Supramolecular polymers

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Supramolecular polymers are a kind of polymers whose monomeric units hold together via highly directional and reversible non-covalent interactions. [1] [2] Unlike conventional bonded polymers, supramolecular polymers engage in a variety of non-covalent interactions that define their properties. These interactions include hydrogen bonding, π-π interaction, metal coordination, and host–guest interaction. [1] Owing to the presence of these reversible noncovalent interactions, supramolecular polymers exhibit dynamic properties such as self-healing. [3]

Supramolecular chemistry is the domain of chemistry concerning chemical systems composed of a discrete number of molecules. The strength of the forces responsible for spatial organization of the system range from weak intermolecular forces, electrostatic charge, or hydrogen bonding to strong covalent bonding, provided that the electronic coupling strength remains small relative to the energy parameters of the component. Whereas traditional chemistry concentrates on the covalent bond, supramolecular chemistry examines the weaker and reversible non-covalent interactions between molecules. These forces include hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi–pi interactions and electrostatic effects.

Polymer substance composed of macromolecules with repeating structural units

A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Due to their broad range of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, viscoelasticity, and a tendency to form glasses and semicrystalline structures rather than crystals. The terms polymer and resin are often synonymous with plastic.

A non-covalent interaction differs from a covalent bond in that it does not involve the sharing of electrons, but rather involves more dispersed variations of electromagnetic interactions between molecules or within a molecule. The chemical energy released in the formation of non-covalent interactions is typically on the order of 1-5 kcal/mol. Non-covalent interactions can be classified into different categories, such as electrostatic, π-effects, van der Waals forces, and hydrophobic effects.

Contents

An example of supramolecular polymers with quadruple hydrogen bonds. Fig 1. An example of supramolecular polymers with quadruple hydrogen bonds.png
An example of supramolecular polymers with quadruple hydrogen bonds.

Properties of supramolecular polymers

Self-healing property

Owing to the nature of supramolecular polymers, the noncovalent interactions make supramolecular polymers more dynamic and reversible. Such properties enable supramolecular polymers to construct a dynamic and reversible network, which are able to develop self-healing materials based on noncovalent bonds. [5] Compared with self-healing materials based on covalent bonds, these supramolecular polymers-based self-healing materials can restore the initial structure and function of polymers before being exposed to damages, and can also undergo repeating damage-heal process.

Optoelectronic property

To achieve the light-to-charge conversion is the prerequisite step in artificial photosynthesis systems. [6] By incorporating electron donors and electron acceptors into the supramolecular polymers, a number of artificial systems, including photosynthesis system, can be constructed. Owing to the existence of more than one interactions (π-π interaction, hydrogen bonding interaction and the like), electron donor and electron acceptor can be held together in a proper proximity to afford long-lived charge separated states. [6] Then a light-to-charge conversion system with faster photoinduced electron transfer and higher electron-transfer efficiency can be achieved in these artificial polymers. [2] [6]

Photosynthesis Biological process to convert light into chemical energy

Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις, synthesis, "putting together". In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.

An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process.

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Electron acceptors are sometimes mistakenly called electron receptors.

Biocompatible property

It is quite common that biomolecules, such as DNA, [7] protein [8] and the like, come into being through various noncovalent interactions in biological system. Likewise, supramolecular polymers assembles themself via a combination of noncovalent interactions. Such formation manner endows supramolecular polymers with features, being more sensitive to external stimuli and able to render reversibly dynamic changes in structures and functions. [9] By modifying monomeric units of supramolecular polymers with water-soluble pendants, bioactive moieties as well as biomarkers, supramolecular polymers can realize various kinds of functions and applications in biomedical field. [10] At the same time, their reversible and dynamic nature make supramolecular polymers bio-degradable, [11] [12] which surmounts hard-to-degrade issue of covalent polymers and makes supramolecular polymers a promising platform for biomedical applications. Being able to degrade in biological environment lowers potential toxicity of polymers to a great extent and therefore, enhances biocompatibility of supramolecular polymers. [13] [14]

Biomolecule molecule that is produced by a living organism

A biomolecule or biological molecule is a loosely used term for molecules and ions that are present in organisms, essential to some typically biological process such as cell division, morphogenesis, or development. Biomolecules include large macromolecules such as proteins, carbohydrates, lipids, and nucleic acids, as well as small molecules such as primary metabolites, secondary metabolites, and natural products. A more general name for this class of material is biological materials. Biomolecules are usually endogenous but may also be exogenous. For example, pharmaceutical drugs may be natural products or semisynthetic (biopharmaceuticals) or they may be totally synthetic.

DNA Molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses

Deoxyribonucleic acid is a molecule composed of two chains that coil around each other to form a double helix carrying the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms and many viruses. DNA and ribonucleic acid (RNA) are nucleic acids; alongside proteins, lipids and complex carbohydrates (polysaccharides), nucleic acids are one of the four major types of macromolecules that are essential for all known forms of life.

Protein biological molecule consisting of chains of amino acid residues

Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, providing structure to cells and organisms, and transporting molecules from one location to another. Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific three-dimensional structure that determines its activity.

A titin-mimicking supramolecular polymers, with quadruple hydrogen bonding between 2-ureido-4[1H]-pyrimidone (UPy) moieties. Fig 2. A titin-mimicking supramolecular polymers.png
A titin-mimicking supramolecular polymers, with quadruple hydrogen bonding between 2-ureido-4[1H]-pyrimidone (UPy) moieties.

Examples of supramolecular polymers

Three types self-healing materials are illustrated here: hydrogen bonding-based, metal coordination-based and π-π interaction-based self-healing supramolecular polymers.

Hydrogen bonding-based self-healing supramolecular polymers

A bivalent poly(isobutylene)s (PIBs) with barbituric acid functionalized at head and tail. [16] Multiple hydrogen bonding existed between the carbonyl group and amide group of barbituric acid enable it to form a supramolecular network. In this case, the snipped small PIBs-based disks can recover itself from mechanical damage after several-hour contact at room temperature.

Barbituric acid or malonylurea or 6-hydroxyuracil is an organic compound based on a pyrimidine heterocyclic skeleton. It is an odorless powder soluble in water. Barbituric acid is the parent compound of barbiturate drugs, although barbituric acid itself is not pharmacologically active. The compound was first synthesised by Adolf von Baeyer.

Carbonyl group functional group

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containing a carbonyl group is often referred to as a carbonyl compound.

Amide group of chemical substances

An amide, also known as an acid amide, is a compound with the functional group RnE(O)xNR′2. Most common are carboxamides, but many other important types of amides are known, including phosphoramides and sulfonamides. The term amide refers both to classes of compounds and to the functional group within those compounds.

Polymers containing coordination complexes

Taking advantage of coordination interactions between catechol and ferric ions, researchers developed pH-controlled self-healing supramolecular polymers. [17] The formation of mono-, bis- and triscatehchol-Fe3+ complexes can be manipulated by pH, of which the bis- and triscatehchol-Fe3+ complexes show elastic moduli as well as self-healing capacity. For example, the triscatehchol-Fe3+ can restore its cohesiveness and shape after being torn.

Catechol chemical compound

Catechol ( or ), also known as pyrocatechol or 1,2-dihydroxybenzene, is an organic compound with the molecular formula C6H4(OH)2. It is the ortho isomer of the three isomeric benzenediols. This colorless compound occurs naturally in trace amounts. It was first discovered by destructive distillation of the plant extract catechin. About 20 million kg is now synthetically produced annually as a commodity organic chemical, mainly as a precursor to pesticides, flavors, and fragrances.

Ferric chemical compound

Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron(III) or Fe3+. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron(II) or Fe2+. Iron(III) is usually the most stable form of iron in air, as illustrated by the pervasiveness of rust, an insoluble iron(III)-containing material. The word ferric is derived from the Latin word ferrum for iron.

pH measure of the acidity or basicity of an aqueous solution

In chemistry, pH is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the base 10 logarithm of the molar concentration, measured in units of moles per liter, of hydrogen ions. More precisely it is the negative of the base 10 logarithm of the activity of the hydrogen ion. At 25 °C, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. The neutral value of the pH depends on the temperature, being lower than 7 if the temperature increases. Pure water is neutral, pH 7 at (25 °C), being neither an acid nor a base. Contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively.

An example of metal coordination-based self-healing supramolecular polymer. After standing for 24 hours, the cut pieces were rejoined. Fig 3. An example of metal coordination-based self-healing supramolecular polymer. After standing for 24 hours, the cut pieces were rejoined.png
An example of metal coordination-based self-healing supramolecular polymer. After standing for 24 hours, the cut pieces were rejoined.

π-π interaction-based self-healing supramolecular polymers

Chain-folding polyimide and pyrenyl-end-capped chains give rise to supramolecular networks. [19]

Potential biomedical applications

With the excellent nature in biodegradation and biocompatibility, supramolecular polymers show great potential in the development of drug delivery, gene transfection and other biomedical applications. [10]

Drug delivery

Multiple cellular stimuli could induce responses in supramolecular polymers. [9] [20] [10] The dynamic molecular skeletons of supramolecular polymers can be depolymerized when exposing to the external stimuli like pH in vivo. On the basis of this property, supramolecular polymers are capable of being a drug carrier. Making use of hydrogen bonding between nucleobases to induce self-assemble into pH-sensitive spherical micelles.

Gene transfection

Effective and low-toxic nonviral cationic vectors are highly desired in the field of gene therapy. [10] On account of the dynamic and stimuli-responsive properties, supramolecular polymers offer a cogent platform to construct vectors for gene transfection. By combining ferrocene dimer with β-cyclodextrin dimer, a redox-control supramolecular polymers system has been proposed as a vector. In COS-7 cells, this supramolecular polymersic vector can release enclosed DNA upon exposing to hydrogen peroxide and achieve gene transfection. [21]

Others

Rationally designed supramolecular polymers-based polymers can simultaneously meet the requirements of aqueous compatibility, bio-degradability, biocompatibility, stimuli-responsiveness and other strict criterion. [10] Consequently, supramolecular polymers can be applied to the biomedical field as a robust system. Other than applications mentioned above, other important and fascinating biomedical applications, like protein delivery, [22] [23] bio-imaging and diagnosis [24] [25] and tissue engineering, [26] [27] are also well developed.

Related Research Articles

Porphyrin

Porphyrins are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent porphyrin is porphine, a rare chemical compound of exclusively theoretical interest. Substituted porphines are called porphyrins. With a total of 26 π-electrons, of which 18 π-electrons form a planar, continuous cycle, the porphyrin ring structure is often described as aromatic. One result of the large conjugated system is that porphyrins typically absorb strongly in the visible region of the electromagnetic spectrum, i.e. they are deeply colored. The name "porphyrin" derives from the Greek word πορφύρα (porphyra), meaning purple.

Anthraquinone chemical compound

Anthraquinone, also called anthracenedione or dioxoanthracene, is an aromatic organic compound with formula C
14H
8O
2. Several isomers are possible, each of which can be viewed as a quinone derivative. The term anthraquinone, however, almost invariably refers to one specific isomer, 9,10-anthraquinone wherein the keto groups are located on the central ring. It is a building block of many dyes and is used in bleaching pulp for papermaking. It is a yellow highly crystalline solid, poorly soluble in water but soluble in hot organic solvents. For instance, it is almost completely insoluble in ethanol near room temperature but 2.25 g will dissolve in 100 g of boiling ethanol.

Azobenzene Chemical compound

Azobenzene is a chemical compound composed of two phenyl rings linked by a N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of similar compounds. These azo compounds are considered as derivatives of diazene (diimide), and are sometimes referred to as 'diazenes'. The diazenes absorb light strongly and are common dyes.

Molecular recognition

The term molecular recognition refers to the specific interaction between two or more molecules through noncovalent bonding such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, halogen bonding, electrostatic and/or electromagnetic effects. In addition to these direct interactions as well solvent can play a dominant indirect role in driving molecular recognition in solution. The host and guest involved in molecular recognition exhibit molecular complementarity.

Self-assembled monolayer

Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases molecules that form the monolayer do not interact strongly with the substrate. This is the case for instance of the two-dimensional supramolecular networks of e.g. perylenetetracarboxylic dianhydride (PTCDA) on gold or of e.g. porphyrins on highly oriented pyrolitic graphite (HOPG). In other cases the molecules possess a head group that has a strong affinity to the substrate and anchors the molecule to it. Such a SAM consisting of a head group, tail and functional end group is depicted in Figure 1. Common head groups include thiols, silanes, phosphonates, etc.

Supramolecular assembly

A supramolecular assembly or "supermolecule" is a well defined complex of molecules held together by noncovalent bonds. While a supramolecular assembly can be simply composed of two molecules, it is more often used to denote larger complexes of molecules that form sphere-, rod-, or sheet-like species. Micelles, liposomes and biological membranes are examples of supramolecular assemblies. The dimensions of supramolecular assemblies can range from nanometers to micrometers. Thus they allow access to nanoscale objects using a bottom-up approach in far fewer steps than a single molecule of similar dimensions.

Macrocycle any chemical compound having a ring composed of at least several atoms (usually minimum of 9–14 atoms)

Macrocycles are often described as molecules and ions containing twelve or more membered ring. Classical examples include the crown ethers, calixarenes, porphyrins, cyclodextrins. Macrocycles describe a large, mature area of chemistry.

Salt bridge (protein and supramolecular)

In chemistry, a salt bridge is a combination of two non-covalent interactions: hydrogen bonding and ionic bonding. Ion pairing is one of the most important noncovalent forces in chemistry, in biological systems, in different materials and in many applications such as ion pair chromatographyga. It is a most commonly observed contribution to the stability to the entropically unfavorable folded conformation of proteins. Although noncovalent interactions are known to be relatively weak interactions, small stabilizing interactions can add up to make an important contribution to the overall stability of a conformer. Not only are salt bridges found in proteins, but they can also be found in supramolecular chemistry. The thermodynamics of each are explored through experimental procedures to assess the free energy contribution of the salt bridge to the overall free energy of the state.

Stacking (chemistry) attractive, noncovalent interactions between aromatic rings

In chemistry, pi stacking refers to attractive, noncovalent interactions between aromatic rings, since they contain pi bonds. These interactions are important in nucleobase stacking within DNA and RNA molecules, protein folding, template-directed synthesis, materials science, and molecular recognition, although new research suggests that pi stacking may not be operative in some of these applications. Despite intense experimental and theoretical interest, there is no unified description of the factors that contribute to pi stacking interactions.

A resorcinarene is a macrocycle, or a cyclic oligomer, based on the condensation of resorcinol (1,3-dihydroxybenzene) and an aldehyde. Resorcinarenes are a type of calixarene.

Cation–pi interaction noncovalent molecular interaction between the face of an electron-rich π system and an adjacent cation; example of noncovalent bonding between a monopole (cation) and a quadrupole (π system)

Cation–π interaction is a noncovalent molecular interaction between the face of an electron-rich π system (e.g. benzene, ethylene, acetylene) and an adjacent cation (e.g. Li+, Na+). This interaction is an example of noncovalent bonding between a monopole (cation) and a quadrupole (π system). Bonding energies are significant, with solution-phase values falling within the same order of magnitude as hydrogen bonds and salt bridges. Similar to these other non-covalent bonds, cation–π interactions play an important role in nature, particularly in protein structure, molecular recognition and enzyme catalysis. The effect has also been observed and put to use in synthetic systems.

Aurophilicity

In chemistry, aurophilicity refers to the tendency of gold complexes to aggregate via formation of weak metallophilic interactions.

Molecular self-assembly

Molecular self-assembly is the process by which molecules adopt a defined arrangement without guidance or management from an outside source. There are two types of self-assembly. These are intramolecular self-assembly and intermolecular self-assembly. Commonly, the term molecular self-assembly refers to intermolecular self-assembly, while the intramolecular analog is more commonly called folding.

Noel Hush Australian chemist

Professor Noel Sydney Hush AO, DSc, FRS, FNAS, FAA, FRACI, FRSN is an Australian chemist at the University of Sydney.

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 covalent bonds. Coordinate bonds are useful in such supramolecular self-assembly because of their versatile geometries. However, there is controversy over calling coordinate bonds noncovalent, as they are typically strong bonds and have covalent character. The combination of a coordination cage and a guest is a type of inclusion compound. Coordination complexes can be used as "nano-laboratories" for synthesis, and to isolate interesting intermediates. The inclusion complexes of a guest inside a coordination cage show intriguing chemistry as well; often, the properties of the cage will change depending on the guest. Coordination complexes are molecular moieties, so they are distinct from clathrates and metal-organic frameworks.

Two-dimensional polymer

A two-dimensional polymer (2DP) is a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges. This recent definition of 2DP is based on Hermann Staudinger's polymer concept from the 1920s. According to this, covalent long chain molecules ("Makromoleküle") do exist and are composed of a sequence of linearly connected repeat units and end groups at both termini.

Ayyappanpillai Ajayagosh is an organic chemist, academic and the director of the National Institute for Interdisciplinary Science and Technology. He is known for his studies on supramolecular assemblies and light induced sensor systems and is an elected fellow of all the three major Indian science academies viz. the National Academy of Sciences, India, Indian National Science Academy and the Indian Academy of Sciences as well as The World Academy of Sciences. The Council of Scientific and Industrial Research, the apex agency of the Government of India for scientific research, awarded him the Shanti Swarup Bhatnagar Prize for Science and Technology, one of the highest Indian science awards for his contributions to Chemical Sciences in 2007. He received the TWAS Prize of The World Academy of Sciences in 2013.


Metallogels are one-dimensional nanostructured materials, which constitute a growing class in the Supramolecular chemistry field. Non-covalent interactions, such as hydrophobic interactions, π-π interactions, and hydrogen bonding, are among the responsible forces for the formation of those gels from small molecules. However, the main driving force for the formation of a metallogel is the metal-ligand coordination. Once the structure has been established, it resists gravitational force when inverted.

References

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