Networking (chemistry)

Polymers before and after crosslinking

In macromolecular chemistry , crosslinking describes reactions in which a large number of individual macromolecules are linked to form a three-dimensional network. The link can be achieved either directly when building the macromolecules or through reactions on existing polymers .

The crosslinking process changes the properties of the crosslinked substances. In general, an increase in the hardness , the toughness , the melting point and a decrease in the solubility are observed. The change increases with the degree of crosslinking , the proportion of crosslinked points based on the total amount of polymer. A cross-linker is characterized by at least two reactive groups. Crosslinkers with two identical reactive groups are called homobifunctional crosslinkers, whereas those with two different groups are called heterobifunctional crosslinkers.

Applications

Examples of direct crosslinking reactions are free-radical polymerizations of monomers with two vinyl functions or polycondensation or polyaddition using monomers with two or more functionalities (e.g. in the case of phenoplasts ). The crosslinking of already existing polymers is also referred to as crosslinking and can either take place via functionalities already present in the polymer by skillfully choosing the reaction conditions (self- crosslinking agents ), or by adding multifunctional, low molecular weight substances, the crosslinking agents . Depending on the degree of crosslinking, the crosslinking of polymers first creates elastomers and, with increasing crosslinking, thermosets as well .

• Examples of the use of crosslinking agents are
  • the addition of sulphurous substances during the vulcanization of rubber , and
  • the curing and gelation of epoxy resins with amines or of polyurethanes with blocked isocyanates .

• Areas of application are
  • in biochemistry, where in the polyacrylamide gel electrophoresis methylene bisacrylamide as homobifunctional crosslinking of the acrylamide is used
  • in affinity chromatography , pulldown assays (such as immunoprecipitation ) and size exclusion chromatography , using dextran or agarose cross-linked with epichlorohydrin .
  • Sucrose-epichlorohydrin copolymers of different molar mass distributions are used in the density gradient centrifugation of blood to isolate the leukocytes contained therein (under brand names such as Ficoll or Histopaque).
  • Dextran-epichlorohydrin copolymers are used to separate types of molecules in chromatography (under brand names such as Sephadex).

Proteins

Collagen fibers in leather are more mobile and more degradable without cross-linking

Schematic presentation of (intracheal) disulfide bridges within a peptide chain of a protein.

Schematic presentation of two (interchenar) disulfide bridges between peptide chains of two proteins.

Biochemical analysis

Similarly, protein-protein interactions and quaternary structures of proteins by cross-linking covalently be fixed so that a subsequent separation by size exclusion chromatography or even under the denaturing conditions of an SDS-PAGE with an identification by Western blot or mass spectrometry can be done. For this purpose, amines of lysine - or arginine side chains by adding glutaraldehyde , imidates (e.g. dimethyl adipimidate, dimethyl pimelimidate or dimethylsuberimidate.), Vinyl sulfone , Phenyldiisothiocyanat or N -Hydroxysuccinimid with carbodiimides , or other Sulfosuccinimid- succinimidyl esters crosslinked while the sulfhydryl groups of Cysteine side chains can be modified by disulfides or maleimide esters. For the coupling of amino to carboxy groups are coupling reagents of the peptide synthesis employed.

Photo- reactive molecules ( aryl azides , diazirines ) can also be used as one of the reactive groups of a crosslinker in proteins ( photoaffinity labeling ) in order to better control the time of crosslinking, since crosslinking is only triggered with UV radiation. Due to the lower selectivity of the radical crosslinkers, the functionality of the protein is often reduced by reaction of the radical crosslinker with important functions (such as an active center or a binding site ). For this reason, photoreactive crosslinkers are mostly used when no or only one amine or sulfhydryl group is available for selective crosslinking, or when subsequent functionality is insignificant.

There are also photoreactive diazirine-containing analogs of the amino acids leucine ( photo-leucine ), methionine and p- benzoyl-phenylalanine , which can already be incorporated into the protein during translation in vivo .

In a label transfer , a cross-linking between two neighboring molecules is used to transmit a signal between these molecules and thereby prove their proximity.

Immobilization

→ Main article : Immobilization (biotechnology)

Often the biological half-life of a protein is increased in the case of crosslinking, by increasing the thermal stability and reducing denaturation (due to the stabilization) as well as a decrease in proteolysis (due to the reduced accessibility for proteases ).

Histology and taxidermy

→ Main article : Fixation (preparation method)

The cross-linking of proteins is used in histology and taxidermy for the fixation of tissues ; the removal of the cross-linking during fixation is called antigen unmasking .

leather

→ Main article : Tanning

The cross-linking of proteins is used industrially in the course of tanning leather z. B. used in glutaraldehyde or fat tanning (synonym chamois tanning). The cross-linking of biogenic lipids with proteins is only used in the fat tanning of leather due to the characteristic smell.

Hair and perm

In nature there is a crosslinking of protein chains via the disulfide bridges of the amino acid cystine z. B. in keratin ( hair ). A permanent wave is chemically (a) the breaking of these disulfide bridges by a reducing agent and after the change in the macroscopic arrangement of the hair (b) the renewed crosslinking via disulfide bridges through an oxidation .

Nucleic acids

→ Main article : DNA networking

DNA-crosslinking substances (e.g. formaldehyde, glutaraldehyde) and crosslinking irradiation (e.g. with UV light ) are mutagens, since the DNA sequence is changed in the event of a faulty DNA repair , which leads to functional losses and - profits can arise that lead to the development of tumors .

Lipids

In addition, less selective free-radical crosslinking reactions are possible, as z. B. with polyunsaturated fats (especially in linseed oil , poppy seed oil , soybean oil , wood oil or fish oil ) naturally occur in the course of verranzens and can lead to resinification . These oils are sometimes incorrectly referred to as drying oils in paint production, but they are cross-linking. The crosslinking of biogenic lipids is due to the characteristic odor only when linseed oil varnish for impregnation of wood , as linseed oil paint used and the Fettgerbung of leather.

Properties of crosslinked plastics

Cross-linked plastics have a wide range of uses. These properties depend on the degree of crosslinking and can therefore be varied:

• good electrical insulation properties,
• high dimensional stability,
• wide temperature limits,
• high chemical resistance,
• a favorable fire behavior and
• Safety reserves for temperature spraying.

Individual evidence

1. ^ Fritz Röthemeyer, Franz Sommer: Kautschuktechnologie , Carl Hanser Verlag Munich Vienna, 2nd edition, 2006, pp. 304-310, ISBN 978-3-446-40480-9 .
2. ^ A. Tiselius, J. Porath, PA Albertsson: Separation and fractionation of macromolecules and particles . In: Science (1963) 141 : 13-20 (3575). PMID 13985156 .
3. ↑ J. Porath, R. Axén: Immobilization of enzymes to agar, agarose, and Sephadex supports . In: Methods Enzymol. (1976) Vol. 44, pp. 19-45. PMID 1021680 .
4. ↑ J. Porath, EB Lindner: Separation methods based on molecular sieving and ion exclusion . In: Nature (1961), Vol. 191, pp. 69-70. PMID 13737223 .
5. ↑ Hans-Dieter Jakubke, Hans Jeschkeit: amino acids, peptides, proteins , Verlag Chemie, Weinheim, p 101, 1982, ISBN 3-527-25892-2 .
6. ^ M. Suchanek, A. Radzikowska, C. Thiele: Photo-leucine and photo-methionine allow identification of protein-protein interactions in living cells . In: Nature Methods . 2, No. 4, 2005, pp. 261-268. doi : 10.1038 / nmeth752 . PMID 15782218 .
7. ↑ Christian Bonten: Plastics Technology Introduction and basics , Hanser Verlag, 2014, ISBN 978-3-446-44093-7 .