Host–guest chemistry

In supramolecular chemistry, host–guest chemistry describes complexes that are composed of two or more molecules or ions that are held together in unique structural relationships by forces other than those of full covalent bonds. Host–guest chemistry therefore encompasses the idea of molecular recognition and interactions through non-covalent bonding, which is critical in maintaining the three-dimensional structure of large molecules, such as proteins, and is involved in many biological processes in which large molecules bind specifically but transiently to one another.

Although non-covalent interactions could be roughly divided into those with more electrostatic or dispersive contributions, there are a few commonly mentioned types of non-covalent interactions: ionic bonding, hydrogen bonding, van der Waals forces and hydrophobic interactions.

Host–guest interaction has raised significant attention since it was discovered. Many biological processes and material designs require the host–guest interaction. There are several typical host molecules, such as cyclodextrin and crown ether.

thumbnail|200px|Crystal structure of a host–guest complex with a p-xylylenediammonium bound within a [[cucurbituril

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thumbnail|200px|A guest [[nitrogen|N2 is bound within a host hydrogen-bonded capsule

]]

Host molecules usually have a pore-like structure that is able to capture a guest molecule. Although called molecules, hosts and guests are often ions. The driving forces of the interaction vary, such as hydrophobic effect and van der Waals forces.

Binding between host and guest can be highly selective, in which case the interaction is called molecular recognition. Often, a dynamic equilibrium exists between the unbound and the bound stating:

<math display="block">H + G\ \rightleftharpoons\ HG</math>

where <math>H</math> denotes "host", <math>G</math> denotes "guest", and <math>HG</math> denotes "host–guest complex".

The host component is often the larger molecule, and it encloses the smaller guest molecule. In biological systems, the terms of host and guest are commonly referred to as enzyme and substrate respectively.

Inclusion and clathrate compounds

thumb|Cd(CN)2·CCl4: [[Cadmium cyanide clathrate framework (in blue) containing carbon tetrachloride (C atoms in gray and disordered Cl positions in green)]]

Closely related to host–guest chemistry are inclusion compounds (also known as an inclusion complexes). Here, a chemical complex in which one chemical compound (the "host") has a cavity into which a "guest" compound can be accommodated. The interaction between the host and guest involves van der Waals bonding. The definition of inclusion compounds is very broad, extending to channels formed between molecules in a crystal lattice in which guest molecules can fit.

Another related class of compounds are clathrates, which often consist of a lattice that traps or contains molecules. The word clathrate is derived from the Latin (), meaning 'with bars, latticed'.

Molecular encapsulation

Molecular encapsulation concerns the confinement of a guest within a larger host. In some cases, true host–guest reversibility is observed. In other cases, the encapsulated guest cannot escape.

thumbnail|200px|Molecular encapsulation of a nitrobenzene bound within a [[hemicarcerand]]

An important implication of encapsulation (and host–guest chemistry in general) is that the guest behaves differently than when in solution. Guest molecules that would react by bimolecular pathways are often stabilized because they cannot combine with other reactants. Compounds, like cyclobutadiene, arynes or cycloheptatetraene, that are normally highly unstable in solution have been isolated at room temperature when molecularly encapsulated. Large metalla-assemblies, known as metallaprisms, contain a conformationally flexible cavity that allows them to host a variety of guest molecules. These assemblies have shown promise as agents of drug delivery to cancer cells.

Encapsulation can control reactivity. 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•. Whereas, the same substrate upon encapsulation reacts to yield the controlled recombination product A-B, and rearranged products (isomers of A-CO-B).

Macrocyclic hosts

Organic hosts are occasionally called cavitands. The original definition proposed by Cram includes many classes of molecules: cyclodextrins, calixarenes, pillararenes and cucurbiturils.

Calixarenes

Calixarenes and related formaldehyde-arene condensates (resorcinarenes and pyrogallolarenes) form a class of hosts that form inclusion compounds. Pillararenes (pillered arenes) are a related family of formaldehyde-derived oligomeric rings. One famous illustration of the stabilizing effect of host–guest complexation is the stabilization of cyclobutadiene by such an organic host.

Cyclodextrins and cucurbiturils

[[File:Pillar5arene Feb2013.png|thumb|Chemical structure of pillar[5]arene]]

Cyclodextrins (CDs) are tubular molecules composed of several glucose units connected by ether bonds. The three kinds of CDs--α-CD (six units), β-CD (seven units), and γ-CD (eight units)--differ in their cavity sizes: 5, 6, and 8 Å, respectively. α-CD can thread onto one PEG chain, while γ-CD can thread onto two PEG chains. β-CD can bind with thiophene-based molecules.

Cucurbiturils are macrocyclic molecules made of glycoluril () monomers, linked by methylene bridges (). The oxygen atoms are located along the edges of the band and are tilted inwards, forming a partly enclosed cavity (cavitand). Cucurbit[n]urils have similar size of γ-CD, which also behave similarly (e.g., one cucurbit[n]uril can thread onto two PEG chains). This capsule is made of two halves, like a plastic easter egg (Figure 6). Salt bridge interactions between the two halves cause them to self-assemble in solution (Figure 7). They are stable even when heated to 60 °C.

Polymeric hosts

Zeolites have open framework structures with cavities where guest species can reside. Zeolites are rigid due to Aluminosilicates being their composition. Many structures are known, some of which are used as catalysts and for separations.

Clathrate compounds, with formula A8B16X30, where A is an alkaline earth metal, B is a group III element, and X is an element from group IV, have been explored for thermoelectric devices. Thermoelectric materials follow a design strategy called the phonon glass electron crystal concept. Low thermal conductivity and high electrical conductivity is desired to produce the Seebeck Effect. 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.

Hofmann clathrates are coordination polymers, with the formula Ni(CN)4·Ni(NH3)2(arene). These materials crystallize with small aromatic guests (benzene, certain xylenes), and this selectivity has been commercially exploited for the separation of these hydrocarbons. Several other organic molecules form clathrates: thiourea, hydroquinone, and Dianin's compound. In this circumstance, α-CD and CB can be used, in which the phosphor serves as a guest to interact with the host. For example, when 4-phenylpyridium derivatives interacted with CB, and copolymerized with acrylamide, the resulting polymer yielded ~2 s of phosphorescence lifetime. Additionally, Zhu et al. used crown ether and potassium ions to modify the polymer and enhance the emission of phosphorescence.

Another technique for evaluating host–guest interactions is calorimetry.

Aspiration applications

Host guest complexation is pervasive in biochemistry. Many protein hosts recognize and hence selectively bind other biomolecules. When the protein host is an enzyme, the guests are called substrates.

Self-healing

thumb|308x308px|Self-healing mechanism of host–guest interaction by a) using host–small-guest molecule and b) host–polymer. Redrawn from source material.

A self-healing hydrogel can be constructed from modified cyclodextrin and adamantane. Another strategy is to use the interaction between the polymer backbone and host molecule (host molecule threading onto the polymer). If the threading process is fast enough, self-healing can also be achieved.

Photolytically sensitive caged compounds have been examined as containers for releasing drugs or reagents.

Encryption

An encryption system constructed by pillar[5]arene, spiropyran and pentanenitrile (free state and grafted to polymer) was created by Wang et al.. After UV irradiation, spiropyran transforms into merocyanine. When 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. This spiropyran–merocyanine transformation can be used for message encryption. Another strategy is based on the metallacages and polycyclic aromatic hydrocarbons. Because of the fluorescence emission differences between the complex and the cages, the information could be encrypted.

Mechanical properties

Although some host–guest interactions are not strong, increasing the amount of the host–guest interaction can improve the mechanical properties of the materials. As an example, threading the host molecules onto the polymer is one of the commonly used strategies for increasing the mechanical properties of the polymer. It takes time for the host molecules to de-thread from the polymer, which can be a way of energy dissipation. Another method is to use the slow exchange host–guest interaction. Though the slow exchange improves the mechanical properties, simultaneously, self-healing properties will be sacrificed.

Sensing

Silicon surfaces functionalized with tetraphosphonate cavitands have been used to singularly detect sarcosine in water and urine solutions.

Traditionally, chemical sensing has been approached with a system that contains a covalently bound indicator to a receptor though a linker. Once the analyte binds, the indicator changes color or fluoresces. This technique is called the indicator–spacer–receptor approach (ISR). In contrast to ISR, indicator-displacement assay (IDA) utilizes a non-covalent interaction between a receptor (the host), indicator, and an analyte (the guest). Similar to ISR, IDA also utilizes colorimetric (C-IDA) and fluorescence (F-IDA) indicators. In an IDA assay, a receptor is incubated with the indicator. When the analyte is added to the mixture, the indicator is released to the environment. Once the indicator is released it either changes color (C-IDA) or fluoresces (F-IDA).

thumb|300px|right|Types of chemosensors: (1) indicator–spacer–receptor (ISR), (2) indicator-displacement assay (IDA)

IDA offers several advantages versus the traditional ISR chemical sensing approach. First, it does not require the indicator to be covalently bound to the receptor. Secondly, since there is no covalent bond, various indicators can be used with the same receptor. Lastly, the media in which the assay may be used is diverse.

thumb|175px|left|Indicator-displacement assay indicators: (1) azure A, (2) thiazole orange

Chemical sensing techniques such as C-IDA have biological implications. For example, protamine is a coagulant that is routinely administered after cardiopulmonary surgery that counteracts the anti-coagulant activity of heparin. In order to quantify the protamine in plasma samples, a colorimetric displacement assay is used. Azure A dye is blue when it is unbound, but when it is bound to heparin it shows a purple color. The binding between Azure A and heparin is weak and reversible. This allows protamine to displace Azure A. Once the dye is liberated it displays a purple color. The degree to which the dye is displaced is proportional to the amount of protamine in the plasma.

F-IDA has been used by Kwalczykowski and co-workers to monitor the activities of helicase in E. coli. In this study they used thiazole orange as the indicator. The helicase unwinds the dsDNA to make ssDNA. The fluorescence intensity of thiazole orange has a greater affinity for dsDNA than ssDNA and its fluorescence intensity is higher when it is bound to dsDNA than when it is unbound.

Conformational switching

A crystalline solid has been traditionally viewed as a static entity where the movements of its atomic components are limited to its vibrational equilibrium. As seen by the transformation of graphite to diamond, solid to solid transformation can occur under physical or chemical pressure. It has been proposed that the transformation from one crystal arrangement to another occurs in a cooperative manner. Most of these studies have been focused in studying an organic or metal-organic framework. In addition to studies of macromolecular crystalline transformation, there are also studies of single-crystal molecules that can change their conformation in the presence of organic solvents. An organometallic complex has been shown to morph into various orientations depending on whether it is exposed to solvent vapors or not.

Environmental applications

Host guest systems have been proposed to remove hazardous materials. Certain calix[4]arenes bind cesium-137 ions, which could in principle be applied to clean up radioactive wastes. Some receptors bind carcinogens.

Alcohol

According to food chemist Udo Pollmer of the European Institute of Food and Nutrition Sciences in Munich, alcohol can be molecularly encapsulated in cyclodextrines, a sugar derivate. In this way, encapsuled in small capsules, the fluid can be handled as a powder. The cyclodextrines can absorb an estimated 60 percent of their own weight in alcohol. A US patent has been registered for the process as early as 1974.