Host–guest chemistry
In biological systems, the analogous terms of host and guest are commonly referred to as enzyme and substrate respectively.
The interaction between the host and guest involves purely van der Waals bonding.
Inclusion Compound: A complex in which one component (the host) forms a cavity or, in the case of a crystal, a crystal lattice containing spaces in the shape of long tunnels or channels in which molecular entities of a second chemical species (the guest) are located.
There is no covalent bonding between guest and host, the attraction being generally due to van der Waals forces.
[10] Yet another related class of compounds are clathrates, which often consisting of a lattice that traps or contains molecules.
[13] An important implication of encapsulation (and host-guest chemistry in general) is that the guest behaves differently from the way it would when in solution.
[16][17] Large metalla-assemblies, known as metallaprisms, contain a conformationally flexible cavity that allows them to host a variety of guest molecules.
For instance, excited state reactivity of free 1-phenyl-3-tolyl-2-proponanone (abbreviated A-CO-B) yields products A-A, B-B, and AB, which result from decarbonylation followed by random recombination of radicals A• and B•.
The original definition proposed by Cram includes many classes of molecules: cyclodextrins, calixarenes, pillararenes and cucurbiturils.
[21] Cyclodextrin (CD) are tubular molecules composed of several glucose units connected by ether bonds.
The oxygen atoms are located along the edges of the band and are tilted inwards, forming a partly enclosed cavity (cavitand).
Due to the phenyl groups and aliphatic chains, the cages inside cryptophanes are highly hydrophobic, suggesting the capability of capturing non-polar molecules.
Based on this, cryptophanes can be employed to capture xenon in aqueous solution, which could be helpful in biological studies.
Crown ethers can also be threaded with slender linear molecules and/or polymers, giving rise to supramolecular structures called rotaxanes.
[11] Silica clathrasil are compounds structurally similar to clathrate hydrates with a SiO2 framework and can be found in a range of marine sediment.
Thermoelectric materials follow a design strategy called the phonon glass electron crystal concept.
When the guest and host framework are appropriately tuned, clathrates can exhibit low thermal conductivity, i.e., phonon glass behavior, while electrical conductivity through the host framework is undisturbed allowing clathrates to exhibit electron crystal.
These materials crystallize with small aromatic guests (benzene, certain xylenes), and this selectivity has been exploited commercially for the separation of these hydrocarbons.
Urea, a small molecule with the formula O=C(NH2)2, has the peculiar property of crystallizing in open but rigid networks.
Ribbons of hydrogen-bonded urea molecules form tunnel-like host into which many organic guests bind.
With nuclear magnetic resonance (NMR) spectra the observed chemical shift value, δ, arising from a given atom contained in a reagent molecule and one or more complexes of that reagent, will be the concentration-weighted average of all shifts of those chemical species.
is the molar absorbance (also known as the extinction coefficient) of the ith chemical species at the wavelength λ, ci is its concentration.
Additionally, Zhu et al used crown ether and potassium ion to modify the polymer, and enhance the emission of phosphorescence.
While these concepts are well established in biological systems, the applications of synthetic host-guest chemistry remains mostly in the realm of aspiration.
When the visible light was shined on the material, the merocyanine close to the pillar[5]arene-free pentanenitrile complex had faster transformation to spiropyran; on the contrary, the one close to pillar[5]arene-grafted pentanenitrile complex has much slower transformation rate.
[41] Silicon surfaces functionalized with tetraphosphonate cavitands have been used to singularly detect sarcosine in water and urine solutions.
[42] Traditionally, chemical sensing has been approached with a system that contains a covalently bound indicator to a receptor though a linker.
[43] In contrast to ISR, indicator-displacement assay (IDA) utilizes a non-covalent interaction between a receptor (the host), indicator, and an analyte (the guest).
For example, protamine is a coagulant that is routinely administered after cardiopulmonary surgery that counter acts the anti-coagulant activity of herapin.
[47][48] A crystalline solid has been traditionally viewed as a static entity where the movements of its atomic components are limited to its vibrational equilibrium.
