Computing: Does life exist because Nature is asymmetric?

MEXICO CITY (apro).- The symmetry and asymmetry of the universe should make us think that this is a profound concept that impacts all manifestations of Nature. And this could even be the reason for life itself. In 1848, Louis Pasteur examined some tartaric acid salts under a microscope and noticed that two types of crystals were formed, one being the mirror image of the other. When he separated the two in water, he formed two solutions that he illuminated with a beam of light. To his surprise, one solution polarized the light to the right while the other to the left.

This led the scientist to develop a theory about molecular structure when he was just 25 years old. Pasteur postulated at that time that two different shapes of salt crystals and their ability to polarize light differently arose from the fact that salt molecules were made of two types: left-handed molecules and right-handed molecules.

In 1857 Pasteur made another remarkable discovery: certain fungi growing in a dish containing an optically inactive solution did not polarize light. But Pasteur then verified the effect by shining a beam of light into the dish. The contaminated solution then appeared to be polarized. The microorganisms had changed an optically inactive solution into an active solution. According to his molecular theory, Pasteur thought that the original solution was optically inactive because it contained an equal number of left- and right-handed molecules. The fungus had reacted chemically with one type, leaving a greater number of left- or right-handed molecules in the solution. This imbalance caused the substance to be optically active.

And then Pasteur realized that the chemistry of life preferred a polarization, something like a preference between left and right. And this was basically equivalent to breaking symmetry and that this was a fundamental element to distinguish between the living and the non-living. Pasteur wrote: “Life manifests itself to us and is a function of the asymmetry of the universe and the consequences of this fact.” And shortly after, in front of the French Academy of Sciences, Pasteur declared his great conjecture: “the universe is asymmetrical.”[1].

But it was Pierre Eugène Marcellin Berthelot (1827-1907)[2]who showed at the same time as Pasteur that yeast secreted an enzyme (called “invertase”), which acted on the sucrose in table sugar. Berthelot showed that sucrose was dextro-rotated but became levo-rotated after exposure to yeast.[3]In other words, these structures had a preference towards left or right.[4].

Pasteur’s conjecture eventually came to fruition, and what’s more, it’s much more widespread than anyone had thought. Science has revealed that mirror symmetry is often absent in Nature. Molecules, as Pasteur found, have this property called “chirality,” which is a preference for left or right. Chemists actually refer to molecules as l-enantiomers (from “levo” – left) and d-enantiomers (from “dextro” – right).[5].

The enantiomeric forms[6]They are found in many substances, both organic and inorganic, and this is essentially crucial for the development of life, specifically when we talk about proteins, which are responsible for the structure and chemical regulation of living cells, such as DNA, which is the molecule that carries the genetic load.

A protein is a molecule, a polymer, that is, a long chain of small molecules which are in this case, a chain of amino acids. And although there are hundreds of amino acids, all proteins are made of the same 20 amino acids. Interestingly, all amino acids, except for glycine, are chiral, that is, they have a preference for left or right, i.e. they are levo or dextro enantiomers. But to make things even stranger, proteins are made exclusively of levorotatory amino acids.[7].

The function of certain proteins, which are called enzymes, is to catalyze biomolecular reactions, that is, to accelerate them, including the synthesis of other proteins. The catalytic capacity of enzymes depends on their three-dimensional structure, which depends, incredibly, on their left-handed amino acid sequence. For example, synthetic amino acid chains made of L and D enantiomers do not rotate in such a way that they are efficient for catalytic activity, or in other words, they cannot form the structure called the alpha helix, which is present in most enzymes.

All of this speaks to the fact that chirality is the key to the molecules of living organisms and that human chemistry, for example, is very sensitive to enantiomeric differences. One case where this was observed was that of thalidomide, a medicine developed by the German company Grünenthal GmbH, a drug that was marketed between 1957 and 1963 as a sedative and to relieve nausea during the first three months of pregnancy.

Unfortunately, however, the drug caused thousands of babies to be born with phocomelia, a congenital anomaly characterized by the absence or excessive shortness of the limbs. It was later discovered that there were two different thalidomides (with the same molecular form), in which the arrangement of the groups in one carbon was changed, something that had not been taken into account until then. It was, therefore, a single molecule with two enantiomers. According to current nomenclature, there were thus the R form (which produced the sedative effect sought) and the S form (which produced effects that caused phocomelia). This discovery led to the stereoisomerism in molecules being taken into account from that moment on, using the R/S system today.[8].

Like proteins, the nucleic acids DNA and RNA are polymers that exist in nature in only one chirality. Each is composed of four types of subunits, each incorporating a group with a specific chirality. DNA and RNA normally form right-handed helices as a result of certain D-sugars. Correct replication of nucleic acids depends on the activity of proteins made from L-amino acids.

Thus, there is a great preference in the chemistry of life for L-amino acids and D-sugars over their mirror counterparts and to make the scenario more complex, everything seems to be explained from the electromagnetic interactions that occur when these processes occur.[9]And as we will see later, parity, symmetry, is not preserved in the world that seems impenetrable, that of quantum mechanics.

[1] Roger A. Hegstrom y Dilip K. Kondepudi, The Handedness of the Universe, Scientific American, Enero 1990.

[2]

[3]

[4]

[5] This notation is inherited from Pasteur’s own studies on the rotation (or polarization) of light.

[6] In chemistry, an enantiomer is also known as an optical isomer and is one of two stereoisomers that are mirror images of each other and cannot be superimposed, that is, they are not strictly identical.

[7] Except perhaps for polypeptides containing dextro-amino acids, although this is an extremely rare case.

[8]

[9] Martin Gardner, The New Ambidextrous Universe, W.H. Freeman and Co., 1990