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The spin-selective crystallization mechanism due to the CISS effect and the experimental setup. (A) As molecules approach a surface, they transiently acquire an induced charge polarization. Due to the CISS effect, the transient charge polarization of a chiral molecule is accompanied by spin polarization. The spin state associated with the charge poles is determined by the handedness of the chiral molecule. Since the magnetic surface itself is spin-polarized, it kinetically favors (similar to a crystal seed) the enantiomer whose transient spin state results in a low-energy spin exchange interaction. The lower energy overlap with the magnetic surface is singlet (red, ) and the upper energy overlap is triplet (blue, ). The energy difference between these two configurations is greater than the ambient temperature, kbT; therefore, the effect manifests itself robustly. (B) Schematic of the setup used in crystallization experiments and a sample microscope image of RAO crystals on a magnetite surface from a direct crystallization experiment. The image shows the magnetite surface as a black background and the needle-shaped conglomerate crystals of RAO formed on the surface as well as the stochastically arranged needle-twined crystals of D- and L-RAO and racemic RAO in the form of a flaky powder suspended in the column of water above the surface. Credit: The progress of science (2023). DOI: 10.1126/sciadv.adg8274

Research conducted by Harvard University may have solved the puzzle of how life became molecularly right-handed. In the paper “Origin of biological homochirality by crystallization of an RNA precursor on a magnetic surface”, published in The progress of scienceresearchers explain how it all could have started with the right kind of rocks.

Molecules can be left-handed, right-handed, or both. RNA and the sugars that make up DNA are right-handed molecules. No one knows why or if there is a reason beyond chance that life began with the right hand.

By analogy, human hands can be right or left, and are mirror images of each other, meaning they cannot be superimposed without looking in the wrong direction. Molecules can have similar structural symmetry.

In much the same way that right-handed people have difficulty with left-handed scissors, or left-handed guitarists have to swap strings and play the instrument backwards, the molecules don’t interact the same way when they’re left- or right-handed. Once started, it makes sense that the building blocks of life continue with the same handedness.

One intriguing idea is that left-handed cosmic rays destroyed left-handed DNA precursors just as life began on Earth.

Ribo-amino oxazoline (RAO) is a crucial RNA precursor for two of the RNA nucleotides, cytosine and uracil. RAO also forms a crystal structure that can be right or left handed which, once the crystal begins to form, right or left handed, only bonds with other molecules of the same handedness.

By placing RAO on magnetite (Fe3OR4), researchers could achieve 100% handedness of RAO crystallization, left or right, depending on the spin exchange interaction and the degree of spin alignment (magnetization) on the active surface.

Earth’s most abundant naturally occurring magnetic mineral, magnetite, would have had many opportunities for interaction with RAO in early times. However, the researchers say the effect is unlikely to occur in contact with solution particles such as mud, but rather on sedimentary rock surfaces.

Even with current discoveries that could unlock two of the four nucleotide components of RNA, two more are still missing. So far, the origin story notes that common and naturally occurring components at room temperature can initiate the process. If the next two are found to have similar requirements, it would indicate that life on any Earth-like planet in the universe could just as easily have started.

More information:
S. Furkan Ozturk et al, Origin of biological homochirality by crystallization of an RNA precursor on a magnetic surface, The progress of science (2023). DOI: 10.1126/sciadv.adg8274

S. Furkan Ozturk et al, Chirality-induced magnetization of magnetite by an RNA precursor, arXiv (2023). DOI: 10.48550/arxiv.2304.09095

About the magazine:
The progress of science


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