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When the protein host is an enzyme, the guests are called substrates. While these concepts are well established in biological systems, the applications of synthetic host-guest chemistry remains mostly in the realm of aspiration. One major exception, being zeolites where host-guest chemistry is their raison d'etre.
Crystal structure of a nitrobenzene bound within a hemicarcerand reported by Cram and coworkers in Chem. Commun., 1997, 1303-1304.. In host–guest chemistry, a carcerand (from Latin carcer 'prison') is a host molecule that completely entraps its guest (which can be an ion, atom or other chemical species) so that it will not escape even at high temperatures. [1]
In terms of supramolecular chemistry, chemosensing is an example of host–guest chemistry, where the presence of a guest (the analyte) at the host site (the sensor) gives rise to recognition event (e.g. sensing) that can be monitored in real time.
Static molecular recognition is likened to the interaction between a key and a keyhole; it is a 1:1 type complexation reaction between a host molecule and a guest molecule to form a host–guest complex. To achieve advanced static molecular recognition, it is necessary to make recognition sites that are specific for guest molecules.
In host–guest chemistry the bistable states of molecular switches differ in their affinity for guests. Many early examples of such systems are based on crown ether chemistry. The first switchable host is described in 1978 by Desvergne & Bouas-Laurent [10] [11] who create a crown ether via photochemical anthracene dimerization.
The other strategy is to take advantage of the so-called "host-guest" assembly. In this case, the monomer is designed to link to a "host" molecule, while the host molecule is in charge of forming the ordered network. The host molecule stays intact during the polymerization. Such strategies simplify the synthesis of monomer. [16]
Coordination cages are used to study guest-guest and host–guest interactions and reactions. In some instance, planar aromatic molecules stack inside of metalloprisms, as can be observed by UV-visible spectroscopy. Metal-metal interactions can also be observed. [15] Mixed valence species have also been trapped inside of coordination cages. [15]
Computer models of CB[5], CB[6], and CB[7]. Top row is the view into the cavity and the bottom is the side view. In host–guest chemistry, cucurbiturils are macrocyclic molecules made of glycoluril (=C 4 H 2 N 4 O 2 =) monomers linked by methylene bridges (−CH 2 −).