The Robert Boyle Lectures (a series of lectures given at the Oxford University Science Club) began in 1892 with The Place in Science and the Character of Robert Boyle . This inaugural talk was dedicated to this famous thinker and philosopher who gives them their name (1627-1691), great of modern physics and chemistry and of whom he is considered one of its main founders, and inventor.
The second of the conferences, The molecular tactics of a crystal , took place on May 16, 1893, and in it, dedicated to the geometry of the structure and modes of molecular grouping in crystals, William Thomson (Lord Kelvin, 1824- 1907) used the term ‘chiral’ for the first time, to refer to objects that are not superimposable with their mirror image: “I call any geometric figure, or group of points chiral, and I say that it has chirality if its image in a flat mirror , ideally realized, it cannot coincide with itself. “
Thus, coming from the Greek ‘hand’, as this is one of the most familiar ‘chiral’ objects that we can find (we can easily prove, and I encourage the reader , that a hand and its image in the mirror are not superimposable no matter how many twists we give it), this term came to be used systematically in physics and chemistry, to refer to two objects (real or figurative) that, one being a mirror image of the other, cannot be superimposed by means of rotation and translation operations only . This, in the chemical field, leads us to the existence of molecules, or groupings of molecules, which are chiral, that is, they are not superimposable with their mirror image.
Molecular chirality, which is essentially due to the distribution of its atoms in a three-dimensional space in such a way that there is no symmetry in the whole, can have its origin in a variety of factors, some as simple as the presence of carbons attached to four different substituents (asymmetric carbons), or more complex factors, such as the folding in space of helicenos, which are ortho-condensed polycyclic aromatic molecules, angularly ringed, capable of generating chirality without the need for asymmetric centers, depending of the folding sense.
However, regardless of the origin of chirality in the molecules, a chiral molecule will be, one of two options, levo (l-) or dextro (d-), depending on whether a solution of her, its plane of oscillation is deviated to the left (l-), or to the right (d-). Therefore, chiral molecules can exist in two isomeric forms, called enantiomers (also optical isomers or specular isomers), one called left-handed and the other right-handed, since it is a fact that, if a solution of a pure chiral molecule (100% , that is, homochiral, a term also coined by Lord Kelvin), produces a turn in one sense, its mirror image (that is, its enantiomer or optical isomer) also pure, will do so in the same amount, but in the opposite sense.
In turn, given any chiral chemical substance, an equimolecular mixture (50/50) of the two possible enantiomers (l- and d-), called a racemic mixture , will not rotate said plane of vibration of polarized light at all, because the Effects of both isomers will be counteracted. In the middle term, when there is an excess of one enantiomer with respect to the other (for example, 40/60), we speak of enantiomeric enrichment, and the plane of the polarized light will be deviated in the direction of the majority enantiomer, be this the l – or d-, in an amount less than that caused by said pure enantiomer.
Why are living systems essentially homochiral?
Well, let’s go to what really interests us and what is the reason for this article. While in general chemistry, like physics, does not differentiate between left and right (that is, as a general rule, with their exceptions, the laws of physics and chemistry do not have preferential directions in space), it turns out that, in In living systems, the opposite occurs: there is a very high preference for homochirality. Living systems almost exclusively prefer some enantiomers, to the detriment of others that they discard. This, and since living systems cannot but obey the same physicochemical rules as the rest of the known universe, it is still in a certain way a contradiction, which is why it has been the subject of much study and intense debate, without being has reached, for now, a definitive solution. It is, therefore, one of the great questions that science has yet to solve.
But let’s see, when we comment that in general neither chemical nor physical phenomena differentiate between left or right, or that, with few exceptions, there are no preferential directions in space … what does this mean in our context? Simply that, as a general rule in the chemical field, the synthesis of any chiral substance, in the absence of external agents, also chiral, will give rise to racemic mixtures (50/50 of both enantiomers), since none of the enantiomers will be favored in their formation with respect to the other. However, and going back to living systems, we said that these are highly homochiral. Thus, for example, it occurs with almost all the elemental constituents of biomolecules, such as the amino acids that make up proteins, always levo (l-), or carbohydrates, always dextro (d-), whether they are found independently as monomers (for example, d-glucose), or as part of biopolymers such as cellulose or in the nucleotides of nucleic acids (DNA and RNA), among others.
Certainly, recently D-amino acids have been found in higher order living organisms, both in the form of amino acids, as peptides and as free proteins, which in some cases seem to have some important physiological functions, and in others they would be related to aging. However, given its small proportion, it only comes to tell us that it is the exception that confirms the rule.
Returning to homochirality in living systems, it should be said that this idea is not new, but has been assumed as a requirement for living beings since more than 150 years ago, when Louis Pasteur (1822-1895) managed to solve the enantiomers of the tartaric acid by crystallization, coming to elucidate that the differences that were appreciated between tartaric acid from living beings (enantiopure, that is, homochiral), from which it was generated when the synthesis in the laboratory was approached (racemic mixture), was not other than the presence of a single optical isomer in the first case (dextrorotatory isomer), and the 50% mixture of the two optical isomers in the second case (dextro and levo isomers at 50%). However, even though much has happened since then, there are still big unresolved questions surrounding this issue: Why are living systems essentially homochiral? Why do I levo and not dextro amino acids? Why do I dextro carbohydrates and I do not have them? We will not leave without giving a few brushstrokes, although brief, on how the state of matter is in relation to these questions.
Regarding the first question, there is enough data today that allow us to affirm without fear of being wrong, that it is a matter of necessity, not so much of chance. Living systems are essentially homochiral out of necessity. Thus, in nature, all living systems have evolved towards maximum efficiency. Only the best, the most efficient, the ones that use the least energy and make the best use of resources, survive. The inefficient is quickly discarded. And it turns out that, in the very high complexity of living organisms, homochirality is essential to achieve that efficiency. To give just one example, if long protein chains were made up of a variety of optical isomers of different amino acids, they would not be able to fold into the appropriate highly specific structures that are required in cell chemistry. This has been proven repeatedly, both by computational calculations and in the laboratory. Therefore, presumably, once the world of l-amino acids was established at the origin of life on our planet, d-amino acids were forever excluded from living systems.
However, regarding the second question, the issue is still not so clear. On the one hand, it is possible to think that the exclusivity of l-amino acids in biological systems is due solely to a matter of chance. That is, from a supposedly heterochiral and racemic primordial soup, that is, in which there would be equal proportions of the optical isomers for each biomolecular ‘brick’, by a process of natural selection directed towards maximum efficiency, in a purely random way , l-amino acids were selected against d-amino acids, and similar for carbohydrates, etc. With a 50% chance, it doesn’t seem unreasonable to think of this as a worthy reason to end the issue.
But the human being, when it comes to science, and especially when it comes to looking at the ultimate origin that has brought us here, is not generally very friendly to content with luck as a primary justification. And in that effort, perhaps a little wanting to curl the loop, explanations have been sought for this original question of l-amino acids in a diverse diversity of physical and chemical phenomena, such as the violation of the CPT conservation principle – charge, parity and time. – in weak nuclear interactions; possible polarized magnetic, electric, or light fields, such as circularly polarized infrared light from the dust cloud in the constellation Orion; or circularly polarized synchrotron radiation emitted by neutron stars, to name a few.
These phenomena, which in themselves generate a naturally asymmetric environment , where not all directions in space are equivalent, have been shown capable of, in their interaction with the matter that makes up the biological bricks, unbalance the energy balance in favor of a certain optical isomer. In fact, certain enantiomeric excesses, of even values of 10%, in the molecular composition of some meteorites that reached Earth point in this direction, such as those found in the Murchison meteorite that fell in Australia in 1969. If we add to this the recently described chirality amplification phenomena that, operating after a slight and momentary spontaneous break in symmetry, can give rise to highly homochiral macroscopic systems without the need for asymmetric external intervention. that it is difficult not to delight.
