cross-link

Vulcanization is an example of cross-linking.

Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers (such as proteins). 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 use of a probe to link proteins together to check 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.

Crosslinks in synthetic polymer chemistry

When polymer chains are linked together by crosslinks, 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 crosslinking the chains together.

In polymer chemistry, when a synthetic polymer is said to be "crosslinked", it usually means that the entire bulk of the polymer has been exposed to the crosslinking method. The resulting modification of mechanical properties depends strongly on the crosslink density. Low crosslink densities raise the viscosities of polymer melts. Intermediate crosslink densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high crosslink densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials.

Formation of crosslinks

Cross-links can be formed by chemical reactions that are initiated by heat, pressure, 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 crosslinks. Cross-linking can also be induced in materials that are normally thermoplastic through exposure to a radiation source, such as electron beam exposure^{[citation needed]}, 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 (eg. 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 tyres. 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.

Crosslinks 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 bonds) between protein chains in hair.

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 tires cannot be recycled easily. A new class of polymers known as thermoplastic elastomers rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as snowmobile tracks, and catheters for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains which 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.

Crosslinks in the biological milieu

In the biological sciences, crosslinking typically refers to a more specific reaction used to probe molecular interactions. For example, proteins (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.^[1]

Crosslinker use in protein study

The interactions or mere proximity of proteins 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^[2] 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 proteins, protein interactions and various parameters of protein function by using differing crosslinkers often with diverse spacer arm lengths. Subunit structure is deduced since crosslinkers only bind 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 NHS-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 caboxyls into amine-reactive isourea intermediates that bind to lysine residues or other available primary amines. SMCC or its water soluble analog, Sulfo-SMCC, are commonly used to prepare antibody-hapten conjugates for antibody development.

In-vivo crosslinking of protein complexes using photo-reactive amino acid analogs was introduced in 2005 by researchers from the Max Planck Institute^[3] 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 angstroms of the photo-reactive amino acid analog (UV cross-linking).

Cross Linker

Cross-linking agents contain at least two reactive groups that are reactive towards numerous groups, including sulfhydryls and amines, and create chemical covalent bonds between two or more molecules. Functional groups that can be targeted with cross-linking agents are primary amines, carboxyls, sulfhydryls, carbohydrates and carboxylic acids. Protein molecules have many of these functional groups and therefore proteins and peptides can be readily conjugated using cross-linking agents. Cross-linking agents are used to study protein structure and function, to anchor proteins to solid supports, preparation of immunogens, immunotoxins, and other conjugated protein reagents.

Cross-linking agents can be divided into groups dependent on the number and similarity of the reactive groups:

• Homobifunctional have two reactive ends that are identical.

• Heterobifunctional have two different reactive ends.

Homobifunctional cross-linkers are used in one-step reactions while the heterobifunctional cross-linkers are used in two-step sequential reactions, where the least labile reactive end is reacted first. Homobifunctional cross-linking agents have the tendency to result in self-conjugation, polymerization, and intracellular cross-linking. On the other hand, heterobifunctional agents allow more controlled two step reactions, which minimizes undesirable intramolecular cross reaction and polymerization.

The most widely used heterobifunctional cross-linking agents are used to couple proteins through amine and sulfhydryl groups. The least stable amine reactive NHS-esters couple first and, after removal of uncoupled reagent, the coupling to the sulfhydryl group proceeds. The sulfhydryl reactive groups are generally maleimides, pyridyl disulfides and a-haloacetyls. Other cross-linkers include carbodiimides, which-link between carboxyl groups (-COOH) and primary amines (-NH2). There are heterobifunctional cross-linkers with one photoreactive end. Photoreactive groups are used when no specific groups are available to react with as photoreactive groups react non-specifically upon exposure to UV light.

It is often desirable to minimize the degree of structural shift due to cross-linking reactions, and more so if the protein molecule is biologically active. Therefore, cross-linking is performed under mild buffer and pH conditions. Depending on the application, the degree of conjugation is also important and an optimal cross-linker to protein ratio must be maintained. The number of target groups on the outer surface of a protein is also important. If the exposed target groups are readily available for conjugation; a lower cross-linker to protein ratio can be used.

Cross-linkers are available with different spacer arm lengths. A cross-linker with a longer space arm may be used where two target groups are further apart. The availability of cross-linkers with different spacer arms allows optimization of cross reaction efficiency. Cross-linkers with short space arms are suitable for intramolecular cross-linking. Cleavable cross-linkers are also available which extends the scope of protein analysis.

Selection of Protein Cross-linkers

The following features are taken into consideration when making selection of a cross-linker:

1. Reagent solubility
2. The nature of reactive groups
3. Homobifunctional or heterobifunctional
4. Photoreactive or thermoreactive groups
5. The length of spacer arm
6. Conjugated product cleavable or not
7. Potential for further labeling
8. Reaction condition needed for conjugation

For additional information, such as cross-linking applications, specific cross-linking reaction conditions and other important cross-linking information, download the Protein Crosslinker technical bulletin.

Double Do™ Cross-linkers

G-Biosciences offers a wide selection of cross-linking reagents as Double Do™ Cross-linkers. The Double Do™ Cross-linkers line of products include the chemical cross-linker, supporting accessories consisting of specific reaction buffers, complete reaction systems, selection guides and protocols.

Double Do™ Cross-linker Selection Guide & Ordering Information

To select a cross-linking reagent several factors need to be considered:

• Reactive Toward: Determines the target residues to be cross-linked, select a reagent that does not interfere with protein’s function.
• Membrane Permeability: For cell surface labeling, select non-membrane permeable reagents.
• Cleavable: For easy release of cross-linked proteins from solid supports or for further downstream applications.
• Reversible: An alternative to cleavable reagents are reversible reagents. For example, ANB-NOS is released by photolysis.
• Steric Hindrance: Bulky groups around the binding site may require reagents with longer spacer arms.

Uses for crosslinked polymers

Synthetically crosslinked polymers have many uses, including those in the biological sciences, such as applications in forming polyacrylamide gels for gel electrophoresis. Synthetic rubber used for tires 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 cosslinked polymers.

Alkyd enamels, the dominant type of commercial oil-based paint, cure by oxidative crosslinking after exposure to air.

See also

• branching
• Cross-linked polyethylene
• Phenol formaldehyde resin and Phenolic resin
• Application in enzyme catalysis: Cross-linked enzyme aggregate
• Crosslinking of DNA
• Fixation (histology)

External links

• Nexam Chemical Focus on unique proprietary crosslinking chemicals.

References