Cross-link
Encyclopedia
Cross-links are bonds that link one polymer
chain to another. They can be covalent bond
s or ionic bond
s. "Polymer chains" can refer to synthetic polymers or natural polymers (such as protein
s). When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of cross-links to promote a difference in the polymers' physical properties. When "crosslinking" is used in the biological milieu, it refers to the use of a probe to link proteins together to check for protein–protein interactions, as well as other creative cross-linking methodologies.
Cross-linking is used in both synthetic polymer chemistry and in the biological sciences. Although the term is used to refer to the "linking of polymer chains" for both sciences, the extent of crosslinking and specificities of the crosslinking agents vary. Of course, with all science, there are overlaps, and the following delineations are a starting point to understanding the subtleties.
In polymer chemistry, when a synthetic polymer is said to be "cross-linked", it usually means that the entire bulk of the polymer has been exposed to the cross-linking method. The resulting modification of mechanical properties depends strongly on the cross-link density. Low cross-link densities decrease the viscosities of polymer melts. Intermediate cross-link densities transform gummy polymers into materials that have elastomer
ic properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials.
s that are initiated by heat, pressure, change in pH, or radiation. For example, mixing of an unpolymerized or partially polymerized resin
with specific chemicals called crosslinking reagents results in a chemical reaction that forms cross-links. Cross-linking can also be induced in materials that are normally thermoplastic
through exposure to a radiation source, such as electron beam exposure, gamma-radiation, or UV light. For example, electron beam processing
is used to cross-link the C type of cross-linked polyethylene
. Other types of cross-linked polyethylene are made by addition of peroxide during extruding (type A) or by addition of a cross-linking agent (e.g. vinylsilane
) and a catalyst during extruding and then performing a post-extrusion curing.
The chemical process of vulcanization
is a type of cross-linking and it changes the property of rubber
to the hard, durable material we associate with car and bike tire
s.
This process is often called sulfur curing, and the term vulcanization
comes from Vulcan
, the Roman
god of fire. However, this is a slow process taking around 8 hours. A typical car tire is cured for 15 minutes at 150°C. However, the time can be reduced by the addition of accelerators such as 2-benzothiazolethiol or tetramethylthiuram disulfide. Both of these contain a sulfur atom in the molecule that initiates the reaction of the sulfur chains with the rubber. Accelerators increase the rate of cure by catalysing the addition of sulfur chains to the rubber molecules.
Cross-links are the characteristic property of thermosetting plastic
materials. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Especially in the case of commercially used plastics, once a substance is cross-linked, the product is very hard or impossible to recycle. In some cases, though, if the cross-link bonds are sufficiently different, chemically, from the bonds forming the polymers, the process can be reversed. Permanent wave
solutions, for example, break and re-form naturally occurring cross-links (disulfide bond
s) between protein chains in hair
.
s cannot be recycled easily. A class of polymers known as thermoplastic elastomer
s rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as snowmobile
tracks, and catheter
s for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains that act as cross-links are reversible, so can be reformed by heat. The stabilising domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters.
The aforementioned process of applying a permanent wave to hair is one example of oxidative cross-linking. In that process the disulfide bonds are reduced, typically using a mercaptan such as ammonium thioglycolate. Following this, the hair is curled and then 'neutralized'. The neutralizer is typically a basic solution of hydrogen peroxide, which causes new disulfide bonds to form under conditions of oxidation, thus permanently fixing the hair into its new configuration.
s (a type of natural polymer) can be cross-linked together using small-molecule crosslinkers. In biological tissue, crosslinks can be induced as disulfide bonds between collagen fibrils. Compromised collagen in the cornea, a condition known as keratoconus, can be treated with clinical crosslinking.
s can be studied by the clever use of crosslinking agents. For example, protein A and protein B may be very close to each other in a cell, and a chemical crosslinker could be used to probe the protein–protein interaction between these two proteins by linking them together, disrupting the cell, and looking for the crosslinked proteins.
A variety of crosslinkers are used to analyze subunit
structure of protein
s, protein interactions and various parameters of protein function by using differing crosslinkers often with diverse spacer arm lengths. Subunit structure is deduced, since crosslinkers bind only surface amino residues in relatively close proximity in the native state
. Protein interactions are often too weak or transient to be easily detected, but, by crosslinking, the interactions can be captured and analyzed.
Examples of some common crosslinkers are the imidoester crosslinker dimethyl suberimidate, the N-Hydroxysuccinimide
-ester crosslinker BS3
and formaldehyde
. Each of these crosslinkers induces nucleophilic attack of the amino group of lysine
and subsequent covalent bonding via the crosslinker. The zero-length carbodiimide
crosslinker EDC functions by converting carboxyls into amine-reactive isourea intermediates that bind to lysine residues or other available primary amines. SMCC or its water-soluble analog, Sulfo-SMCC, is commonly used to prepare antibody-hapten conjugates for antibody development.
In-vivo crosslinking of protein complexes using photo-reactive amino acid analog
s was introduced in 2005 by researchers from the Max Planck Institute
In this method, cells are grown with photoreactive diazirine
analogs to leucine
and methionine
, which are incorporated into proteins. Upon exposure to ultraviolet light, the diazirines are activated and bind to interacting proteins that are within a few ångström
s of the photo-reactive amino acid analog (UV cross-linking).
gels for gel electrophoresis
. Synthetic rubber used for tire
s is made by crosslinking rubber through the process of vulcanization
. Also most rubber articles are cross-linked to make them more elastic. Hard-shell kayaks are also often manufactured with crosslinked polymers.
Alkyd enamels, the dominant type of commercial oil-based paint, cure by oxidative crosslinking after exposure to air.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
chain to another. They can be covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s or ionic bond
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
s. "Polymer chains" can refer to synthetic polymers or natural polymers (such as protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s). When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of cross-links to promote a difference in the polymers' physical properties. When "crosslinking" is used in the biological milieu, it refers to the use of a probe to link proteins together to check for protein–protein interactions, as well as other creative cross-linking methodologies.
Cross-linking is used in both synthetic polymer chemistry and in the biological sciences. Although the term is used to refer to the "linking of polymer chains" for both sciences, the extent of crosslinking and specificities of the crosslinking agents vary. Of course, with all science, there are overlaps, and the following delineations are a starting point to understanding the subtleties.
Cross-links in synthetic polymer chemistry
When polymer chains are linked together by cross-links, they lose some of their ability to move as individual polymer chains. For example, a liquid polymer (where the chains are freely flowing) can be turned into a "solid" or "gel" by cross-linking the chains together.In polymer chemistry, when a synthetic polymer is said to be "cross-linked", it usually means that the entire bulk of the polymer has been exposed to the cross-linking method. The resulting modification of mechanical properties depends strongly on the cross-link density. Low cross-link densities decrease the viscosities of polymer melts. Intermediate cross-link densities transform gummy polymers into materials that have elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
ic properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials.
Formation of cross-links
Cross-links can be formed by chemical reactionChemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s that are initiated by heat, pressure, change in pH, or radiation. For example, mixing of an unpolymerized or partially polymerized resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...
with specific chemicals called crosslinking reagents results in a chemical reaction that forms cross-links. Cross-linking can also be induced in materials that are normally thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
through exposure to a radiation source, such as electron beam exposure, gamma-radiation, or UV light. For example, electron beam processing
Electron beam processing
Electron beam processing or electron irradiation is a process which involves using electrons, usually of high energy, to treat an object for a variety of purposes. This may take place under elevated temperatures and nitrogen atmosphere...
is used to cross-link the C type of cross-linked polyethylene
PEX
Cross-linked polyethylene, commonly abbreviated PEX or XLPE, is a form of polyethylene with cross-links. It is formed into tubing, and is used predominantly in hydronic radiant heating systems, domestic water piping and insulation for high tension electrical cables...
. Other types of cross-linked polyethylene are made by addition of peroxide during extruding (type A) or by addition of a cross-linking agent (e.g. vinylsilane
Vinylsilane
Vinylsilane, also called vinyl silane or ethenyl silane, is an organic chemical with chemical formula or CH2=CH–SiH3. It is a derivative of silane....
) and a catalyst during extruding and then performing a post-extrusion curing.
The chemical process of vulcanization
Vulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
is a type of cross-linking and it changes the property of rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
to the hard, durable material we associate with car and bike tire
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
s.
This process is often called sulfur curing, and the term vulcanization
Vulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
comes from Vulcan
Vulcan (mythology)
Vulcan , aka Mulciber, is the god of beneficial and hindering fire, including the fire of volcanoes in ancient Roman religion and Roman Neopaganism. Vulcan is usually depicted with a thunderbolt. He is known as Sethlans in Etruscan mythology...
, the Roman
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...
god of fire. However, this is a slow process taking around 8 hours. A typical car tire is cured for 15 minutes at 150°C. However, the time can be reduced by the addition of accelerators such as 2-benzothiazolethiol or tetramethylthiuram disulfide. Both of these contain a sulfur atom in the molecule that initiates the reaction of the sulfur chains with the rubber. Accelerators increase the rate of cure by catalysing the addition of sulfur chains to the rubber molecules.
Cross-links are the characteristic property of thermosetting plastic
Thermosetting plastic
A thermosetting plastic, also known as a thermoset, is polymer material that irreversibly cures. The cure may be done through heat , through a chemical reaction , or irradiation such as electron beam processing.Thermoset materials are usually liquid or malleable prior to curing and designed to be...
materials. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Especially in the case of commercially used plastics, once a substance is cross-linked, the product is very hard or impossible to recycle. In some cases, though, if the cross-link bonds are sufficiently different, chemically, from the bonds forming the polymers, the process can be reversed. Permanent wave
Permanent wave
A permanent wave, commonly called a perm, involves the use of chemicals to break and reform the bonds of the hair. The hair is washed and wrapped on a perm rod and waving lotion is applied with a base. This solution creates a chemical reaction that softens the inner structure of the hair by...
solutions, for example, break and re-form naturally occurring cross-links (disulfide bond
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
s) between protein chains in hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
.
Physical cross-links
Chemical covalent cross-links are stable mechanically and thermally, so once formed are difficult to break. Therefore, cross-linked products like car tireTire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
s cannot be recycled easily. A class of polymers known as thermoplastic elastomer
Thermoplastic elastomer
Thermoplastic elastomers , sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers which consist of materials with both thermoplastic and elastomeric properties...
s rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as snowmobile
Snowmobile
A snowmobile, also known in some places as a snowmachine, or sled,is a land vehicle for winter travel on snow. Designed to be operated on snow and ice, they require no road or trail. Design variations enable some machines to operate in deep snow or forests; most are used on open terrain, including...
tracks, and catheter
Catheter
In medicine, a catheter is a tube that can be inserted into a body cavity, duct, or vessel. Catheters thereby allow drainage, administration of fluids or gases, or access by surgical instruments. The process of inserting a catheter is catheterization...
s for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains that act as cross-links are reversible, so can be reformed by heat. The stabilising domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters.
Oxidative cross-links
Many polymers undergo oxidative cross-linking, typically when exposed to atmospheric oxygen. In some cases this is undesirable and thus polymerization reactions may involve the use of an antioxidant to slow the formation of oxidative cross-links. In other cases, when formation of cross-links by oxidation is desirable, an oxidizer such as hydrogen peroxide may be used to speed up the process.The aforementioned process of applying a permanent wave to hair is one example of oxidative cross-linking. In that process the disulfide bonds are reduced, typically using a mercaptan such as ammonium thioglycolate. Following this, the hair is curled and then 'neutralized'. The neutralizer is typically a basic solution of hydrogen peroxide, which causes new disulfide bonds to form under conditions of oxidation, thus permanently fixing the hair into its new configuration.
Crosslinks in the biological sciences
In the biological sciences, crosslinking typically refers to a more specific reaction used to probe molecular interactions. For example, proteinProtein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s (a type of natural polymer) can be cross-linked together using small-molecule crosslinkers. In biological tissue, crosslinks can be induced as disulfide bonds between collagen fibrils. Compromised collagen in the cornea, a condition known as keratoconus, can be treated with clinical crosslinking.
Crosslinker use in protein study
The interactions or mere proximity of proteinProtein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s can be studied by the clever use of crosslinking agents. For example, protein A and protein B may be very close to each other in a cell, and a chemical crosslinker could be used to probe the protein–protein interaction between these two proteins by linking them together, disrupting the cell, and looking for the crosslinked proteins.
A variety of crosslinkers are used to analyze subunit
Protein subunit
In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...
structure of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, protein interactions and various parameters of protein function by using differing crosslinkers often with diverse spacer arm lengths. Subunit structure is deduced, since crosslinkers bind only surface amino residues in relatively close proximity in the native state
Native state
In biochemistry, the native state of a protein is its operative or functional form. While all protein molecules begin as simple unbranched chains of amino acids, once completed they assume highly specific three-dimensional shapes; that ultimate shape, known as tertiary structure, is the folded...
. Protein interactions are often too weak or transient to be easily detected, but, by crosslinking, the interactions can be captured and analyzed.
Examples of some common crosslinkers are the imidoester crosslinker dimethyl suberimidate, the N-Hydroxysuccinimide
N-Hydroxysuccinimide
N-Hydroxysuccinimide is a compound with a molecular weight of 115.09 and a melting point of 95 °C.As it is slightly acidic, it is an irritant to skin, eyes and mucous membranes....
-ester crosslinker BS3
BisSulfosuccinimidyl suberate
Bissulfosuccinimidyl suberate is a crosslinker used in biological research. It is a water-soluble version of disuccinimidyl suberate.- Crosslinkers :...
and formaldehyde
Formaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...
. Each of these crosslinkers induces nucleophilic attack of the amino group of lysine
Lysine
Lysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
and subsequent covalent bonding via the crosslinker. The zero-length carbodiimide
Carbodiimide
A carbodiimide or a methanediimine is a functional group consisting of the formula RN=C=NR. Carbodiimides hydrolyze to form ureas, which makes them uncommon in nature.-Carbodiimide formation:...
crosslinker EDC functions by converting carboxyls into amine-reactive isourea intermediates that bind to lysine residues or other available primary amines. SMCC or its water-soluble analog, Sulfo-SMCC, is commonly used to prepare antibody-hapten conjugates for antibody development.
In-vivo crosslinking of protein complexes using photo-reactive amino acid analog
Photo-reactive amino acid analog
Photo-reactive amino acid analogs for in-vivo crosslinking of protein complexes were introduced in 2005 by researchers from the Max Planck Institute. In this method, cells are grown with photoreactive diazirine analogs to leucine and methionine, which are incorporated into proteins...
s was introduced in 2005 by researchers from the Max Planck Institute
Max Planck Society
The Max Planck Society for the Advancement of Science is a formally independent non-governmental and non-profit association of German research institutes publicly funded by the federal and the 16 state governments of Germany....
In this method, cells are grown with photoreactive diazirine
Diazirine
Diazirines are a class of organic molecules consisting of a carbon bound to two nitrogen atoms, which are double-bonded to each other, forming a cyclopropene-like ring...
analogs to leucine
Leucine
Leucine is a branched-chain α-amino acid with the chemical formula HO2CCHCH2CH2. Leucine is classified as a hydrophobic amino acid due to its aliphatic isobutyl side chain. It is encoded by six codons and is a major component of the subunits in ferritin, astacin and other 'buffer' proteins...
and methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
, which are incorporated into proteins. Upon exposure to ultraviolet light, the diazirines are activated and bind to interacting proteins that are within a few ångström
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
s of the photo-reactive amino acid analog (UV cross-linking).
Uses for crosslinked polymers
Synthetically crosslinked polymers have many uses, including those in the biological sciences, such as applications in forming polyacrylamidePolyacrylamide
Polyacrylamide is a polymer formed from acrylamide subunits. It can be synthesized as a simple linear-chain structure or cross-linked, typically using N,N-methylenebisacrylamide. Polyacrylamide is not toxic...
gels for gel electrophoresis
Gel electrophoresis
Gel electrophoresis is a method used in clinical chemistry to separate proteins by charge and or size and in biochemistry and molecular biology to separate a mixed population of DNA and RNA fragments by length, to estimate the size of DNA and RNA fragments or to separate proteins by charge...
. Synthetic rubber used for tire
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
s is made by crosslinking rubber through the process of vulcanization
Vulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
. Also most rubber articles are cross-linked to make them more elastic. Hard-shell kayaks are also often manufactured with crosslinked polymers.
Alkyd enamels, the dominant type of commercial oil-based paint, cure by oxidative crosslinking after exposure to air.
See also
- BranchingBranching (chemistry)In polymer chemistry, branching occurs by the replacement of a substituent, e.g., a hydrogen atom, on a monomer subunit, by another covalently bonded chain of that polymer; or, in the case of a graft copolymer, by a chain of another type...
- Cross-linked polyethylene
- Phenol formaldehyde resinPhenol formaldehyde resinPhenol formaldehyde resins include synthetic thermosetting resins such as obtained by the reaction of phenols with formaldehyde. Sometimes the precursors include other aldehydes or other phenol. Phenolic resins are mainly used in the production of circuit boards...
and Phenolic resin - Application in enzyme catalysis: Cross-linked enzyme aggregateCross-linked enzyme aggregateIn biochemistry, a cross-linked enzyme aggregate is an immobilized enzyme prepared via crosslinking. They can be used as stereoselective industrial biocatalysts.-Background:Enzymes are proteins that catalyze chemical reactions...
- Crosslinking of DNACrosslinking of DNACrosslinks in DNA occur when various exogenous or endogenous agents react with two different positions in the DNA. This can either occur in the same strand or in the opposite strands of the DNA . Crosslinks also occur between DNA and protein...
- Fixation (histology)Fixation (histology)In the fields of histology, pathology, and cell biology, fixation is a chemical process by which biological tissues are preserved from decay, thereby preventing autolysis or putrefaction...