Creationism in Crisis - Another Mystery of Abiogenesis May Have Been Solved

Creationism in Crisis

Another Mystery of Abiogenesis May Have Been Solved

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Chirality in a generic amino acid (-NH 2 : amine,-COOH: caboxyl,-R: rest of the molecule). (a) Left-handed (LH) enantiomer: with thumb along the C→H axis, it takes the left hand for the fingers to point from COOH to NH 2 through R. (b) Righthanded (RH) enantiomer: with thumb along the C→H axis, it takes the right hand for the fingers to point from COOH to NH 2 through R.

Picture: Wikimedia Commons

New Study Provides Novel Insights into the Cosmic Evolution of Amino Acids - University of Tsukuba

One of the enduring problems in biology is explaining why just about all organic molecules in biological organisms have the same chirality. Creationists often latch onto this problem as evidence of something that science "can't explain", as though not (yet) being able to explain something renders the entire body of science wrong, so their "God did it!" superstition wins by default.

Of course, creationism can't explain it either other than by declaring it to be one of their creator god's mysteries.

But, properly understood, chirality is evidence of common descent since the earliest molecules from which self-replicating organisms evolved had one particular chirality - a property which has been inherited by all its descendants. The mystery is why one chirality came to dominate so completely to the exclusion of the other.

Chirality arises because an atom of carbon can form a molecule with four different groups which can be in one of two stereo-spacial arrangements, a levo (L) and a dextro (D) form, known as enantiomers. All amino acids from which proteins are built, and all fatty acids, are L-enantiomers.

Chiral molecules also display 'optical isomerism' in that the different enantiomers rotate the plane of polarisation in polarised light in opposite directions. 'L' enantiomers rotate it to the right 'd' (dextro rotation) and 'D' enantiomers rotate it to the left 'l' ( levorotation).

Molecular chirality refers to the property of asymmetry in a molecule. Chiral molecules are those that cannot be superimposed onto their mirror images. In other words, they possess a non-superimposable mirror image relationship, much like our left and right hands. The mirror images of chiral molecules are called enantiomers. Chirality plays a crucial role in biology for several reasons:

1. Biochemical Processes: Chirality is essential for many biological processes. Biomolecules such as proteins, nucleic acids (DNA and RNA), and sugars (carbohydrates) are chiral. The specific arrangement of atoms in these molecules affects their interactions with other molecules, including enzymes, receptors, and other proteins. Chiral biomolecules exhibit different biological activities based on their three-dimensional structure, and this influences their ability to bind to specific target molecules.
2. Enzyme Specificity: Enzymes, which are biological catalysts, often exhibit high specificity towards chiral molecules. Enzymes can distinguish between the two enantiomers of a chiral substrate, selectively binding and catalyzing reactions with only one of them. This selectivity is crucial for the proper functioning of metabolic pathways and other biochemical reactions in living organisms.
3. Drug Design: Chirality is an important consideration in pharmaceutical drug design. Many drugs are chiral molecules, and the different enantiomers can have different effects in the body. For example, one enantiomer may be therapeutically effective, while the other enantiomer may be ineffective or even produce adverse effects. The infamous case of thalidomide, a drug prescribed to pregnant women in the 1950s, highlighted the importance of considering chirality in drug development due to its severe teratogenic effects caused by one enantiomer.
4. Sensory Perception: Chirality is involved in our sense of smell and taste. Receptors in our nasal cavity and taste buds can distinguish between different enantiomers of chiral molecules, leading to distinct sensory experiences. For example, the two enantiomers of carvone, a compound found in caraway seeds, smell like spearmint and caraway, respectively. This phenomenon explains why two enantiomers can have drastically different smells or tastes.

Understanding molecular chirality is crucial for various fields, including pharmacology, biochemistry, organic chemistry, and drug development. It enables scientists to design effective drugs, comprehend molecular interactions, and explore the intricate mechanisms underlying biological processes.

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Biological molecules primarily exhibit the same chirality, also known as homochirality, due to the mechanisms involved in their synthesis and evolution. The specific reasons for this homochirality are not yet fully understood, but several theories have been proposed:

1. Prebiotic Origins: The origins of life and the emergence of biomolecules are thought to have occurred through prebiotic chemistry. Some scientists speculate that the initial selection of a particular chirality may have been a random event. Once a chiral bias was established, it could have been perpetuated through subsequent chemical reactions and self-replication.
2. Selective Chemical Reactions: Chiral molecules can undergo selective reactions with other chiral molecules. It is possible that early chemical reactions on Earth favored one chirality over the other due to specific interactions or catalytic effects, leading to the predominance of one enantiomeric form.
3. Self-Replication and Evolution: Chirality could have played a role in the self-replication and evolution of early biomolecules. Self-replication is a fundamental process in life, and the inheritance of chirality during replication could have led to the propagation of a specific chiral form. Over time, this selective advantage may have contributed to the dominance of one chirality in biological systems.
4. Selective Pressures: Once homochirality was established in early life forms, selective pressures could have reinforced the dominance of a specific chirality. Chiral molecules that interacted more effectively with enzymes, receptors, and other biomolecules would have had a higher fitness advantage, leading to the prevalence of that chirality in subsequent generations.

It is important to note that the exact mechanisms responsible for the homochirality observed in biological systems are still under investigation. Scientists continue to study the origins of life, the chemical processes involved, and the factors that led to the selection of a specific chirality.

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Now, Japanese scientists working at the University of Tsukuba, have shown that there may be an explanation in the form of amino acids formed in deep space and brought to earth in meteorites. If these formed the precursors to the first self-replicating molecules, then all descendants of those molecules would have the same chirality.

As the Tsukuba University press release explains:

Scientists perform computational simulations for biological molecules detected in meteorites to clarify the origin of life on Earth.

The Murchison meteorite
The Murchison meteorite is a famous meteorite that fell near Murchison, Victoria, Australia, in 1969. It is one of the most studied meteorites due to its high content of organic compounds, including amino acids, sugars, and nucleobases, which are the building blocks of life.

The Murchison meteorite is particularly relevant to the chirality problem in biology because it contains both left-handed (L) and right-handed (D) enantiomers of amino acids. This discovery was significant because in terrestrial life, proteins are composed exclusively of L-amino acids, while sugars are predominantly D-enantiomers. This phenomenon is known as the "homochirality of life."

The presence of both enantiomers in the Murchison meteorite suggests that extraterrestrial sources, such as meteorites, may have contributed to the delivery of chiral organic molecules to Earth. It raises intriguing questions about the origin of homochirality in terrestrial life and whether it has extraterrestrial origins.

Several hypotheses have been proposed to explain the homochirality of life, and the Murchison meteorite provides evidence to support some of these theories. Here are a few possibilities:

1. Asymmetric Synthesis: Chiral molecules may have been synthesized through asymmetric reactions occurring on early Earth. The presence of both enantiomers in the Murchison meteorite suggests that chiral molecules could have been formed in space and then delivered to Earth, providing a starting point for the development of homochirality.
2. Amplification Mechanisms: Amplification mechanisms involving autocatalysis or self-replication could have led to the selective amplification of one enantiomer over the other. The presence of chiral molecules in meteorites suggests that these mechanisms could have been initiated or enhanced by extraterrestrial sources.
3. Chiral Symmetry Breaking: Physical processes occurring in space, such as circularly polarized light, could have contributed to the chiral symmetry breaking, leading to the dominance of one enantiomer over the other. The Murchison meteorite's findings support the idea that extraterrestrial environments played a role in creating chiral asymmetry.

While the Murchison meteorite does not provide a definitive answer to the chirality problem in biology, it offers valuable insights into the potential sources and mechanisms that could have contributed to the development of homochirality on Earth. Continued study of meteorites and extraterrestrial chemistry can help shed light on the origins of life and the emergence of chirality in biological systems.

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All biological amino acids on Earth appear exclusively in their left-handed form, but the reason underlying this observation is elusive. Recently, scientists from Japan uncovered new clues about the cosmic origin of this asymmetry. Based on the optical properties of amino acids found on the Murchison meteorite, they conducted physics-based simulations, revealing that the precursors to the biological amino acids may have determined the amino acid chirality during the early phase of galactic evolution.

If you look at your hands, you will notice that they are mirror images of each other. However, no matter how hard you try to flip and rotate one hand, you will never be able to superimpose it perfectly over the other. Many molecules have a similar property called "chirality," which means that the "left-handed" (L) version of a molecule cannot be superimposed onto its "right-handed" (D) mirror image version. Even though both versions of a chiral molecule, called "enantiomers," have the same chemical formula, the way they interact with other molecules, especially with other chiral molecules, can vary immensely.

Interestingly, one of the many mysteries surrounding the origin of life as we know it has to do with chirality. It turns out that biological amino acids (AAs)—the building blocks of proteins—on Earth appear only in one of their two possible enantiomeric forms, namely the L-form. However, if you synthesize AAs artificially, both L and D forms are produced in equal amounts. This suggests that, at some early point in the past, L-AAs must have come to dominate a hetero-chiral world. This phenomenon is known as "chiral symmetry breaking."

Against this backdrop, a research team led by Assistant Professor Mitsuo Shoji from University of Tsukuba, Japan, conducted a study aimed at solving this mystery. As explained in their paper published in The Journal of Physical Chemistry Letters, the team sought to find evidence supporting the cosmic origin of the homochirality of AAs on Earth, as well as iron out some inconsistencies and contradictions in our previous understanding.

"The idea that homochirality may have originated in space was suggested after AAs were found in the Murchison meteorite that fell in Australia in 1969," explains Dr. Shoji. Curiously enough, in the samples obtained from this meteorite, each of the L-enantiomers was more prevalent than its D-enantiomer counterpart. One popular explanation for this suggests that the asymmetry was induced by ultraviolet circularly polarized light (CPL) in the star-forming regions of our galaxy. Scientists verified that this type of radiation can, indeed, induce asymmetric photochemical reactions that, given enough time, would favor the production of L-AAs over D-AAs. However, the absorption properties of the AA isovaline are opposite to those of the other AAs, meaning that the UV-based explanation alone is either insufficient or incorrect.

Taken together, our findings suggest that ANs underlie the origin of the homochirality. More specifically, irradiating AN precursors with R-CP Lyα radiation lead to a higher ratio of L-enantiomers. The subsequent predominance of L-AAs is possible via reactions induced by water molecules and heat.

Assistant Professor Mitsuo Shoji

Against this backdrop, Dr. Shoji's team pursued an alternate hypothesis. Instead of far-UV radiation, they hypothesized that the chiral asymmetry was, in fact, induced specifically by the CP Lyman-α (Lyα) emission line, a spectral line of hydrogen atom that permeated the early Milky Way. Moreover, instead of focusing only on photoreactions in AAs, the researchers investigated the possibility of the chiral asymmetry starting in the precursors to the AAs, namely amino propanals (APs) and amino nitriles (ANs).

Through quantum mechanical calculations, the team analyzed Lyα-induced reactions for producing AAs along the chemical pathway adopted in Strecker synthesis. They then noted the ratios of L- to D-enantiomers of AAs, APs, and ANs at each step of the process.

The results showed that L-enantiomers of ANs are preferentially formed under right-handed CP (R-CP) Lyα irradiation, with their enantiomeric ratios matching those for the corresponding AAs. "Taken together, our findings suggest that ANs underlie the origin of the homochirality," remarks Dr. Shoji. "More specifically, irradiating AN precursors with R-CP Lyα radiation lead to a higher ratio of L-enantiomers. The subsequent predominance of L-AAs is possible via reactions induced by water molecules and heat."

The study thus brings us one step closer to understanding the complex history of our own biochemistry. The team emphasizes that more studies focused on ANs need to be conducted on future samples from asteroids and comets to validate their findings. "Further analyses and theoretical investigations of ANs and other prebiotic molecules related to sugars and nucleobases will provide new insights into the chemical evolution of molecules and, in turn, the origin of life," concludes an optimistic Dr. Shoji.

The team's findings are published, open access, in the Journal of Physical Chemistry Letters:

Graphical Abstract

Abstract

High enantiomeric excesses (ee’s) of l-amino acids, including non-proteinogenic amino acid isovaline (Iva), were discovered in the Murchison meteorite, but the detailed molecular mechanism responsible for the observed ee of amino acids remains elusive and inconsistent, because Iva has an inverted circular dichroism (CD) spectrum with respect to α-H amino acids, e.g., alanine. To address this issue, we resort to accurate ab initio calculations for amino acids and their precursors in the Strecker synthesis. We evaluated their photolysis-induced ee in the range 5–11 eV including the Lyman alpha emission line (Lyα), the typical intensive 10.2 eV radiation ascribed to the early phase of galactic evolution. We show that only the aminonitrile precursors are characterized by positive ee in the Lyα region, explaining why right-handed circularly polarized Lyα (R-CP-Lyα) induces homologous l-amino acids. This study shows that the homochirality of amino acids is produced at the aminonitrile precursors stage.

Shoji, Mitsuo; Kitazawa, Yuya; Sato, Akimasa; Watanabe, Natsuki; Boero, Mauro; Shigeta, Yasuteru; Umemura, Masayuki
Enantiomeric Excesses of Aminonitrile Precursors Determine the Homochirality of Amino Acids.
The Journal of Physical Chemistry Letters (2023); 14(13) 3243-3248. DOI: 10.1021/acs.jpclett.2c03862.

Copyright: © 2023 The authors.
Published by American Chemical Society, Open access.
Reprinted under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International license (CC BY-NC-ND 4.0)

The creationist cult needs to work doubly hard to keep their dupes ignorant of science like this and to give them coping strategies for when they do accidentally stumble across it. But then creationists are normally as keen to dismiss the science they don't like as they are to accept the lies and deceptions fed to them by their cult leaders, because the most terrifying thing a creationist can imagine is wondering if they could be wrong. Anything, including a display of ignorance and a pretense of infantile stupidity, is preferable to wondering if they could be wrong.

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