By Antonino Puglisi
This post is meant to be a sequel of the one previously published in this blog entitled “A surprising LEGO world” in which I argued that molecules are holistic entities and can never be considered “just-a-bunch-of-atoms”. When a new molecule is formed, in fact, some of its properties are often new and unpredictable in comparison to those of its constituent “building blocks”.
In our surprising LEGO world some more fascinating things happen when we move just beyond the molecular level and we enter into what is called the realm of supramolecular chemistry1. In a nutshell, every molecule, with its very unique arrangement of atoms connected through chemical bonds in space, could be seen as a set of information. It is essentially the shape as well as the spatial arrangement of its various chemical groups that determine the way a molecule interacts with others. Now, when more molecules come together at a low level of energy (that is, without the destruction and formation of a strong covalent bond) a so-called supramolecular assembly is formed. This essentially works as if each interacting molecule “read” the set of information of the other and, according to rules based on shape complementarity and formation of weak chemical bonds (such as hydrogen bonds or van der Waals interactions, pi-pi stacking, etc.), they could either form a new supramolecular assembly or not (see for instance the example in the figure). This process in chemistry is known as molecular recognition and a lot of biology going on in nature is based on it.
This new branch of chemistry was formally launched by the French chemist Jean-Marie Lehn (b.1939) who received the Nobel Prize in Chemistry in 1987 for his pioneering work in this field.
The exploration of chemistry beyond the molecular level in the last fifty years has opened up an exciting new world made up of tiny molecular devices often with very high technological value. The properties of these supramolecular systems have surprised scientists as they often exhibited new emergent properties that were unpredictable and not shared by their isolated components.2, 3
This has urged chemists to look at their own science more and more from a non-reductionist standpoint. In the words of another Nobel Prize winner for Chemistry (2016), Sir Fraser Stoddard:
The time has come for us to embrace complexity (…) and put much more of our effort into studying complex mixtures of interacting molecules. An excellent reason for responding positively to the intellectual challenge posed by systems chemistry is that complexity very often gives rise to emergent properties that are not present in the components of a complex mixture but come to light only as a result of interactions between molecules.4
A real intellectual challenge indeed! The advent of supramolecular chemistry has ushered in a new age of chemistry in which, among other things, the vocabulary of the scientific publications has been enriched by words like elegance and beauty.
1. Cf. J.M. Lehn, Supramolecular Chemistry – Concepts and Perspectives, Wiley-VCH, Weinheim 1995.
2. Forgan, Ross S., et al. “Nanoporous carbohydrate metal–organic frameworks.” Journal of the American Chemical Society 134.1 (2011): 406-417.
3. Tayi, Alok S., et al. “Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes.” Nature 488.7412 (2012): 485.
4. J.F. Stoddart, From Supramolecular to Systems Chemistry: Complexity Emerging out of Simplicity, in «Angew. Chem. Int. Ed.» 2012, 51, pp. 12902-12903. https://onlinelibrary.wiley.com/doi/full/10.1002/anie.201209124