supramolecular chemistry (rus. супрамолекулярная химия) — an area of chemistry focused on the study of the supramolecular structures (assemblies consisting of two or more molecules, stabilised by intermolecular interactions); chemistry of molecular assemblies and intermolecular bonds (defined by Jean-Marie Lehn).

Description

Traditional chemistry is based on covalent atomic bonds. At the same time the synthesis of complex nanosystems and molecular devices that are used in nanotechnology require more than just the covalent chemistry features because such systems may contain several thousand atoms. This is where intermolecular interactions come in: they can help to combine individual molecules into complex assemblies, called supramolecular structures.

The simplest example of supramolecular structures is a host-guest complex. The host (receptor) is usually a large organic molecule with a cavity in the centre and a guest is a simple molecule or ion. For example, different size cyclic polyesters (crown ethers) are strongly bound by alkali metal ions (Fig. 1).

Supramolecular structures normally have the following properties:

1. The host has more than one binding site. With crown ethers, this role is played by oxygen atoms, which have lone electron pairs.

2. Complementarity: the host’s and the guest’s geometric structure and electronic properties are mutually complementing. In case of crown ethers, this results in the cavity diameter matching the ion radius. Complementarity allows the host to carry out selective binding of the guests of a particular structure only. In supramolecular chemistry, this phenomenon is called molecular recognition (Fig. 2).

3. Complexes with a large number of bonds between the complementary host and guest have a high structural organisation.

Supramolecular structures are very common in nature. All reactions in living organisms occur involving enzymes, catalysts of protein nature. Enzymes are the perfect host molecules. The active sites of each enzyme are arranged so as to accept the only substance (substrate) which corresponds to it in size and energy; no reaction will take place between the enzyme and other substrates.

Another example of biochemical supramolecular structures is a DNA molecule, where two polynucleotide chains complement each other through multiple hydrogen bonds. Each chain is both a guest and a host for the other chain at the same time. The main types of non-covalent interactions that form supramolecular structures are: ion, ion-dipole , van der Waals , hydrophobic interactions and hydrogen bonds. All non-covalent interactions are weaker than covalent bonds – their energy rarely reaches 100 kJ/mol but a large number of bonds between the guest and the host enable the high stability of supramolecular assemblies. Non-covalent interactions are weak on the individual basis but are collectively strong.

Supramolecular assemblies can form at random. This phenomenon is called self-assembly. It is a process where small molecular components spontaneously join together and form much larger and more complex supramolecular assemblies. Self-assembly reduces the entropy of the system, ΔS < 0, so to make the process spontaneous, meaning to make the process have the negative Gibbs energy:

ΔG = ΔH - TΔS < 0,

the following must be true ΔH < 0 and |ΔH| > |TΔS|. In other words, self-assembly is accompanied with the release of large amounts of heat. The main driver underpinning self-assembly is the tendency of chemical systems to lower the Gibbs energy by forming new chemical bonds, the enthalpy effect prevails over entropy here. The main classes of supramolecular compounds are cavitands, cryptands, calixarenes, host-guest complexes, rotaxanes, catenanes, and clathrates. Micelles, liposomes and liquid crystals can also be referred to supramolecular structures.

Supramolecular chemistry methods are widely used in chemical analysis, medicine, catalysis and photochemistry. Supramolecular structures form the basis for a great number of modern technologies, such as the extraction of biologically active substances, the creation of photo-and chemosensors, molecular electronic devices, the development of nanocatalysts, the synthesis of materials for nonlinear optics and the simulation of complex biological processes (Biomimetics).

Illustrations

Fig. 1.
Fig. 1. "Host-guest" complexes formed from crown ethers and ions of alkaline metals.
Fig. 2. Molecular recognition using hydrogen bonds.
Fig. 2. Molecular recognition using hydrogen bonds.

Author

  • Eremin Vadim V.

Sources

  1. Lehn J.-M. Supramolecular Chemistry: Concepts and Perspectives. - Morrisville, NC: Wiley-VCH, 1995. - 271 pp.
  2. Steed J. W., Atwood J. L. Supramolecular Chemistry. 2nd Ed. — J. Wiley & Sons: Chichester, 2009. — 745 pp.

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