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Host–guest chemistry

From Wikipedia, the free encyclopedia

In supramolecular chemistry,[1] 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 encompasses the idea of molecular recognition and interactions through non-covalent bonding. Non-covalent bonding is critical in maintaining the 3D 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 few commonly mentioned types of non-covalent interactions: ionic bonding, hydrogen bonding, van der Waals forces and hydrophobic interactions.[2]

Host-guest interaction has raised dramatical attention since it was discovered. It is an important field, because many biological processes require the host-guest interaction, and it can be useful in some material designs. There are several typical host molecules, such as, cyclodextrin, crown ether, et al.

Crystal structure of a host–guest complex with a p-xylylenediammonium bound within a cucurbituril [3]
A guest N2 is bound within a host hydrogen-bonded capsule [4]

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 purely 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.

IUPAC definition

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.[5]

Overview

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One is the "host molecules", which usually have "pore-like" structure that is able to capture some other molecules. The other one is the "guest molecules", which are generally smaller than the host molecules, and capable of binding the host molecules. The driving forces of the interaction might vary, such as hydrophobic effect, chelate effect, van der Waals force, et al.[6] Different bindings will provide variant properties for the materials, i.e., stimuli-responsiveness, self-healing, matrix rigidification. As a consequence, the host-guest interaction can be applied for self-healing materials, stimuli-responsive materials, room-temperature phosphorescence (RTP), improvement of mechanical properties, et al. The sizes of the host and guest molecules play an essential role in the interactions, and some typical examples of the host interactions will be discussed as follows.[6][7][8][9]

Host–guest chemistry is a branch of supramolecular chemistry in which a host molecule binds a so-called guest molecule or ion. The two components of the complex interact by non-covalent forces, most commonly by hydrogen-bonding. Binding between host and guest can be highly selective, in which case the interaction is called molecular recognition. Often, a dynamic equilibrium exist between the unbound and the bound states:

H ="host", G ="guest", HG ="host–guest complex"

The "host" component is often the larger molecule, and it encloses the smaller, "guest", molecule. In biological systems, the analogous terms of host and guest are commonly referred to as enzyme and substrate respectively.[10]

Main types of macrocyclic hosts

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Calixarenes

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Calixarenes and related formaldehyde-arene condensates are one class of hosts that form inclusion compounds. One famous illustration is the adduct with cyclobutadiene, which otherwise is unstable.[11]

Cyclodextrins

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Cyclodextrin (CD) are tubular molecules composed of several glucose units connected by ether bonds. The three kinds of CDs, α-CD (6 units), β-CD (7 units), and γ-CD (8 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 2 PEG chains. β-CD can bind with thiophene-based molecule.[6]

Cyclodextrins are well established hosts for the formation of inclusion compounds.[1][2][3] Illustrative is the case of ferrocene which is inserted into the cyclodextrin at 100 °C under hydrothermal conditions.[12]

Cyclodextrin also forms inclusion compounds with fragrances. As a result, the fragrance molecules have a reduced vapor pressure and are more stable towards exposure to light and air. When incorporated into textiles the fragrance lasts much longer due to the slow-release action.[13]

Cryptophanes

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a) Structure of Cryptophanes. b) Structure of Resorcinarenes and Pyrogallolarenes. c) Structure of cucurbit[n]urils. Redrawn from.[6]

The structure of cryptophanes contain 6 phenyl rings, mainly connected in 4 ways . 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.[6]

Resorcinarenes and similar molecules

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One of the classic structures of resorcinarenes and pyrogallolarenes is shown below. Because of the phenol group, some hydrogen bonds are foromed among the molecules. Sometimes, the binding ratio of the host and guest could reach 2 : 1.[6] Cucurbit[n]urils have similar size of γ-CD, which also behave similarly (e.g., 1 cucurbit[n]uril can thread onto 2 PEG chains).[6]

Crown ethers and cryptands

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a) Structure of 18-crown-6. b) Threading of crown ether and 1,2,3-triazole (rotaxane). Redrawn from [3]. c) Inclusion of a-CD and polyethylene glycol (PEG) d) Threading of b-cyclodextrin and thiophene-based molecule. Redrawn from.[6]

Crown ethers are well known for their ability to bind metal caions. For example, 12-crown-4, 15-crown-5, 18-crown-6, 21-crown-7, and 24-crown-8 interact with potassium, sodium, ammonium, and calcium ions, respectively.[6] Beyond ionic guest, crown ethers also bind to some neutral molecules, e.g., 1, 2, 3- triazole. Crown ethers can also be threaded with slender linear molecules and/or polymers, giving rise to supramolecular structures called rotaxanes. Given that the crown ethers are not bound to the chains, they can move up and down the threading molecule.[9] Crown ether complexes (and those of Cryptands) are not however consider to be inclusion complexes since the guest is bound by forces stronger than van der Waals bonding.

Thermodynamics of host-guest interactions

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When the host and guest molecules combine to form a single complex, the equilibrium is represented as

and the equilibrium constant, K, is defined as

where [X] denotes the concentration of a chemical species X (all activity coefficients are assumed to have a numerical values of 1). The mass-balance equations, at any data point,

where and represent the total concentrations, of host and guest, can be reduced to a single quadratic equation in, say, [G] and so can be solved analytically for any given value of K. The concentrations [H] and [HG] can then derived.

The next step in the calculation is to calculate the value, , of a quantity corresponding to the quantity observed . Then, a sum of squares, U, over all data points, np, can be defined as

and this can be minimized with respect to the stability constant value, K, and a parameter such the chemical shift of the species HG (nmr data) or its molar absorbency (uv/vis data). This procedure is applicable to 1:1 adducts.

Experimental techniques

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Set of NMR spectra from a host–guest titration
Typical ultraviolet–visible spectra for a host–guest system

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. Chemical exchange is assumed to be rapid on the NMR time-scale.

Using UV-vis spectroscopy, the absorbance of each species is proportional to the concentration of that species, according to the Beer–Lambert law.

where λ is a wavelength, is the optical path length of the cuvette which contains the solution of the N compounds (chromophores), is the molar absorbance (also known as the extinction coefficient) of the ith chemical species at the wavelength λ, ci is its concentration. When the concentrations have been calculated as above and absorbance has been measured for samples with various concentrations of host and guest, the Beer–Lambert law provides a set of equations, at a given wavelength, that which can be solved by a linear least-squares process for the unknown extinction coefficient values at that wavelength.

Other techniques includ fluorescent intensity and calorimetry

Aspiration applications

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Self-healing

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Self-healing mechanism of host-guest interaction by a) using host-small guest molecule and b) host-polymer. Redrawn from [14][15]

A self-healing hydrogel constructed from modified cyclodextrin and adamantane .[14][16] 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.[15]

Room-temperature phosphorescence

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Host-guest structures can provide a rigid matrix that protects emitters from being quenched, extending the lifetime of phosphoresce.[17] In this circumstance, α-CD and CB could be used,[18][19] in which the phosphor is served as a guest to interact with the host. For example, 4-phenylpyridium derivatives interacted with CB, and copolymerize with acrylamide. The resulting polymer yielded ~2 s of phosphorescence lifetime. Additionally, Zhu et al used crown ether and potassium ion to modify the polymer, and enhance the emission of phosphorescence.[20]

Stimuli-responsive materials

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Many guest molecules are photo-responsive.[21]

Encryption

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An encryption system constructed by pillar[5]arene, spiropyran and pentanenitrile (free state and grafted to polymer) was constructed by Wang et al. After UV irradiation, spiropyran would transform into merocyanine. 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. This spiropyran-merocyanine transformation can be used for message encryption.[22] Another strategy is based on the metallacages and polycyclic aromatic hydrocarbons.[23] Because of the fluorescnece emission differences between the complex and the cages, the information could be encrypted.

Mechanical property

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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.[16][24][25] 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.[26]

Sensing

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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).[27] 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).[28]

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.[29]

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 counter acts the anti-coagulant activity of herapin. 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 herapin, 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.[30]

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 increases when it is bound to dsDNA than when it is unbound.[31][32]

Conformational switching

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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.[33][34] Most of these studies have been focused in studying an organic or metal-organic framework.[35][36] 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.[37]

Environmental applications

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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 binds carcinogens.[38][39]

References

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