Host–guest Chemistry
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In
supramolecular chemistry Supramolecular chemistry refers to the branch of chemistry concerning Chemical species, chemical systems composed of a integer, discrete number of molecules. The strength of the forces responsible for spatial organization of the system range from w ...
, host–guest chemistry describes complexes that are composed of two or more
molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s or
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s that are held together in unique structural relationships by forces other than those of full
covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
s. 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 Ionic bonding is a type of chemical bonding that involves the Coulomb's law, electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in io ...
,
hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...
ing,
van der Waals forces In molecular physics and chemistry, the van der Waals force (sometimes van der Waals' force) is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical ele ...
and
hydrophobic interactions The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and to be excluded by water. The word hydrophobic literally means "water-fearing", and it describes the segregation of water and nonpolar ...
. Host-guest interaction has raised significant 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''. "Host molecules" usually have "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 might 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 Supramolecular chemistry refers to the branch of chemistry concerning Chemical species, chemical systems composed of a integer, discrete number of molecules. The strength of the forces responsible for spatial organization of the system range from w ...
. Often, a
dynamic equilibrium In chemistry, a dynamic equilibrium exists once a reversible reaction occurs. Substances initially transition between the reactants and products at different rates until the forward and backward reaction rates eventually equalize, meaning the ...
exists between the unbound and the bound states: :H + G \rightleftharpoons\ HG :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 An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
and
substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...
respectively.


Inclusion and clathrate compounds

Closely related to host–guest chemistry are inclusion compounds (also known as an inclusion complexes). Here, a chemical complex in which one
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
(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. Yet another related class of compounds are
clathrate A clathrate is a chemical substance consisting of a lattice (group), lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin language, Latin (), meaning 'with bars, Crystal structure, latticed'. Most clathrate ...
s, which often consist of a lattice that traps or contains molecules.Atwood, J. L. (2012) "Inclusion Compounds" in ''Ullmann's Encyclopedia of Industrial Chemistry''. Wiley-VCH, Weinheim. The word ''clathrate'' is derived from the
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
(), 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. 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. Guest molecules that would react by bimolecular pathways are often stabilized because they cannot combine with other reactants. The spectroscopic signatures of trapped guests are of fundamental interest. Compounds normally highly unstable in solution have been isolated at room temperature when molecularly encapsulated. Examples include
cyclobutadiene Cyclobutadiene is an organic compound with the formula . It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is ...
,
aryne In organic chemistry, arynes and benzynes are a class of highly Reactivity (chemistry), reactive chemical Chemical species, species derived from an aromatic ring by removal of two substituents. Arynes are examples of didehydroarenes (1,2-didehydro ...
s or cycloheptatetraene. Large metalla-assemblies, known as
metallaprism Coordination cages are three-dimensional ordered structures in solution that act as hosts in host–guest chemistry. They are self-assembled in solution from organometallic precursors, and often rely solely on noncovalent interactions rather than co ...
s, 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 Cram may refer to: * Cram (surname), a surname, and list of notable persons having the surname * Cram.com, a website for creating and sharing flashcards * ''Cram'' (Australian game show), a television show * ''Cram'' (game show), a TV game show ...
includes many classes of molecules:
cyclodextrins Cyclodextrins are a family of cyclic oligosaccharides, consisting of a macrocyclic ring of glucose subunits joined by α-1,4 glycosidic bonds. Cyclodextrins are produced from starch by enzymatic conversion. They are used in food, pharmaceuti ...
,
calixarene A calixarene is a macrocycle or cyclic oligomer based on a methylene-linked phenols. With hydrophobic cavities that can hold smaller molecules or ions, calixarenes belong to the class of cavitands known in host–guest chemistry. Nomenclature ...
s, pillararenes and
cucurbituril In host–guest chemistry, 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 ...
s.


Calixarenes

Calixarene A calixarene is a macrocycle or cyclic oligomer based on a methylene-linked phenols. With hydrophobic cavities that can hold smaller molecules or ions, calixarenes belong to the class of cavitands known in host–guest chemistry. Nomenclature ...
s and related formaldehyde-arene condensates ( resorcinarenes and pyrogallolarenes) form a class of hosts that form inclusion compounds. A related family of formaldehyde-derived oligomeric rings are pillararenes (pillered arenes). One famous illustration of the stabilizing effect of host-guest complexation is the stabilization of
cyclobutadiene Cyclobutadiene is an organic compound with the formula . It is very reactive owing to its tendency to dimerize. Although the parent compound has not been isolated, some substituted derivatives are robust and a single molecule of cyclobutadiene is ...
by such an organic host.


Cyclodextrins and cucurbiturils

Cyclodextrin Cyclodextrins are a family of cyclic oligosaccharides, consisting of a macrocycle, macrocyclic ring of glucose subunits joined by α-1,4 glycosidic bonds. Cyclodextrins are produced from starch by enzyme, enzymatic conversion. They are used in ...
s (CDs) 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. Cyclodextrins are well established hosts for the formation of inclusion compounds. Illustrative is the case of
ferrocene Ferrocene is an organometallic chemistry, organometallic compound with the formula . The molecule is a Cyclopentadienyl complex, complex consisting of two Cyclopentadienyl anion, cyclopentadienyl rings sandwiching a central iron atom. It is an o ...
which is inserted into the cyclodextrin at 100 °C under hydrothermal conditions.
Cucurbituril In host–guest chemistry, 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 ...
s are macrocyclic molecules made of glycoluril ()
monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...
s linked by
methylene bridge In chemistry, a methylene bridge is part of a molecule with formula . The carbon atom is connected by single bonds to two other distinct atoms in the rest of the molecule. A methylene bridge is often called a methylene group or simply methylene, ...
s (). The oxygen atoms are located along the edges of the band and are tilted inwards, forming a partly enclosed cavity ( cavitand). Cucurbit rils have similar size of γ-CD, which also behave similarly (''e.g.'', 1 cucurbit ril can thread onto 2 PEG chains).


Cryptophanes

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.


Crown ethers and cryptands

Crown ether In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (). The most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., . Impor ...
s bind cations. Small crown ethers, e.g. 12-crown-4 bind well to small ions such as Li+ and large crowns, such as 24-crown-8 bind better to larger ions. Beyond binding ionic guests, 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
rotaxane A rotaxane () is a mechanically interlocked molecular architecture consisting of a dumbbell-shaped molecule which is threaded through a macrocycle (see graphical representation). The two components of a rotaxane are kinetically trapped since ...
s. Given that the crown ethers are not bound to the chains, they can move up and down the threading molecule. Crown ether complexes of metal cations (and the corresponding complexes of
Cryptand In chemistry, cryptands are a family of synthetic, bicyclic and polycyclic, multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for ...
s) are not considered to be inclusion complexes since the guest is bound by forces stronger than van der Waals bonding.


Polymeric hosts

Zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a meta ...
s have open framework structures with cavities in which guest species can reside.
Aluminosilicate Aluminosilicate refers to materials containing anionic Si-O-Al linkages. Commonly, the associate cations are sodium (Na+), potassium (K+) and protons (H+). Such materials occur as minerals, coal combustion products and as synthetic materials, of ...
s being their composition, zeolites are rigid. Many structures are known, some of which are considerably useful as catalysts and for separations. Silica clathrasil are compounds structurally similar to clathrate hydrates with a SiO2 framework and can be found in a range of marine sediment. Clathrate compounds with formula ''A''8''B''16''X''30, where ''A'' is an
alkaline earth metal The alkaline earth metals are six chemical elements in group (periodic table), group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar p ...
, ''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 The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...
and high electrical conductivity is desired to produce the
Seebeck Effect The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
. 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 In inorganic chemistry, Hofmann clathrates refers to materials with the formula Ni(CN)2(NH3)(C6H6). These materials are a type of coordination polymer that have properties of inclusion compounds. They have attracted attention because they can be ...
are
coordination polymer Coordination may refer to: * Coordination (linguistics), a compound grammatical construction * Coordination complex, consisting of a central atom or ion and a surrounding array of bound molecules or ions ** A chemical reaction to form a coordinat ...
s 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 exploited commercially for the separation of these hydrocarbons. Metal organic frameworks (MOFs) form clathrates.
Urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...
, a small molecule with the formula , has the peculiar property of crystallizing in open but rigid networks. The cost of efficient molecular packing is compensated by hydroge-bonding. Ribbons of hydrogen-bonded urea molecules form tunnel-like host into which many organic guests bind. Urea-clathrates have been well investigated for separations. Beyond urea, several other organic molecules form clathrates:
thiourea Thiourea () is an organosulfur compound with the formula and the structure . It is structurally similar to urea (), with the oxygen atom replaced by sulfur atom (as implied by the '' thio-'' prefix). The properties of urea and thiourea differ s ...
,
hydroquinone Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. It has two hydroxyl groups bonded to a benzene ring in a ''para' ...
, and Dianin's compound.


Thermodynamics of host-guest interactions

When the host and guest molecules combine to form a single complex, the equilibrium is represented as :H + G \leftrightharpoons HG and the equilibrium constant, K, is defined as :K=\frac where 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, :T_H = + K /math> :T_G = + K /math> where T_G and T_H represent the total concentrations, of host and guest, can be reduced to a single quadratic equation in, say, and so can be solved analytically for any given value of K. The concentrations and Gcan then derived. : = T_H - T_G + /math> : G= K /math> The next step in the calculation is to calculate the value, X^_i, of a quantity corresponding to the quantity observed X^_i. Then, a sum of squares, U, over all data points, np, can be defined as :U=\sum_ (X^_i -X^_i)^2 and this can be minimized with respect to the stability constant value, K, and a parameter such as 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

With
nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
(NMR) spectra the observed
chemical shift In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of ...
value, , 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 Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative log ...
of each species is proportional to the concentration of that species, according to the
Beer–Lambert law The Beer–Bouguer–Lambert (BBL) extinction law is an empirical relationship describing the attenuation in intensity of a radiation beam passing through a macroscopically homogenous medium with which it interacts. Formally, it states that the ...
. :A_\lambda = \ell\sum_^N \varepsilon_ c_i where λ is a wavelength, \ell is the optical path length of the cuvette which contains the solution of the ''N'' compounds (
chromophore A chromophore is the part of a molecule responsible for its color. The word is derived . The color that is seen by our eyes is that of the light not Absorption (electromagnetic radiation), absorbed by the reflecting object within a certain wavele ...
s), \varepsilon_ is the molar absorbance (also known as the extinction coefficient) of the ''i''th chemical species at the wavelength λ, ''c''i 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 can be solved by a linear least-squares process for the unknown extinction coefficient values at that wavelength. Host-guest structures can be probed by their luminescence. A rigid matrix protects emitters from being quenched, extending the lifetime of phosphorescence. In this circumstance, α-CD and CB could be used, 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 Acrylamide (or acrylic amide) is an organic compound with the chemical formula CH2=CHC(O)NH2. It is a white odorless solid, soluble in water and several organic solvents. From the chemistry perspective, acrylamide is a vinyl-substituted primary ...
. 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. Another technique for evaluating host-guest interactions is
calorimetry In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in '' state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reac ...
.


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. While these concepts are well established in biological systems, the applications of synthetic host-guest chemistry remain mostly in the realm of aspiration. One major exception are zeolites where host-guest chemistry is their ''raison d'etre.''


Self-healing

A self-healing hydrogel 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.


Encapsulation and release: fragrances and drugs

Cyclodextrin forms inclusion compounds with
fragrance An aroma compound, also known as an odorant, aroma, fragrance, flavoring or flavor, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficien ...
s which are more stable towards exposure to light and air. When incorporated into textiles the fragrance lasts much longer due to the slow-release action. Photolytically sensitive caged compounds have been examined as containers for releasing a drug or reagent.


Encryption

An
encryption In Cryptography law, cryptography, encryption (more specifically, Code, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the inf ...
system constructed by pillar rene, 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 rene-free pentanenitrile complex had faster transformation to spiropyran; on the contrary, the one close to pillar rene-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 property

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 Sarcosine, also known as ''N''-methylglycine, or monomethylglycine, is a amino acid with the formula CH3N(H)CH2CO2H. It exists at neutral pH as the zwitterion CH3N+(H)2CH2CO2−, which can be obtained as a white, water-soluble powder. Like some ...
in water and
urine Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...
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). 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. Chemical sensing techniques such as C-IDA have biological implications. For example,
protamine Protamines are small, arginine-rich, nuclear proteins that replace histones late in the haploid phase of spermatogenesis and are believed essential for sperm head condensation and DNA stabilization. They may allow for denser packaging of DNA ...
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. F-IDA has been used by Kwalczykowski and co-workers to monitor the activities of
helicase Helicases are a class of enzymes that are vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic double helix, separating the two hybridized ...
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 A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...
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 renes 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 Munich is the capital and most populous city of Bavaria, Germany. As of 30 November 2024, its population was 1,604,384, making it the third-largest city in Germany after Berlin and Hamburg. Munich is the largest city in Germany that is no ...
, alcohol can be molecularly encapsulated in
cyclodextrin Cyclodextrins are a family of cyclic oligosaccharides, consisting of a macrocycle, macrocyclic ring of glucose subunits joined by α-1,4 glycosidic bonds. Cyclodextrins are produced from starch by enzyme, enzymatic conversion. They are used in ...
es, 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 A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an sufficiency of disclosure, enabling discl ...
has been registered for the process as early as 1974.Preparation of an Alcohol Containing Powder
General Foods Corporation March 31, 1972


See also

* Cryptophane


Further reading

* * * * *


References

{{DEFAULTSORT:Host-guest chemistry Supramolecular chemistry Equilibrium chemistry