground state of a hydrogen atom. This early phase is categorized as Lyα emitters (LAEs) (Shibuya et al., 2014).

In this study, we inspect the arising of amino acid ee up to the 11 eV light region covering the LAE spectra by resorting to a high accurate theoretical methodology. In particular, we focus on optical properties and photolysis-induced ee by the circularly polarized Lyα (CP-Lyα) for three key amino acids; alanine (Ala), Aba and Val. All these amino acids were found in the Murchison meteorite, and their %ee were reported to be 1.2, 0.4, and 2.2, respectively (Cronin and Pizzarello, 1999).

2. Methods

In our computational approach, we first searched for the most stable conformations of the three targeted amino acids (l-Ala, l-Aba, and l-Val). To this aim, we make use of the random connection method (Shoji et al., 2022b) to evaluate all the possible conformations under fixed bond connections in their zwitterionic forms. The geometry optimization and vibrational frequency calculations were done via quantum chemical methods within a B3LYP//6–311++G** level with the COSMO solvation procedure. Our former calculations have already shown that the B3LYP hybrid functional is able to reproduce total energies with an accuracy comparable to the CCSD(T)/cc-pVTZ level and is applicable to optimizations, a large number of samplings, and larger molecules (Shoji et al., 2022a, 2022b; Hashimoto et al., 2023; Mishima et al., 2023). CD spectra were calculated at the symmetry-adapted cluster-configuration interaction (SAC-CI)/cc-pVTZ theoretical level for the l-amino acids. A total of 150 excited states were included to evaluate the spectra up to 12 eV. The excited states of amino acids result as a combination of electronic and vibrational excitations. Because of this combination, the excitation spectrum of an amino acid features a certain bandwidth rather than separate discrete peaks at ultra-low temperature in space. We assumed a Gaussian distribution with a 0.30 eV bandwidth to compare with experimental absorption and CD spectra (Yabana and Bertsch, 1999; Seino et al., 2006). To compare with the experimental spectra of amino acids (Kaneko et al., 2009; Tanaka et al., 2010b), absorption and CD spectra are calibrated by assuming as references the values ɛ ₀(6.6 eV) = 1.0 and Δɛ ₀(6.6 eV) = 1.0 in L-Ala. Relative values with respect to these references are used for a quantitative discussion. Further theoretical details are provided in the Supplementary Data.

All the calculations were performed using the Gaussian16 program package (Frisch et al., 2019). Molecular structures were illustrated by using the visual molecular dynamics program (Humphrey et al., 1996).

3. Results

The most stable conformations of the three _L-amino acids considered are shown in Fig. 1A. In the zwitterionic form, amino acids take only one conformation, and their conformational flexibility is due to side chains. Therefore, the conformations of their most stable state are structurally very similar. Specifically, Ala has one conformer, while both Aba and Val have three different conformers. The relative energies of the Aba conformers are 0.436 and 1.182 kcal/mol, while those of Val are 1.185 and 1.789 kcal/mol as obtained at the B3LYP//6–311++G** theoretical level with zero-point energy correction included. The relative Aba energies of 0.436 and 1.182 kcal/mol contribute to the stability of the lowest energy conformer by 6.4% and 0.06% at the upper limit temperature of molecular clouds, 80 K. This suggests that the conformational contributions of these amino acids are minimal in molecular clouds. However, by comparing with the experimental CD spectra, observed at T = 298 K, their contribution ratios increase to 47.9% and 13.6%, respectively. Hence, we computed the CD spectra by including all the conformers. In the ongoing discussion, we first present the results obtained for the photoabsorption and CD features at 0 K, since this is a crucial reference temperature in molecular cloud conditions, and then we focus on these same spectra at higher temperatures up to 298 K providing an insightful comparison with experimental CD spectra.

FIG. 1.

(A) Three l-amino acids in their most stable conformation, (B) their relative absorption and relative CD spectra at 0 K as obtained from the SAC-CI/cc-pvTZ approach. The intensities of the red (CD) and the blue curves (absorption) are reported on the left and right vertical axes as indicated by the corresponding arrows. CD, circular dichroism; SAC-CI, symmetry-adapted cluster–configuration interaction.

In the most stable conformers of Fig. 1A, the photoabsorption (ɛ) spectra and CD difference of molar absorptivity between left- and right-CPL; Δɛ = ɛ(L) − ɛ(R), were calculated at the SAC-CI/cc-pVTZ theoretical level. For l-Ala, the calculated CD spectrum is peaked at 6.7 and 9.8 eV, which correspond to the experimental CD peaks at 6.8 and 10.2 eV (Tanaka et al., 2010a). To compensate for the small photoenergy differences, we introduced the following calibration equation (Eq. 1), $y = 1.0978 x - 0.5171$ (1)

where x is the photon energy in eV unit before calibration, and y is the resulting scaled value. By applying Eq. 1, the peak positions of the calculated CD spectrum shift to 6.8 and 10.2 eV, thus becoming closer to the experimental values. In the following discussions, the Eq. 1 is applied to all calculations. The refined photoabsorption and CD spectra for l-Ala, l-Aba, and l-Val are shown in Fig. 1B.

For a better comparison with the experimental data, the computed absorption and CD spectra at higher temperatures are reported in Fig. 2. For l-Ala, calculated CD and absorption spectra are in good agreement with the experimental data (black dotted curve in Fig. 2). Analogously, the experimental characteristic CD peaks at 6.8 eV (182 nm), 8.0 eV (155 nm), and 10 eV (122 nm) are well reproduced by our calculations and located at 6.8 eV (181 nm), 8.4 eV (148 nm), and 10.2 eV (121 nm), respectively. The two CD absorption peaks at 6.8 and 10.2 eV are both very pronounced and characterized by a positive and negative amplitude, respectively. The CD spectra computed for l-Aba and l-Val have very similar features with a large positive peak at 6.8 eV and a large negative one at 10.2 eV. By applying the proper thermal correction to the CD spectra of l-Aba, we observed a small shift of the 10.1 eV peak toward 10.2 eV, consistent with the experimental CD spectra which is located at a slightly lower value (9.5 eV). The thermal corrections of the absorption and CD spectra resulting from conformations of these amino acids in higher energy states are nearly negligible at temperatures below 298 K. The similarity of the CD spectra of amino acids is because their most stable conformations are common in their zwitterionic form. The CD spectra show that not only the positive CD peak around 6.8 eV region in the FUV region but also the native CD peaks around 10 eV region are present in all these l-amino acids. All the absorption and CD spectra in ground and higher energy conformations are reported in Supplementary Fig. S1 in the Supplementary Data.

FIG. 2.

Relative absorption and CD spectra of the three l-amino acids targeted in this study at different temperatures as obtained by the SAC-CI/cc-pvTZ calculations. Thermal corrected spectra of the absorption and CD by Maxwell–Boltzmann distributions are shown for l-Aba and l-Val. Absorption spectra are shown in blue (T = 0 K) and pink (T = 298 K). CD spectra are shown in red (T = 0 K), orange (T = 80 K) and green (T = 298 K). Experimental CD spectra, observed in the 5–10 eV region, are shown as dotted black curves. Aba, 2-aminobutyric acid; Val, valine.

Under the enantioselective photolysis, %ee has been estimated from the molecular absorption coefficients as follows: $|% e e_{g}| \geq (1 - {(1 - ξ)}^{\frac{|g|}{2}}) \times 100$ (2a) $g = \frac{Δ δ}{δ} = 2 \frac{δ_{L} - δ_{R}}{δ_{L} + δ_{R}}$ (2b)

where ξ is the extent of reaction and g the anisotropy factor (Balavoine et al., 1974). Equation 2a indicates that %ee increases as the photolysis proceeds (ξ approaching the value of 1.0). By ξ = 0.9999, the computed %ee values (%ee_g ) are plotted in Fig. 3 (green curves), and the g value is the only variable determining %ee. The g value is obtained by dividing Δɛ by ɛ as expressed by Eq. 2b. Therefore, photons characterized by small ɛ values give large %ee values. Given this situation, values of %ee of about 6.8 eV become dominant compared to other photon energies. Nonetheless, it seems that this argument does not hold for the wide photon range considered in this study. Indeed, in our case, ξ is expected to depend on ɛ(ν). By making use of the original derivation of ξ (Balavoine et al., 1974), we obtain

FIG. 3.

Enantiomeric excesses at 0 K (%ee_g ) estimated by using Eq. 2a is shown in green and the analogous quantity %ee _CD estimated by using Eq. 4 is in red color; LAE spectra and their convolution (LAE × %ee _CD) are indicated in each panel. We recall that |%ee _CD| predicts higher ee than |%ee_g | in the Lyα region (10.2 eV) compared with the 6.6 eV region. Regions sharing positive and negative %ee values are colored in pink and cyan, respectively. The positive %ee region, in all the cases analyzed here, corresponds to the range 6.5–7.3 eV, whereas the negative %ee regions are located at 5.6–6.2, 7.4–8.3, and 9.5–11 eV. ee, enantiomeric excess; LAE, Lyman-α emitter; Lyα, Lyman-α.

ξ = 1 - \frac{1}{2} (e^{- k_{L} t} + e^{- k_{R} t})

(3a)

= 1 - \frac{1}{2} (e^{- δ (1 + \frac{g}{2}) t^{,}} + e^{- δ (1 - \frac{g}{2}) t^{,}})

(3b)

\approx 1 - e^{- δ t^{,}} (1 + \frac{1}{2} {(δ t^{,} \frac{g}{2})}^{2})

(3c)

\approx 1 - e^{- δ t^{,}}

(3d)

where k is the reaction rate constant, proportional to the molar absorptivity (ɛ), and t′ is the scaled time. Substituting Eq. 3d into Eq. 2a, we obtain the following formula: $|% e e_{C D}| \geq (1 - e^{- \frac{|Δ δ|}{2} t^{,}}) \times 100$ (4)

In Eq. 4, %ee depends on the time (t′) and Δɛ, but not on ɛ. Equation 4 indicates that for small values of ɛ, the photolysis proceeds slowly but large value of g determines the %ee progress, whereas for large values of ɛ, the photolysis proceeds faster, while smaller g values decrease substantially the %ee contribution. Consequently, ɛ does not influence directly %ee. Details of the simulation performed are provided in Supplementary Fig. S2 in the Supplementary Data. By making use of Eq. 4, the %ee values (%ee _CD) have been calculated and graphically shown in Fig. 3 (red curves).

We made use of the newly derived ee formulation given by Eq. 4 (%ee _CD) by setting an appropriate t′ to generate a %ee value similar to the one given by the original formulation of Eq. 2a (%ee _g) in l-Ala at 6.6 eV. Photon energies with an identical %ee sign are colored in pink and cyan for positive and negative regions, respectively, in Fig. 3. A negative %ee sign is commonly obtained for three regions: 5.6–6.2 eV (220–201 nm), 7.4–8.3 eV (168–150 nm), and 9.5–11 eV (131–112 nm). Positive %ee values have been found only in the 6.5–7.3 eV (190–169 nm) range. The absolute value |%ee _CD| in the 10.2 eV turns out to be substantially enhanced and comparable to the one of 6.8 eV region. The %ee _CD trends shown in our plots provide a new basis to rationalize the ee induction upon photolysis and corroborate the importance of the CD spectra in the 10 eV region in amino acids.

To complete our analysis, we calculated the convolutions of the derived %ee _CD spectra with an LAE spectrum to verify the generation of %ee by CP-Lyα (Fig. 3). To this aim, ee employed a spectrum composed of eight LAEs at a redshift near the value z = 2.2 of the luminosity z, as a typical average spectrum of LAEs (Shibuya et al., 2014). The background continuum component (hν < 10.17 eV and hν > 10.23 eV) far from the intense LAE peak at 10.19 eV (121.66 nm) were not integrated since the diffuse background continuum is expected to be nonpolarized. The computed %ee values, with the LAE spectrum integrated within the ∼0.0016 eV interval, are summarized in Table 1.

Table 1.

Calculated Enantiomeric Excesses of the Amino Acids (%ee _g/%ee _CD) Generated by Asymmetric Photo-Dissociation Reactions at the Photon Energies of 6.6 and 10.2 eV and Lyman-α Emitter Spectra

	6.6 eV	10.2 eV	LAE
Ala	0.76/0.79	−0.13/−0.74	−0.13/−0.74
Aba	0.21/0.23	−0.14/−1.14	−0.14/−1.13
Val	0.37/0.42	−0.08/−0.66	−0.08/−0.65

Aba = 2-aminobutyric acid; Ala = alanine; CD = circular dichroism; LAE = Lyman-α emitter; Val = valine.

The %ee values obtained by using the LAE spectrum compare very well with the %ee values estimated by 10.2 eV monochromatic light, as the LAE spectrum is considerably sharp with respect to the CD spectra of amino acids. All the three α-amino acids have in common an enhanced %ee not only at 6.8 eV but also at 10.2 eV region, and the signs of the induced %ee are inverted at 6.8 and 10.2 eV, respectively. By taking into consideration the fact that the CPL irradiation directly induces an amino acid excitation and its photo-dissociation, our results indicate that 10.2 eV left handed CPL (L-CPL) is responsible for the suppression of d-amino acids to a larger extent than l-amino acids. In other words, left-handed CP-Lyα enhances l-form ee (ee_L = ee) through the asymmetric photo-dissociation of amino acids.

4. Discussion

Former studies reported that ee_L is created by the right handed CPL (R-CPL) irradiation in the amino acid photosynthesis experiments (De Marcellus et al., 2011; Modica et al., 2014). Specifically, Modica et al. (2014) inferred that 10.2 eV R-CPL produces 1.04% ee _L of Ala and 6.6 eV L-CPL produced 0.71%ee _L, respectively. Although the signs they obtained for the ee from the photosynthesis experiments are inconsistent with our theoretical results, the spectral peaks of the experimental CD spectra show a high correlation with our theoretical results in the photon energy. We expect that the experimental CPL radiation may induce a compositive photosynthesis which generates not only the amino acids but also its derivative or even other chiral molecules that exist in interstellar environment. Generated chiral molecules may play a key role as the origin of chiral asymmetry of amino acids in the formation history. Amino acid precursors might be a solution to explain the origin of homochirality (Shoji et al., 2023). Hori et al. (2022) reported that propylene oxide, a sole chiral molecule detected in space, has a negative CD absorption in the Lyα region. To answer these questions, further understanding regarding the synthetic mechanisms of biomolecules (i.e., synthetic reactions via radical species [Sato et al., 2018]), molecular mechanisms of the enhancement of ee (Watanabe et al., 2023), and even kinetics of various interstellar molecules on interstellar dust surface are required. Our study proposes the proper direction as a starting point of insight into the origin of homochirality.

On a speculative basis, all galactic bulges including the one in the Milky Way are thought to undergo a process of the LAE phase (Mori and Umemura, 2006). The Lyα emission on the LAE phase is 10⁵–10⁶ times stronger than in an individual star-forming region. Moreover, the bulge on the LAE phase is thought to be composed of substantial dust contents (Hayes et al., 2013). Hence, the scattering and dichroic extinction by dust grains, aligned along a given direction of the interstellar magnetic field, can produce an intense CPL in the Lyα band. In an interstellar molecular cloud, ammonium, aldehyde, and hydrogen cyanide are created, and these react with each other to form biomolecules on icy dust upon ultraviolet (UV) irradiation (Nuevo et al., 2007). Amino acids are synthesized from predecessor molecules through a number of chemical evolutional processes occurring on icy dust surface in interstellar medium (Kayanuma et al., 2017; Sato et al., 2018). Regarding the observation data, CPL has been detected in star-forming regions in the Milky Way (Bailey, 1998; Fukue et al., 2010). In NGC6334 nebula, a strong circular polarization (CP) with a magnitude of 20% was found, and it was ascribed as the cause of dust scattering and dichroic extinction (Kwon et al., 2013). Although this CP is observed in the infrared range, an analogous signal in the UV region is expected. Fukushima et al. (2020) simulated the CP generation and resulted in the dependency of the CP degree on the relative distribution of dust grain in the forward position.

The LAE phase in the Milky Way might correspond to an early starburst phase that occurred several billion years ago in the bulge component (Mori and Umemura, 2006). Our solar system is located at 28,000 light-years away from the galactic center in the disk component. Therefore, the intense Lyα emission was likely to have reached our planet from the solid angle collimated toward the galactic center in the whole sky. Peaky Lyα radiation could have been circularly polarized by scattering and dichroic extinction by dust grains aligned to the interstellar magnetic field in the proto-solar system (Whitney and Wolff, 2002). As the CP-Lyα was sharp enough, it could act as a galactic waveband filter. We suggest that these studies support the idea that the ancient polarized UV may have a crucial role to generate enantiomeric excess on the early biomolecule (represented as amino acids). It may be the trigger to the breaking of racemic state and thus could be the origin of homochirality in the Solar System.

5. Conclusions

Our theoretical and computational approach has been shown to be able to reproduce with appreciable accuracy the experimental CD spectra of three amino acids, l-Ala, l-Aba, and l-Val. Not only the positive CD magnitude around 6.8 eV is correctly obtained but also the negative one around 10 eV is preserved in the temperature range typical of a molecular cloud (0–80 K). Moreover, we propose a new equation (Eq. 4) extending the applicability of the calculation of ee to a wide wavelength range, showing that the ee induction upon photolysis is large in the 10 eV region and comparable to the one in the 6.8 eV region, which was so far considered to be most effective. In the early phase of the galactic evolution, the brightest spectral line of Lyα was emitted, then circular polarization occurs because of dust scattering and dichroic extinction. As the CP-Lyα is sharp enough, it can act as a galactic waveband filter. Hence, our calculations provide clear evidence of the crucial role played by CP-Lyα in the origin of biomolecular homochirality on Earth.

Footnotes

Author Disclosure Statement

No competing financial interests exist for any of the authors.

Funding Information

This research has been supported by the research projects (1) JST, PRESTO grant number JPMJPR19G6, Japan, (2) TIA collaborative research program “Kakehashi” and (3) JSPS KAKENHI grant numbers JP19H00697, JP20H05453, JP20H05088, and JP22H04916. Computational resources were partially supported by the Multidisciplinary Cooperative Research Program in CCS, University of Tsukuba. M.B. thanks the HPC Center at the University of Strasbourg funded by the Equipex Equip@Meso project and the CPER Alsacalcul/Big Data, and the Grand Equipement National de Calcul Intensif (GENCI) under allocation DARI-A0120906092 and A0140906092.

Supplementary Material

Supplementary Data

Associate Editor: Christopher McKay

Abbreviations Used

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Supplementary Material

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Origin of Homochirality in Amino Acids Induced by Lyman-α Irradiation in the Early Stage of the Milky Way

Abstract

1. Introduction

2. Methods

3. Results

5. Conclusions

Footnotes

Author Disclosure Statement

Funding Information

Supplementary Material

Abbreviations Used

References

Supplementary Material