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Many molecules have two versions that mirror each other, but organisms use only one of them. New research suggests that this asymmetry of living molecules may be caused by the interaction between electrons and magnetic surfaces.

(by Chen Qiang / tr. by Robert Taylor)

In 1848, the Frenchman Louis Pasteur was a young chemist, many years before he invented pasteurization, but he discovered something strange about crystals formed when cooking wine for too long. Half of these crystals are tartaric acid, a salt that forms naturally on the walls of the barrel. The other half of the crystal has the same atomic composition as tartaric acid, but the arrangement of atoms is mirror symmetrical and cannot coincide with each other.

This molecular property was later called chirality, similar to the fact that our left and right hands are mirrored and cannot coincide with each other. A left-handed and right-handed version of a molecule that is enantiomer to each other.

Pasteur discovered not only the chirality of some molecules, but also the biological significance of chirality. Cooking produces tartaric acid which is a mixture of two enantiomers because the boiling process allows the formation of the same number of left-handed and right-handed versions. But in the natural crystals in the barrel, all tartaric acid molecules are right-handed, because the grapes used to make wine produce only one enantiomer.

Why are life molecules asymmetrical? As we know, the molecules that make up life are asymmetric. That is, when life uses chiral molecules, they use only one enantiomer. For example, the sugar molecules that make up DNA are right-handed, and the amino acids that make up proteins are left-handed. If the wrong enantiomer is added to the drug, it may sometimes be toxic or even life-threatening.

Why does life choose one chirality over the other? Scientists believe that something must have happened during the origin of life, leading to the asymmetry of life molecules. But scientists have not yet decided what these events are and whether they are inevitable or accidental.

Scientists have put forward many hypotheses in an attempt to solve the mystery. However, these hypotheses are not quite consistent with the actual geological conditions. Moreover, although there are some hypotheses that can explain the single chirality of a living molecule, there is no hypothesis to explain why the whole life system is biased towards a chirality.

Recently, a research team has published a series of papers that provide a new insight into how the asymmetry of life molecules appears. Their hypotheses not only link geophysics, geochemistry and biochemistry together, but also are supported by relevant experiments.

The origin of the hypothesis that chiral molecules "disperse" electrons can be traced back to more than 20 years of research. At the time, Ron Naaman, a chemical physicist at the Weizmann Institute of Science in Israel, and his team discovered the key effects of chiral molecules. This effect involves the spin of electrons, which is a quantum property. There are two possible values for electron spin, usually referred to as "up" and "down", or denoted by "+ 1amp 2" and "- 1max 2". Electron spin cannot be understood in classical physics because it is not really the rotation of electrons around their axis, but a pure quantum effect.

To better understand Naaman's findings, imagine throwing a Frisbee that touches the wall of the hallway while flying. If the Frisbee touches the right-hand wall, it will bounce forward only if it rotates clockwise; otherwise, it will bounce backwards. If you let the Frisbee touch the left-hand wall, the opposite will happen.

Similarly, chiral molecules "disperse" electrons according to the direction of their spin rotation. In this way, electrons with the same spin will eventually gather at one pole of the chiral molecule. In addition, the left-handed and right-handed versions of a molecule are the opposite of the electron spins gathered at their respective poles.

However, the redistribution of electrons will affect the interaction between chiral molecules and other molecules, because electrons with opposite spins attract each other, while electrons with the same spins repel each other. Naman et al named this phenomenon "chiral induced spin selectivity" (CISS) effect.

Therefore, when a chiral molecule approaches a magnetic surface, if the molecule and the surface have opposite spins, they will be pulled closer together. If the spin is the same, they will repel each other. (because other chemical interactions are also taking place, molecules cannot simply flip to rearrange themselves. Therefore, different chiral molecules can be screened on the magnetic surface.

In 2011, Naaman's team, in collaboration with the University of Munster in Germany, measured the spin of electrons passing through double-stranded DNA, confirming the CISS effect. Since then, scientists have begun a series of studies on the effect and its possible applications. For example, Naaman's team found that the CISS effect could be used to remove impurities from biological drugs or to remove the wrong enantiomers from drugs to prevent major side effects.

The team led by Harvard astronomer Dimitar Sasselov (Dimitar Sasselov) and his graduate student S Fulcan Oztuk (S. Furkan Ozturk) believed that the CISS effect also had an important influence on the asymmetry of life molecules, so they worked with Naaman and others to start the research.

The magnetic surface allows the same chiral molecule to crystallize Naaman's team has shown that they can use the magnetic surface to screen an enantiomer of the molecule and make them form crystals that contain only the same chirality. In addition, John Sutherland (John Sutherland) of the MRC Molecular Biology Laboratory in the UK and his team have found that a RNA precursor called "ribose aminoxazoline" (RAO) can synthesize two of the four nucleotides of RNA, and can be crystallized in this way. They found that once the enantiomers attracted to the magnetic surface formed a seed, the crystals preferred to combine with more of the same enantiomers to grow.

Based on these experiments, Sasselov and Oztuk put forward a hypothesis. They believe that in the early formation of the earth, the surface of the water body rich in minerals was magnetized by the earth's magnetic field at that time, and the resulting magnetic surface played a selective role in molecules, making it easier for one chiral molecule to bind together than another. This initiates the process of making biomolecules such as RNA and amino acids asymmetrical.

The team conducted research in a laboratory at Harvard University. They poured a solution with a magnetic surface into a petri dish and injected it with equal amounts of left-handed and right-handed RAO molecules. Then they put the petri dishes on magnets and put them together in the refrigerator, waiting for the first crystals to form. Initially, they found that 60% of the crystals were uniformly chiral. When they repeated the experiment, they succeeded in producing 100% crystals with the same chirality.

They also found that if the solution surface is magnetized in one direction, the resulting crystals are right-handed; if the solution surface is magnetized in the other direction, the resulting crystals are left-handed.

But there is still a big problem with the experiment: the magnetic field of the magnet they used in the experiment is about 6500 times that of the earth's magnetic field. As a result, Oztuk went to the Weizman Institute of Science in November last year and conducted a follow-up experiment with Naaman and others. They did not use an external magnetic field at all in the experiment, and found that when chiral molecules were adsorbed on a magnetic surface, the local magnetic field they generated on the surface was about 50 times stronger than the geomagnetic field. This local magnetic field makes it easier for the crystal to continue to grow without an external magnetic field.

RNA gives priority to the production of proteins with opposite chirality, but the crux of the question is how this effect spreads throughout the molecular network. In a paper published in the August issue of the Journal of Chemical Physics (The Journal of Chemical Physics), Oztuk, Sasselov, Sutherland and others proposed a model of how this effect spreads through molecular networks before the origin of life.

Their research shows that analogs of right-chiral RNA molecules bind to left-chiral amino acids about 10 times faster than to right-chiral amino acids. This finding suggests that a chiral RNA gives priority to the production of opposing chiral proteins, as happens in nature.

Oztuk said that although the study does not directly explain why the nucleotides preferred for life are right-handed and amino acids are left-handed, they can show that in the process of biomolecules becoming asymmetrical, the decisive factor is the Earth's magnetic field.

Other scientists welcomed their research, saying it would inspire more people to study the link between the CISS effect and biomolecule asymmetry. However, some scientists believe that only when we see the CISS effect leading to RNA aggregation can we really find the answer.

Sutherland's team is still trying to prove that the other two types of nucleotides can also be made from RAO molecules, and Oztuk hopes to repeat these experiments in a more natural environment. In short, their hypothesis needs more evidence and support. However, it undoubtedly provides a new perspective and train of thought for solving the mystery of asymmetry of life molecules.

References:

Https://www.science.org/doi/10.1126/sciadv.adg8274

Https://pubs.aip.org/aip/jcp/article/159/6/061102/2905827/The-central-dogma-of-biological-homochirality-How

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