An Interesting Discussion on the Origins of Chemical Chirality via the Asymmetry of the Weak...
...Nuclear Force.
Chirality is the property most easily demonstrable by one's hands. If one puts one's hands together as people who pray do, one can see that they are mirror images of one another, more or less, but they cannot be superimposed upon one another.
This property is found in most of the molecules responsible for biology, i.e. molecular biology.
In a laboratory if one wishes to make a chiral molecule, one must put chiral molecules somewhere in the reaction mixture; a very common approach is to use chiral catalysts. If one does not include chirality in the reaction mixture, one will get a racemic mixture, an exact 50:50 mixture of both "hands."
Living systems are quite capable of producing chiral molecules, usually catalyzed by enzymes, all of which are chiral since they are made of amino acids. Of the 20 "normal" amino acids, 19 are chiral; one, glycine, the simplest amino acid, is not.
One of the great mysteries in science is the question of how chirality first arose. It is known from things like the Murchinson meteorite that chirality exists in space, so living systems are not required to produce it.
I've been pondering this question most of my adult life; but I do not expect an absolutely definitive mechanism to be shown in my life time. On the other hand, maybe some progress is being made.
There is one chiral nuclear reaction, associated with the weak nuclear force, the force responsible for beta decay. It has always seemed to me that this might be a key.
I came across a paper today, directed by a news feed, that touches on the subject in support of the thesis that asymmetric nuclear decay is responsible for the origins of chirality.
It's this one: Origin of Chirality in the Molecules of Life, J. A. Cowan and R. J. Furnstahl, ACS Earth and Space Chemistry 2022 6 (11), 2575-2581.
The introduction to the paper:
Nature often demonstrates a preference for specific symmetries, at least at the most fundamental level of molecular symmetry reflected by the building blocks of life (D-isomers for nucleic acids and L-isomers for amino acids). (1) Once an inherent preference for one chiral form over another has been established, it is not difficult to understand, from the chemical principles of chiral induction and diastereomeric selection, how that chirality is propagated to downstream molecular products (including lipids and other important metabolites) resulting from complex biosynthetic pathways, (3−6) often involving natural protein- or RNA-based catalysts with their own enantiomeric preferences. Consequently, the more fundamental question can be stated as, “how did an intrinsic preference for one chiral form initially arise?”.
Prior work on this problem has been well detailed by Guijarro and Yus, (1) which provides a valuable overview of the primary mechanisms by which chirality in life molecules has been proposed. Both spontaneous symmetry breaking during crystallization (7) and seeding mechanisms from extraterrestrial meteorites and comets (8−10) are possible pathways for initiation of chiral selectivity, but in all of these cases, prior establishment of a specific enantiomeric form is required. Over the past decade, the major research emphasis has focused on the mechanisms and pathways that could promote amplification or selection of selected enantiomers by kinetic criteria. (11−17) The molecules under investigation include both natural biomolecules, (15,18) especially amino acids, and regular organic compounds. (4,6,12,13,16,19−22)
Experimentally, there are a number of principle kinetic factors that sustain autocatalytic generation of a selected enantiomer. (23−26) Primarily, these depend on the difference in activation energies for a reaction that proceeds through a nominal “diastereomeric” transition state to produce one or other of the two possible enantiomeric products (Figure S4). In contrast to normal mechanisms of chiral induction, where a chiral catalyst leads to the “diastereomeric” transition state, in this case, it is the influence of the intrinsically chiral weak nuclear force that distinguishes the two possible transition states and enantiomeric preference...
Unfortunately, I'm very stressed for time, and can't spend a lot of time going over the paper, but apparently - I didn't know this - the presence of heavy elements can enhance energy differences in the transition states between prochiral intermediates, and the authors propose that an interaction with asymmetric radiation in reactions catalyzed by divalent group 2 elements, Ca+2, Sr+2, and Ba+2, which are known catalysts in a class of reactions known as aldol condensations.
Another brief excerpt:
These enhancements notwithstanding, the aim of this work was to re-establish the relevance of parity violation as a likely mechanism for the origin of molecular chirality in the molecules of life. Such a view has been criticized based on the magnitude of the effect and the lack of viable mechanisms for chiral discrimination to be manifested. (31,60) In this paper, we have addressed both of these concerns and demonstrated that a combination of metal-promoted catalysis and viable chemical pathways can readily account for chiral selectivity in the context of the RNA world model over evolutionary time frames.
Although Earth is estimated to be around 4.5 billion years old, it is unreasonable to take that entire time frame over which to establish a chiral preference. Estimates of when life first appeared on earth vary, but evidence suggests that the appearance of simple bacteria most likely occurred ∼3.5–4.0 billion years ago, providing a time window of 0.5–1.0 billion years for chirality to emerge in the fundamental building blocks of life. Given the aforementioned kinetic criteria and time frames, an energy difference exceeding 10–19 au is desirable. Although this places Ca2+ at the edge of the limit for a chirality-selecting catalytic cofactor, both Sr2+ and Ba2+ lie very much within the realm of feasibility. Because conservative estimates have been used in all calculations, and considering the sensitivity of chiral selectivity to rate constants and especially calculations of ΔEPNC, it would be unwise to completely discount a possible role of Ca2+ in chiral discrimination.
Aldol reactions involving formaldehyde, a common compound in the universe, give rise to simple sugars and it is sugars that are responsible for the chirality of RNA. RNA is thought by many researchers to be the primordial molecule, the "RNA World" thesis, since it was discovered that RNA not only is an information molecule, but it also can play a role in catalysis.
Interesting, I think.
Have a happy Friday.
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