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Journal of Creation 36(3):67–73, December 2022

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The origin of L-amino acid enantiomeric excess: part 1—by preferential photo-destruction using circularly polarized light?

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A hypothetical astronomical source of UV right-circularly polarized light (r-CPL) is the favoured evolutionary theory for the origin of homochiral amino acids. Astronomers have not been able to find polarized UV light anywhere in the relevant region of space, however. Furthermore, the c. 1% enantiomeric excess (ee) claimed for some amino acids based on optimized synchrotron experiments is misleading. 99.99% to 99.999% photo-destruction of racemic D- and L-amino acid (AA) mixtures would be necessary, using specific wavelengths (λ) to capitalize on minute differences in D and L absorption. Realistically, however, a minute excess of the surviving enantiomer would have been sometimes positive and other times negative over a range of absorbing λ, tending to cancel out any preference. Realistically, the hypothetical destructive r-CPL absorption would have occurred over a wide range of wavelengths, producing far lower than 1% eeL even after >99.99% photo-destruction. Upon arriving in trace concentrations on a putative still hot, post–Late Heavy Bombardment lifeless Earth, the <<1% eeL extra-terrestrial AAs would have been further diluted and over time increased the quantity of perfectly racemic AAs.

Homochiral biomolecules such as DNA, RNA and proteins are indispensable for cells. Various naturalist hypotheses have been proposed for the origin of homochirality.1

One often encounters claims in the origin-of-life literature that significant L-enantiomeric excess can be produced naturally in proteinaceous amino acids (AAs) by circularly polarized light (CPL) in the UV wavelength range. In 2020 Glavin et al. claimed in a review on chiral asymmetry:2

“An avenue that has shown great promise is the preferential synthesis or destruction of a single enantiomer by exposure to ultraviolet circularly polarized light (UV-CPL).3”

The often-repeated claims rarely, if ever, provide any details to help determine whether there is any merit to this view, so we will examine and evaluate the key foundational experiments here.

Mechanisms to obtain enantiomeric excess

Buchardt described three different photochemical mechanisms for inducing an enantiomeric excess: asymmetric destruction, partial photoresolution, and asymmetric synthesis.4 In each case the yield depends on the anisotropy factor g, defined below.4 We will focus on asymmetric destruction in this paper. It is the favoured explanation for most pro-evolutionists. In the next parts to this series, we will discuss suggestions on how CPL could photochemically lead to slight enantiomeric enhancements of AAs.5,6

Bonner and colleagues were the first to report preferential destruction of an AA enantiomer using CPL.7 Leucine (Leu) was selected for study since its D- and L-enantiomers have among the greatest difference in absorptivity (ε) with respect to CPL of all proteinogenic AAs.8 Preliminary experiments were conducted to find the best wavelength, λ, which would be both strongly absorbed by leucine and maximize the difference in selectivity of absorbance with respect to CPL handedness:

Δε = εl-CPL – εr-CPL,   (1)

where l-CPL refers to left-handed circularly polarized light, and r-CPL to right-handed circularly polarized light. It was found that racemic leucine samples irradiated using a laser tuned to λ = 212.8 nm produced the largest enantiomeric excesses (ee’s).7

At this wavelength, r-CPL was found to preferentially decompose the D-leucine enantiomer, and l-CPL the L-leucine counterpart. The eeD produced was 1.98% after 59% sample photo-destruction, and eeL was 2.50% after 75% photo-destruction.7

The favoured hypothesis for enantiomeric excess assumes chiral photons, found in interstellar and circumstellar UV CPL, selectively destroyed the enantiomer having the larger absorption coefficient, ε, photolytically.3,9-16 The asymmetric photoreactions are proposed to have occurred in interstellar space in ices17,18 and precursors of carbonaceous meteorites before arriving on the early earth.19-21

One astronomical model assumes that neutron stars behave as synchrotrons, emitting CPL of opposite chirality above and below the circulation plane of the charge that circles its core.22 (Man-made synchrotrons are experimental devices which emit CPL above and below the circulation plane having opposite chirality.) This led to the notion that a neutron star may have aimed its right-circularly polarized beam toward Earth, and its left-circularly polarized beam 180° away into space.22 Of course, there could have been other neutron stars with different orientations, so this theoretical effect would partially or fully cancel.23

Key experiments producing enantiomeric excess using UV circularly polarized light

UV electromagnetic radiation ranges from a wavelength of 10 to 400 nm. Carefully planned laboratory experiments have produced small ee’s using ultraviolet circularly polarized light (UV CPL).24 The excesses inevitably require photo-destruction of >>99% of the starting materials, thus demanding impossibly high initial concentrations at some location before delivery to Earth.24

Some amino acid enantiomers absorb left- or right-CPL with slightly different strength (anisotropy) at various wavelengths. This difference is called the differential absorptivity, Δε, and the relative difference is called the anisotropy factor, g:9

g = Δε/ε.    (2)

The enantiomeric excess resulting from preferential photolysis using CPL is believed to depend only on the anisotropy g and extent of reaction, ξ.11 The exact relationship is shown in Eqn (3), from which the values in table 1 were calculated:9

equation3

In a key study in 2012, Prof. Meierhenrich and colleagues published anisotropy spectra and gL values for a wavelength range between 130 and 350 nm (see figure 1 and table 1).

Anisotropy UV spectra
Figure 1. Anisotropy UV spectra (thick lines) and eeL inducible by either left or right CPL at ξ = 0.9999 (thin lines). A: valine, B: alanine.9 After figure in ref. (9).
Table 1. L-Enantiomeric excess (%) predicted for some α-amino acids as a function of the irradiation wavelength, λ (nm), using 100% right-circularly polarized light. Extent of photolysis reaction ξ = 0.9999. Based on a table found in ref. 9.
table1

The amino acids studied all have one UV-light-absorbing chromophore, the carboxylate anion. The spectra were measured using films of amorphous amino acids sublimated under controlled conditions with CPL provided by a synchrotron radiation facility at Aarhus University in Denmark.9 The intention was to model interstellar conditions, where organic molecules are believed to sublimate and condense repetitively. 9 Figure 1 also shows the corresponding predicted eeL values (thin lines, right axis, with values in %).

Until these experiments, only single-wavelength anisotropy values had been reported for AAs in aqueous solution.25 It is important to recognize that the sign and magnitude of g for most proteinogenic AAs differ according to the wavelength (λ) of the CPL.9

The eeL values in figure 1 and table 1 confirm that relative absorbances of CPL by D- and L-AAs have a mirror image relationship. The relative response is not identical, however, and depends on the wavelength.

As part of this study, the amino acid film on the MgF2 support was turned around the axis of the electromagnetic synchrotron radiation and the spectra were measured at 0°, 90°, 180° and 270°. The results at these four positions were nearly identical and anisotropy spectra of separately prepared samples reported to be reproducible. 9

The reason that leucine is used to claim that enantiomeric excesses could arise naturally is easy to understand. Meinert et al. point out that

“The highest value of g measured for these selected amino acids is 0.024 for L-leucine, which is almost one order of magnitude greater than for all other amino acids investigated in this study.”9

If the goal of such studies had been to discredit the relevance of CPL for origin of life purposes the researchers would have picked almost any of the other biologically relevant AAs.

Asymmetric photolytic destruction

The outcome of enantioselective photolysis of a racemic mixture by CPL is thought to depend on two competitive reactions with different rate constants, kR and kS, for the R and S enantiomer, respectively.11 The rate constants are proportional to the absorptivity (εR and εS), and the selectivity of the enantioselective photolysis depends on the difference between kR and kS, which can be expressed as g, as shown in equation (4): 9,26

equation4

The eeL values reported in figure 1 and table 1 were calculated using the measured anisotropy g and extent of reaction ξ = 0.9999, using right-handed CPL. Since equation (4) shows that g is proportional to εR – εS, increasing g implies greater εR > εS, which results in more destructive photolysis of (R)-AA. Proportionally more surviving (S)-AA is, by definition, an increase in eeL. (All proteinogenic (S)-AAs are L-AAs except for L-cysteine, which is classified as (R)-cysteine). Conversely, negative values of g lead to lower eeL. Using left-handed CPL would reverse all these effects.

Infrared circularly polarized light

Absorption spectra of solid films
Figure 2. Absorption spectra of solid films of α-L-amino acids in their zwitterionic state: a) alanine, b) serine, c) valine, d) leucine, e) isovaline, f) proline. The light-absorbing chromophore is the carboxyl group. Based on a figure from the Supplementary Materials for ref. (9).

Meierhenrich et al. mention in their seminal paper that the Orion molecular cloud interstellar radiation is partly circularly polarized in the infrared wavelength region.12 (The infrared spectral region lies in the λ = 780 to 1 mm range). For purposes of generating eeL IR light can’t excite amino acid electrons to the high energy states needed for photolysis. The Supporting Information section confirms that for the proteinaceous AAs examined, for λ > 220 nm, ε drops rapidly to zero (see figure 2). 9

Astronomers have reported, however, that no polarized UV light could be found anywhere in the relevant region of space.23 One group reported in 2005 that they seem to have observed circularly polarized light originating from the region of Orion but only in (irrelevant) infrared frequencies. Infrared light is of too low energy to cause deracemization of the racemic amino acids.23

Discussion

Glavin et al. noted that the degree of circular polarization (CP) is negligible for stars of mass such as our Sun, but mentioned that some infrared light from the Orion star formation region seems to possess up to 17% polarization for higher mass stars.2 Therefore, they speculate that CP light having a single sign may have been delivered from a neighbouring massive star, which preferentially destroyed and/or synthesized enantioenriched amino acids. The correct enantiomer would then be incorporated inside comets and asteroids and delivered elsewhere.2

“After calculating the specific g values at varying wavelengths, amino acids such as alanine, leucine, and isovaline were predicted to yield enantiomeric excesses ranging from 2 to 5% in quantitative asymmetric photolytic reactions (99.999% extent of reaction, ξ); 9,27 those values are in agreement with the L-enantiomeric excesses found for amino acids present in interstellar ice analogs5,6,14,28 and those found in carbonaceous chondrites.”29-31

Claims of agreement between predicted and measured effects from the distant past in the pro-evolution literature always merit careful examination. Alleged small eeL for alanine and leucine from unambiguously uncontaminated extra-terrestrial sources are controversial at best. (Isovaline is not found in proteins.) Early studies on the Murchison meteorite shortly after it landed in Australia in 1969 and before opportunities for contamination arose reported the absence of an eeL and used this as evidence that the AAs found were indigenous to the meteorite. I have noticed a correlation between increasing eeL with time since the meteorite landed. This could be caused by two factors: increasing opportunities for terrestrial contamination and the fact that the most pristine samples, i.e. least likely to have experienced contamination shortly after crashing, were selected first for analysis.

To illustrate the laboratory challenges in determining D/L proportions, in one report Modica et al. irradiated a mixture of H2O, 13CH3OH and NH3 to show how AAs could form in outer space.6 The only source of carbon was from methanol which contained only 13C. After some laboratory processing, their mass spectra revealed the presence of 12C fragments, which they attributed primarily to contamination from biological amino acid, despite their careful efforts to avoid this. AAs extracted from meteorites lie in the ppm to ppb concentration range, and biological contamination can only introduce L-AAs, so unfortunately only an infinitesimally small amount would produce incorrect measured eeL values.

Furthermore, the authors of the original work never predicted AAs eeL values of 2–5%.9,27 In fact, Meierhenrich did state: 9

“We note that the positions of the extrema as well as their intensity in the newly reported anisotropy spectra differ from those in previously described circular dichroism spectra.”

In other words, depending on laboratory details, different g values can be obtained. These are technically difficult experiments to perform. In their Supporting Information the authors claim that “Anisotropy spectra of separately prepared samples are reproducible” but no details were provided.9 Inevitably in these kinds of experiments the estimated errors claimed are based only on repeated gas chromatography (GC) measurements and not repetition of the full experiments. How significant should doubts be about the size of the estimated g values? When Modica et al. applied unpolarized light (UPL) at 122 nm to racemic mixtures, they unexpectedly found enantiomeric excesses for the proteinogenic AAs examined: for alanine eeL = 0.46 ± 0.36, and valine eeL = –0.30 ± 0.44.6 This makes no sense physically and indicates non-neglectable experimental errors are present.

Even small error in g values at various wavelengths can have a considerable effect on the estimated ee values which result after photo-destroying 99.99% to 99.999% of the initial sample.

We see that there is considerable doubt about both the levels of AA eeL from extra-terrestrial sources and producible from photo-destruction. On the same page as the quote above, Glavin et al. admit:2

“However, UV-CPL-induced enantiomeric excesses observed thus far tend to be significantly smaller than the observed enantiomeric excesses in meteorites.”

But this confuses more than enlightens. Enantiomeric excesses of what chemicals in which meteorites? Significantly, unlike α-hydrogen amino acids, the direction of ees produced by isovaline was not affected by irradiation wavelength, but only the light polarization.2 In other words, eeL would always be positive or negative for CPL of a given handedness, which is consistent with L-isovaline excess often being reported in meteorites.

The studies on producing an ee using CPL provide interesting examples on how evolutionists design experiments which maximize the chances of obtaining the results they hope to find. The approach is not “Let’s set up experiments which reflect plausible starting conditions and see what results.” We will critically examine the two reasons anything worth publishing was obtained at all:

  1. The AAs with the highest g factors were selected for study, from which the single best but different wavelengths were used for each AA to predict the highest eeL theoretically possible.
  2. Absurdly high levels of photo-destruction were assumed.

Hypothetical enantiomeric excess based on single optimal wavelength

The CPL generated from experimental equipment is spectrally broadband, and this would be the case also for putative CPL produced extra-terrestrially in the UV range.6 The absorbances of r-CPL by the five proteinogenic L-AAs reported in table 1 (entries 1–5) were sometimes stronger, other times weaker than their D-AA counterparts at different wavelengths. Therefore, even if no annulling l-CPL were also present, exposure of a racemic mixture of D and L of the same AA to 100% r-CPL would result in a cancelling out effect over the relevant UV wavelength spectrum. The true outcome would be very low overall average eeL for each AA, see table 1. In fact, the average eeL’s predicted turn out to be negative using the data points shown for all the AAs except leucine!

Proteins require almost pure L-enantiomers for all the AA residues and 5%–10% randomly distributed D-AAs would destroy most proteins.32 But note that, for the examples in table 1, only at one wavelength (λ ≈ 185 nm) would a positive eeL value be induced in all the five tested proteinaceous amino acids. 9 The average eeL at λ ≈ 185 nm was only 1.5%, once again due primarily to only the atypical leucine, see table 1. Incidentally, in a review of AAs found in the extensively studied Murchison meteorite, Koga and Naraoka reported that only trace amounts of Leu had been reported (D-Leu: 80 ppb; L-Leu: 1058 ppb).33 Large L/D proportions like this are certain to reflect terrestrial contamination, especially taking into account that Leu is the most common AA found in human proteins.34-36 Our review of the literature showed Leu is not claimed to have been found in, for example, three key Antarctic CR chondrite meteorites (EET 92042, GRA 95229, and GRO 95577);37 Tagish Lake meteorite samples;38 Sutter’s Mill Carbonaceous Chondrite;39,40 nor in Aguas Zarcas.41

According to table 1, the overall eeL average is –0.18%, taking all five proteinaceous and two non-proteinaceous AAs into account, and only if 99.99% of the initial AAs were to have photolyzed. Excluding the non-proteinaceous AA data leads to an overall eeL average of only +0.35% but this includes an abnormally high contribution of 11.31% for L-leucine at λ = 220 nm.9 Since there could be multiple sources of polarized UV light and a wide range of absorbing wavelengths, a reasonable prediction would be that no or an insignificant net eeL would be produced on average naturally via photo-destruction.

Hypothetical enantiomeric excess requires high photo-destruction level

Enantiomeric excess of leucine decreases
Figure 3. Enantiomeric excess of leucine decreases rapidly with amount of photo-destruction, ξ using UV circularly polarized light. L-Leucine shown in blue. After figure in ref. (9).

There is no justification for why racemized AAs would have been exposed to only, or almost only, light of the ‘correct’ wavelength, nor why 99.99% to 99.999% photolysis would have occurred before arriving on Earth. What prevented 99.999% photolysis from becoming 100% amino acid destruction? The evolutionist is faced with a dilemma. To obtain a measurable level of eeL such as 1%, the total amount of AA surviving photo-destruction and arriving on Earth would be insignificant << (100% – 99.999%). But to deliver more AA, the eeL would be even closer to perfectly racemic. Figure 3 uses Leu, which displays the greatest g known for AAs, to illustrate how decreasing the amount of photo-destruction c. 1%, from ξ = 99.99% to 99% about halves the eeL.9

Has a possible solution for enantiomeric excess been found?

The short wavelengths needed for absorption by ordinary amino acids could not have penetrated the putative prebiotic terrestrial atmosphere (having high carbon dioxide content).42 When might AAs having enantiomeric excess have been produced? Evolutionists believe life could not have survived an alleged Late Heavy Bombardment between 4.0 and 3.8 billion years ago. Any AAs surviving this period being added or produced as the earth cooled over the next millions of years would have been thoroughly racemized.43 Any slow influx of very dilute extra-terrestrial AAs having an insignificant eeL would have been immediately diluted by the already racemic AAs and would themselves have also racemized over time.

Assuming millions of years merely increases the amount of potentially contaminating racemized AAs, making it ever more difficult for large homochiral peptides to form. Homochiral peptides in water racemize faster than they can elongate for thermodynamic and kinetic reasons, and ever greater proportions of environmental racemic AAs would only have made matters worse over time.44

In conclusion, no astronomical source of r-CPL has been found despite much effort, and the theoretical speculations do not provide a plausible natural solution to the origin of an eeL for amino acids.

Posted on homepage: 22 December 2023

References and notes

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