Theoretical origin of life: homochirality huge problem (Introduction)

by David Turell , Thursday, August 10, 2023, 16:12 (261 days ago) @ David Turell

All proteins in life are either right or left-handed, and to function are more than 200 units long:

https://evolutionnews.org/2023/08/origin-of-life-the-challenge-of-achieving-homochirali...

"The chemical properties of mirror imaged compounds are equivalent for all practical purposes. The production of these building blocks in a prebiotic world, using very simple compounds as starting material, would therefore be expected to result in a racemic mixture, or 50/50 distribution, of these building blocks. This has already been confirmed from analysis of organic compounds retrieved from an asteroid.

"Producing polymers consisting of only one configuration from this mixture is obligatory if a prebiotic origin of life on Earth is be convincingly explained to the scientific community. Proposing chemical schemes that produce polypeptides (proteins) or RNA using homochiral building blocks is the task at hand. Performing such a feat in solution appears unfeasible, so the prebiotic soup approach has been abandoned by many origin of life (OOL) scientists. A better alternative, widely investigated, is the use of mineral surfaces where adsorption of organic chemicals could theoretically select preferentially one configuration of a pair of chiral molecules.

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"...many other laboratories undertook similar approaches, testing out different mineral surfaces and a variety of reaction conditions. The highest achievers managed to produce polypeptides up to decamers. It became clear from these studies that the longer the polypeptide produced, the harder it was to retrieve it from the mineral surface, as it was tightly bound via a multitude of chemical bonding forces spread over a long chain of amino acids. Success at making long polypeptides seemingly dooms the polymer to remain fixed to the surface instead of being released into solution as needed by life.

"A major flaw with the studies described above is that they did not attempt to account for how homochiral selectivity could be enforced by this route. Synthesizing polypeptides with a racemic mixture of amino acids offers prebiotic life nothing in terms of functionality. For proteins to assume specific and reproducible structures, a homochiral set of amino acids must be used.

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"Among the 20 amino acids, only 11 possess at least three functional groups required for appropriate spatial positioning to the mineral interface. The remaining nine have only two, one carboxyl and one amine group. This leaves little room for attaining homochirality for all amino acids as needed to construct proteins.

"To give this laboratory credit, they did mention that alanine, valine, and lysine do not exhibit chiral selection on calcite. The lack of data for most remaining amino acids leads one to believe minerals in general are another dead end for the homochirality problem that OOL protein researchers are trying to solve.

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"The carbohydrate ribose, required to make RNA, presents a highly problematic situation. Ribose has four chiral centers, unlike amino acids with just one. Therefore, D-ribose found in RNA has seven other chemical partners differing in their spatial arrangement of atoms.

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"The tighter binding of ribose, by virtue of its hydroxyl groups all pointing to the mineral surface, would permit tighter retention where the other sugars would proceed more rapidly through this chromatographic separation.

"There are several problems with this innovative proposal. Setting up a chromatographic separation does not happen naturally. It requires a specific design where all molecules must enter the chromatographic medium simultaneously. In other words, an external agent is required to carry out this exercise. Assuming that chromatography proceeds as hoped, the last 5-carbon sugar eluting from the chromatographic medium will be D- and L-ribose. There is still a racemic mixture of ribose that must be contended with. Finally, as discussed in my earlier article, appropriately linking a nucleobase and finally phosphate to D-ribose is an extremely difficult task without the use of specifically engineered catalysts, e.g., enzymes. This is highly unlikely to happen in a prebiotic scenario.

"I have addressed here the potential minerals proposed by OOL researchers to abiotically synthesize the first functional biopolymers. Simple principles discounting these proposals can be applied to other comparable scenarios in this area, but that would require a much longer and more technical article. Suffice it to say there are sound counterarguments to the plethora of schemes that OOL researchers devise in trying to account for how life could have emerged abiotically. Critically examining these schemes is the job with which scientists on the other side of the field are tasked."

Comment: to fit here I have omitted much complex organic chemistry discussion. To get around these problems the labs use off-the-shelf purified left or right molecules. OOL researchers never note these problems.