In 1848, French chemist Louis Pasteur discovered that some molecules essential for life exist in mirror image forms, much like our left and right hands. Today, we know biology chooses just one of these “chiral” forms: DNA, RNA, and their building blocks are all right-handed, whereas amino acids and proteins are all left-handed. Pasteur, who saw hints of this selectivity, or “homochirality,” thought magnetic fields might somehow explain it, but its origin has remained one of biology’s great mysteries. Now, it turns out Pasteur may have been onto something.
In three new papers, researchers suggest magnetic minerals common on early Earth could have caused key biomolecules to accumulate on their surface in just one mirror image form, setting off a positive feedback that continued to favor the same form. “It’s a real breakthrough,” says Jack Szostak, an origin of life chemist at the University of Chicago who was not involved with the new work. “Homochirality is essential to get biology started, and this is a possible—and I would say very likely—solution.”
Chemical reactions are typically unbiased, yielding equal amounts of right- and left-handed molecules. But life requires selectivity: Only right-handed DNA, for example, has the correct twist to interact properly with other chiral molecules. To get life, “you’ve got to break the mirror, or you can’t pull it off,” says Gerald Joyce, an origin of life chemist and president of the Salk Institute for Biological Studies.
Over the past century, researchers have proposed various mechanisms for skewing the first biomolecules, including cosmic rays and polarized light. Both can cause an initial bias favoring either right- or left-handed molecules, but they don’t directly explain how this initial bias was amplified to create the large reservoirs of chiral molecules likely needed to make the first cells. An explanation that creates an initial bias is a good start, but “not sufficient,” says Dimitar Sasselov, a physicist at Harvard University and a leader of the new work.
Hints of another option date to 1999, when researchers led by Ron Naaman, a chemical physicist at the Weizmann Institute of Science, found that the electrons in opposite chiral forms of a molecule have contrasting patterns of spin, a magnetic property. Later experiments revealed that the spin differences can cause chiral molecules to interact differently with magnetic materials, in which electron spins are aligned to create magnetic forces. For example, Naaman and his colleagues found that left-handed peptides (short amino acid chains) might bind to a magnetic surface while right-handed ones are repelled. But this finding, too, did not explain how the initial bias could be amplified.
A glimmer of an amplification mechanism emerged in 2009. Researchers led by Matthew Powner and John Sutherland at the University of Manchester were studying possible origins of RNA, which many researchers think was a central player in the origin of life. They were intrigued by a molecule called ribo-aminooxazoline (RAO), which they discovered could react to form two of RNA’s nucleotide building blocks. RAO is among a rare class of crystals that enforce a single chirality: Once a crystal starts to grow from either right- or left-handed versions of the molecule, only molecules with the same chirality can bind to the structure. Such crystals, if they started with an initial bias, could have caused chiral RAO to build up.
Now, Sasselov and his colleagues have put these two pieces together. They wondered whether magnetic surfaces might favor a single RAO chiral form. To find out, they turned to magnetite, a magnetic mineral that is common in Earth’s crust. They applied a strong external magnetic field, aligning electron spins in the magnetite and strengthening its magnetism. When they exposed the magnetite surface to a solution containing an equal mix of right- and left-handed RAO molecules, 60% of those that settled on top were of a single handedness. This created a crystalline seed that caused additional like-handed RAOs to bind, eventually forming pure single-handed RAO crystals, the researchers reported last week in Science Advances. When they flipped the field’s orientation and repeated the experiment, crystals with the opposite handedness took shape. “It’s a really cool effect and a way to break the symmetry,” says Powner, now at University College London.
One concern is that the applied magnetic field was some 6500 times stronger than Earth’s own field, cautions Noémie Globus, a physicist at the University of California, Santa Cruz, whose own work supports cosmic rays as the source of life’s chiral bias. “It requires conditions that are quite unrealistic,” she says.
But previous reports show that magnetite subjected only to Earth’s natural magnetic field can still cause an initial, though smaller, bias toward one form of a chiral molecule. And Sasselov and his colleagues reported in a 13 April arXiv preprint that when pure chiral RAO crystals were placed on top of magnetite, the alignment of the electron spins in the crystals forced more and more electron spins in the underlying magnetic material to align, creating a positive feedback. “It’s self-enhancing and increases the persistence of the bias” toward one molecular form, says team member Furkan Ozturk, a Harvard Ph.D. student.
The chiral RAO in turn imposes its handedness on the RNA building blocks it generates, and Sasselov’s team has now shown that the effects cascade to other biological molecules. In a report accepted last week in The Journal of Chemical Physics they show that once an excess of chiral RNA is formed, known chemical reactions could pass on this chiral bias, templating amino acids and proteins with the opposite handedness and ultimately fostering other chiral molecules essential to cell metabolism. “There is no solution out there that solves all the steps out there that this does,” Szostak says.
The quest that began with Pasteur isn’t quite over, though. One loose end, Sasselov acknowledges, is that RAO has only been shown to lead to the synthesis of two of RNA’s four nucleotides, cytosine and uracil. It isn’t known to produce the other two, adenine and guanine, although Sasselov says there’s a “big push” to search for RAO reactions that could do it. If they can, the mystery of biological handedness might be another step closer to being solved.
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June 14, 2023 at 03:32AM
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