Theoretical origin of life: problem of chirality solved? (Introduction)

by David Turell , Friday, March 01, 2024, 16:13 (57 days ago) @ David Turell

The usual over hyped hope:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/01_march_2024/41767...

"There’s a bias at the heart of life, and its origin is an enduring mystery. Nearly all the amino acid building blocks of proteins today exist in mirror-image forms, like right- and left-handed gloves. But life only uses left-handed ones, even though both forms should have been equally abundant during the planet’s early days and can readily link up in the lab. Something must have tipped the balance toward lefties in the primordial soup and preserved the bias ever since.

"Now, a trio of U.S. researchers proposes a novel explanation. This week in Nature, they report that by monitoring the formation rates of amino acid pairs, called dipeptides, they’ve found multiple mechanisms that ultimately promote dipeptides whose two members share the same handedness.

“'It’s quite convincing,” says Gerald Joyce, a pioneering origin of life chemist and president of the Salk Institute for Biological Studies who was not involved with the study. Researchers next hope to learn whether the same mechanisms skew larger peptides and proteins toward left-handedness—and whether it can explain the opposite bias in RNA and DNA, whose bases have sugars that are inevitably right-handed. If so, the new mechanisms could explain how life itself took on one mirror-image form and not the other.

***

"But as Blackmond and her colleagues looked more deeply, the news got better. In a series of experiments, the Scripps researchers started with skewed proportions of L and D amino acids—for example, 60% Ls and 40% Ds. The L,D and D,L heterochiral dipeptides formed most quickly, and as they did they pulled equal numbers of L and D amino acids out of the mix. Because of the baseline bias, eventually a predominance of Ls remained in the pool of unreacted amino acids, raising the likelihood of forming fully lefthanded dipeptides. “It’s like a domino effect,” Powner says. The first heterochiral reaction eventually encourages more homochirals to form. “And it’s a general process that works with all amino acids,” Powner says. Joyce adds: “It’s just math.” (my bold)

"Follow-up experiments suggested a second bias that amplifies the effect. The team found that heterochiral dipeptides precipitate out of a solution more quickly than homochiral ones, speeding the way to a relative abundance of either homochiral L,L or D,D pairs, depending the starting mix. Just why this precipitation bias occurs isn’t yet clear, Blackmond says. However, Joyce says, together with the other effect, “it beautifully fits the [experimental] data.” Blackmond adds: “The wrong answer turned out to be the right answer to get us to homochirality.”

"For now, this push toward a particular handedness has only been shown with dipeptides. But Blackmond says preliminary work suggests the same biasing process unfolds when the sulfur catalysts stitch short peptides together into longer peptide chains.

"Joyce thinks it’s possible that the same sort of math may also help explain how life’s genetic molecules gained their handedness. “This could happen with all kinds of other things, like RNA,” he says. Perhaps it was just a statistical coin flip that caused an original bias toward building blocks of one handedness to form, Joyce says. “But once that coin flipped it caused other coins to flip.'”

Comment: just the same sort of over-hyped interpretation yesterday's entry complained about. Getting to first base doesn't mean you can reach home plate. Starting with a biased beginning is just a designer trick, not what nature offers which is always 50/50 L/D.