Дисульфидтік байланыс және оның химиялық қасиеттері

Functional group with the chemical structure R−S−S−R′

In chemistry, a disulfide (or disulphide in British English) is a compound containing a |R\s'S\sS'\sR′ functional group or the S22− anion. The linkage is also called an SS bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorganic chemistry, the anion appears in a few rare minerals, but the functional group has tremendous importance in biochemistry. Disulfide bridges formed between thiol groups in two cysteine residues are an important component of the secondary and tertiary structure of proteins. Compounds of the form |R\sS\sS\sH are usually called persulfides instead.

Disulfides have a C S S C dihedral angle approaching 90°. The S S bond length is 2.03 Å in diphenyl disulfide, similar to that in elemental sulfur. Two kinds of disulfides are recognized, symmetric and unsymmetric. Symmetrical disulfides are compounds of the formula |RSSR. Most disulfides encountered in organo sulfur chemistry are symmetrical disulfides. Unsymmetrical disulfides (also called heterodisulfides or mixed disulfides) are compounds of the formula |RSSR'. Unsymmetrical disulfide are less common in organic chemistry, but many disulfides in nature are unsymmetrical. Illustrative of a symmetric disulfide is cystine.

In thiol–disulfide exchange, a thiolate group |\sS displaces one sulfur atom in a disulfide bond |\sS\sS\s. The original disulfide bond is broken, and its other sulfur atom is released as a new thiolate, carrying away the negative charge. Meanwhile, a new disulfide bond forms between the attacking thiolate and the original sulfur atom. Thiolates, not thiols, attack disulfide bonds. Hence, thiol–disulfide exchange is inhibited at low pH (typically, below 8) where the protonated thiol form is favored relative to the deprotonated thiolate form. (The pKa of a typical thiol group is roughly 8.3, but can vary due to its environment.) Thiol–disulfide exchange is the principal reaction by which disulfide bonds are formed and rearranged in a protein. The rearrangement of disulfide bonds within a protein generally occurs via intra protein thiol–disulfide exchange reactions; a thiolate group of a cysteine residue attacks one of the protein's own disulfide bonds. This process of disulfide rearrangement (known as disulfide shuffling) does not change the number of disulfide bonds within a protein, merely their location (i. e., which cysteines are bonded). Disulfide reshuffling is generally much faster than oxidation/reduction reactions, which change the number of disulfide bonds within a protein. The oxidation and reduction of protein disulfide bonds in vitro also generally occurs via thiol–disulfide exchange reactions. Typically, the thiolate of a redox reagent such as glutathione, dithiothreitol attacks the disulfide bond on a protein forming a mixed disulfide bond between the protein and the reagent. This mixed disulfide bond when attacked by another thiolate from the reagent, leaves the cysteine oxidized. In effect, the disulfide bond is transferred from the protein to the reagent in two steps, both thiol–disulfide exchange reactions. The in vivo oxidation and reduction of protein disulfide bonds by thiol–disulfide exchange is facilitated by a protein called thioredoxin. This small protein, essential in all known organisms, contains two cysteine amino acid residues in a vicinal arrangement (i. e., next to each other), which allows it to form an internal disulfide bond, or disulfide bonds with other proteins. As such, it can be used as a repository of reduced or oxidized disulfide bond moieties.

Disulfide bonds play an important protective role for bacteria as a reversible switch that turns a protein on or off when bacterial cells are exposed to oxidation reactions. Hydrogen peroxide (H2O2) in particular could severely damage DNA and kill the bacterium at low concentrations if not for the protective action of the SS bond. Archaea typically have fewer disulfides than higher organisms.

In eukaryotic cells, in general, stable disulfide bonds are formed in the lumen of the RER (rough endoplasmic reticulum) and the mitochondrial intermembrane space but not in the cytosol. This is due to the more oxidizing environment of the aforementioned compartments and more reducing environment of the cytosol (see glutathione). Thus disulfide bonds are mostly found in secretory proteins, lysosomal proteins, and the exoplasmic domains of membrane proteins. There are notable exceptions to this rule. For example, many nuclear and cytosolic proteins can become disulfide crosslinked during necrotic cell death. Similarly, a number of cytosolic proteins which have cysteine residues in proximity to each other that function as oxidation sensors or redox catalysts; when the reductive potential of the cell fails, they oxidize and trigger cellular response mechanisms. The virus Vaccinia also produces cytosolic proteins and peptides that have many disulfide bonds; although the reason for this is unknown presumably they have protective effects against intracellular proteolysis machinery. Disulfide bonds are also formed within and between protamines in the sperm chromatin of many mammalian species.

As disulfide bonds can be reversibly reduced and re oxidized, the redox state of these bonds has evolved into a signaling element. In chloroplasts, for example, the enzymatic reduction of disulfide bonds has been linked to the control of numerous metabolic pathways as well as gene expression. The reductive signaling activity has been shown, thus far, to be carried by the ferredoxin thioredoxin system, channeling electrons from the light reactions of photosystem I to catalytically reduce disulfides in regulated proteins in a light dependent manner. In this way chloroplasts adjust the activity of key processes such as the Calvin–Benson cycle, starch degradation, ATP production and gene expression according to light intensity. Additionally, It has been reported that disulfides plays a significant role on redox state regulation of Two component systems (TCSs), which could be found in certain bacteria including photogenic strain. A unique intramolecular cysteine disulfide bonds in the ATP binding domain of SrrAB TCs found in Staphylococcus aureus is a good example of disulfides in regulatory proteins, which the redox state of SrrB molecule is controlled by cysteine disulfide bonds, leading to the modification of SrrA activity including gene regulation.

Over 90% of the dry weight of hair comprises proteins called keratins, which have a high disulfide content, from the amino acid cysteine. The robustness conferred in part by disulfide linkages is illustrated by the recovery of virtually intact hair from ancient Egyptian tombs. Feathers have similar keratins and are extremely resistant to protein digestive enzymes. The stiffness of hair and feather is determined by the disulfide content. Manipulating disulfide bonds in hair is the basis for the permanent wave in hairstyling. Reagents that affect the making and breaking of S−S bonds are key, e. g., ammonium thioglycolate. The high disulfide content of feathers dictates the high sulfur content of bird eggs. The high sulfur content of hair and feathers contributes to the disagreeable odor that results when they are burned.

Cystinosis is a condition where cystine precipitates as a solid in various organs. This accumulation interferes with bodily function and can be fatal. This disorder can be resolved by treatment with cysteamine. Cysteamine acts to solubilize the cystine by (1) forming the mixed disulfide cysteine cysteamine, which is more soluble and exportable, and (2) reducing cystine to cysteine.

Thiosulfoxides are orthogonally isomeric with disulfides, having the second sulfur branching from the first and not partaking in a continuous chain, i. e. >S=S rather than −S−S−. Disulfide bonds are analogous but more common than related peroxide, thioselenide, and diselenide bonds. Intermediate compounds of these also exist, for example thioperoxides (also known as oxasulfides) such as hydrogen thioperoxide, have the formula R1OSR2 (equivalently R2SOR1). These are isomeric to sulfoxides in a similar manner to the above; i. e. >S=O rather than −S−O−. Thiuram disulfides, with the formula (R2NCSS)2, are disulfides but they behave distinctly because of the thiocarbonyl group. Compounds with three sulfur atoms, such as CH3S−S−SCH3, are called trisulfides, or trisulfide bonds.

Disulfide is also used to refer to compounds that contain two sulfide (S2−) centers. The compound carbon disulfide, CS2 is described with the structural formula i. e. S=C=S. This molecule is not a disulfide in the sense that it lacks a S S bond. Similarly, molybdenum disulfide, MoS2, is not a disulfide in the sense again that its sulfur atoms are not linked.

The vulcanization of rubber results in crosslinking groups which consist of disulfide (and polysulfide) bonds; in analogy to the role of disulfides in proteins, the S−S linkages in rubber strongly affect the stability and rheology of the material. Although the exact mechanism underlying the vulcanization process is not entirely understood (as multiple reaction pathways are present but the predominant one is unknown), it has been extensively shown that the extent to which the process is allowed to proceed determines the physical properties of the resulting rubber namely, a greater degree of crosslinking corresponds to a stronger and more rigid material. The current conventional methods of rubber manufacturing are typically irreversible, as the unregulated reaction mechanisms can result in complex networks of sulfide linkages; as such, rubber is considered to be a thermoset material.

Due to their relatively weak bond dissociation energy (in comparison to C−C bonds and the like), disulfides have been employed in covalent adaptable network (CAN) systems in order to allow for dynamic breakage and reformation of crosslinks. By incorporating disulfide functional groups as crosslinks between polymer chains, materials can be produced which are stable at room temperature while also allowing for reversible crosslink dissociation upon application of elevated temperature. The mechanism behind this reaction can be attributed to the cleavage of disulfide linkages (RS−SR) into thiyl radicals (2 RS•) which can subsequently reassociate into new bonds, resulting in reprocessability and self healing characteristics for the bulk material. However, since the bond dissociation energy of the disulfide bond is still fairly high, it is typically necessary to augment the bond with adjacent chemistry that can stabilize the unpaired electron of the intermediate state. As such, studies usually employ aromatic disulfides or disulfidediamine (RNS−SNR) functional groups to encourage the dynamic dissociation of the S−S bond; these chemistries can result in the bond dissociation energy being reduced to half (or even less) of its prior magnitude. In practical terms, disulfide containing CANs can be used to impart recyclability to polymeric materials while still exhibiting physical properties similar to that of thermosets. Typically, recyclability is restricted to thermoplastic materials, as said materials consist of polymer chains which are not bonded to each other at the molecular level; as a result, they can be melted down and reformed (as the addition of thermal energy allows the chains to untangle, move past each other, and adopt new configurations), but this comes at the expense of their physical robustness. Meanwhile, conventional thermosets contain permanent crosslinks which bolster their strength, toughness, creep resistance, and the like (as the bonding between chains provides resistance to deformation at the macroscopic level), but due to the permanence of said crosslinks, these materials cannot be reprocessed akin to thermoplastics. However, due to the dynamic nature of the crosslinks in disulfide CANs, they can be designed to exhibit the best attributes of both of the aforementioned material types. Studies have shown that disulfide CANs can be reprocessed multiple times with negligible degradation in performance while also exhibiting creep resistance, glass transition, and dynamic modulus values comparable to those observed in similar conventional thermoset systems.