When admiring the beauty of flowers, we are also struck by their symmetry-petals all identical, and organized in an ensemble which can be divided in two equal parts by an imaginary plane of symmetry, a pattern which is also at the basis of the bilateral symmetry of most living beings. Symmetry is also the basis of our unconscious perception of beauty, and we could say that is one of the very basic features of nature, of our world. But instead, funny enough, we should say that the intrinsic nature of all major components of life is molecular asymmetry, namely precisely the lack of symmetry – a notion for which often the parent expression of “chirality” is used.
The asymmetric world
To understand this very important concept, we have to use a little chemistry – which I will keep to a very basic level. Let me simply say that all bio-molecules in nature – amino acids, sugars, lipids, nucleic acids, proteins and all their derivatives – contain carbon as the basic atomic component, the C-atom. The C-atom under normal conditions is “tetravalent”, namely has the capability of binding four different atoms, or groups of atoms. Imagine to have a point from which four lines depart so as to form a tetrahedron – see figure 1. Thus, we have methane, CH4, or methanol, CH3-OH, or chloroform, CHCl3, where H indicates the atom of hydrogen, Cl that of chlorine, and O is obviously oxygen – and many other molecules. This is all very easy.
Things become however more complex and interesting when the four ligands of the C-atom are all different from each other – see figure 3. Because then you can form two molecular forms which are different from each other. What does “different from each other” means? It means, that the two figures are not superimposable with each other. They are actually mirror images of each other. They are like the two hands, which are obviously different from each other – see figure 2. And when this happens, the molecules in question (or the hands) have no plane of symmetry, they are asymmetric, they are “chiral”, from the Greek chiros, hand. The two molecular mirror images are “optical isomers”, and more specifically they are called “enantiomers”.
And now the basic point is the following: that all important molecules of life – amino acids, sugars, phospholipids, nucleotides – and the corresponding macromolecules, proteins, nucleic acids, polysaccharides… are all asymmetric molecules, in which namely the C-atom is bound to four chemically different groups – see figure 4.
The two enantiomers of an amino acid, usually indicated as L- and D- (for levo and dextro, and there is a different nomenclature which calls them R and S); have all the same physical and chemical properties, and, importantly, they have then the same energy and stability, and therefore the same probability of occurrence. When the chemist synthesizes one of these compounds in the lab by normal synthetic means, he will obtain always a “racemic mixture”, namely a 50:50 mixture of the two.
But… it is not so in nature. In nature, we have only L-amino acids, namely only one kind of chirality (with extremely few exceptions which are not important here). Why is it so? We simply do not know, this is one of the open questions in the origin of life. Determinism or contingency? Even if we do not know the answer, we know that this asymmetry is one fundamental raison for the existence of life on Earth.
To understand this point, let us consider a molecule which has two asymmetry centres, for example the dipeptide of Alanine, (Ala)2. This can exist in four different optical isomers, 22, namely DD, LL, LD and DL. Here, DD and LL are enantiomers with each other(mirror images which are not superimposable) and so are LD and DL. If you now consider a small peptide, (Ala)10, this one in principle can exist in 210=1024 different optical isomers – each with its own melting point, refractive index, solubility, etc. The differences may be small, but in principle all these 1024 isomers are all different molecules.
And now consider a peptide hormone consisting of ten different amino acids in an ordered sequence, say Ala-Tyr-Ala-His-Trp-Phe-Ala-Glu-Ser-Ser (where Tyr, His, Trp, Phe, Glu, Ser, are the amino acids Tyrosine, Histidine, Tryptophan, Phenylalanine, Glutamic acid, Serine). In nature, this hormone would be a single compound, with all amino acids in the L configuration. But: if both L- and D-amino acids were permitted in nature, and the biosynthesis would take place with all of them, this sequence would be present in 1024 different ways! And take a small protein, of the dimension of insulin, with a sequence of 50 amino acids. If both L- and D- were permitted in the synthesis of nature, we would have 250 different compounds, which is one followed by 15 zeroes.
Nature reduces this astronomic number to just one, by using the trick of having only the L form. What a trick! Certainly, one of the most important structural principles to bring about order in the molecular world of our life. Life as we know it would be impossible with a chaotic mixture of proteins, or of hormones – we have life because we have a precise molecular recognition at all levels, and the prerequisite for this, is the existence of single, very precise and unique structures.
The asymmetric carbon atom is not the only element of chirality in the molecular world of our life. The macromolecules like proteins, nucleic acids, and polysaccharides like amylose and cellulose, tend to assume helical conformations. Famous of course is the double helix of DNA – but not everybody is aware of the fact that the helix is per se a chiral object – see for that figure 5. If you make by metal wire a left-handed helix, and a right-handed one, you have two enantiomers, two mirror images which are not superimposable.
The double-stranded helix of our DNA (in its most common conformation under the conditions found in cells) is formed by two right handed macromolecules (see figure 6, right panel), which are not optical isomers, they are chemically different objects, but complementary to each other, in terms of the nucleotide complementary pair, Adenine (A) with thymine (T), cytosine (C) with guanine (G). So, the stretch ATTGC of one strand, is faced by the sequence TAACG of the other strand. In proteins, famous is the alpha-helix, which is also right-handed.
To this regard, something a little more difficult perhaps to understand: the protein right-handed alpha-helix, as showed in left panel of figure 6, has L-amino acids only; and its enantiomer (the mirror image) therefore is not a left-handed helix with L-amino acids, but a left-handed helix with D-amino acids. The right-handed helix with L-amino acids is more stable than the left-handed helix with L-amino acids – and this is why in nature practically only the right-handed alpha helix found. Now, from the world of macromolecules, let us make a big jump to the macroscopic world of flowers and animals mentioned in the opening of this article.
Look at the perfect symmetrical structures of plants, insects, higher animals. Having first realized that asymmetry is the key of structure at the molecular level, the question arises spontaneously: how can the molecular asymmetry-chirality and the macroscopic symmetry co-exist? It is again an operation of nature to warrant order. And in this case, this is achieved by economizing the degree of information needed to build up the macrostructure. In the language of an engineer, to build a flower like that shown in the main figure, formed by eight identical petals, you need the bits of information for only one petal, then the information of “repeat”. From the genetic point of view, you need only the genes to make one petal, and this is much more economic than the alternative, to make the same flower dimensions with petals all different from each other. It is the same principle by which a constructor builds a wall using a series of equal bricks.
It can be said then, looking at the few examples of symmetry in plants and animals (as shown in figures 7, 8 and 9), that both asymmetry and symmetry are used by nature to serve the general principle of order. The order that for our perception is both simplicity and beauty.
Text by Pier Luigi Luisi and Angelo Merante.
Angelo Merante is an artist based in the heart of ancient Magna Graecia (near Catanzaro, Italy). He is one among the founders of the Inism, an avant-garde movement started in Paris, 1980. He took part in several international art exhibitions, mostly in Europe, and is published author of theoretical and creative papers. His works span from abstract and asemic poetry to visual art, including painting, photography and computer graphics. Coming from life sciences studies, he worked from 1990 to 2010 with different research groups in "Rome Sapienza" and "Roma Tre" Universities. Since 2004, he is a coworker of professor Pier Luigi Luisi.