# What Is a Disulfide Bond

A disulfide bond example is a type of bond where the bonding occurs through sulfide atoms attached to each other. In this article we are going to go into more detail about some important disulfide bond examples.

Disulfide bonds can be formed by the process of oxidation, reduction, or isomerization. Disulfide bonds are commonly found in proteins (secretory and extracellular domains). The main function of disulfide bonds is the stabilization of tertiary/quaternary proteins .

## Allicin

Allicin is found/obtained from garlic and is called organosulfur (compound). Allicin is not readily present in garlic, it is produced in response to damage to plant tissue. It involves the action of enzymes such as allinase on the constituent alliin (non-proteinogenic) amino acids. The structure of allicin is said to be deduced by Seebach, Stoll (in 1948).

The structure consists of six atoms of carbon, ten atoms of hydrogen, one atom of oxygen and two atoms of sulphur. Speaking of linkage, it consists of a sulfur and oxygen bond (in which sulfur has created a positive charge and oxygen is assumed to have a negative charge. There is also a disulfide bond (a sulfur-sulfur bond) in the structure). Appearance Allicin appears to be oily type, yellow (slight/faint) colored liquid. Its smell is similar to that of garlic.

The observed density is about 1.112 g cm. is -3 . It can be produced/synthesized by oxidation of polysulfides (analogs) or can be obtained from garlic extract (purified). Allicin has found many applications in pharmaceutical chemistry.

Speaking of its antibacterial properties, it can work against both gram-positive and negative bacteria and many more (even multidrug-resistant). Can work against protozoa (intestinal) and acts as antiparasitic. The reason behind its antimicrobial activity is the reaction with various enzymes (with thiol group) like thioredoxin reductase etc.

## disulfur dichloride (S2Cl2)

Its synonym is dimeric sulfenic chloride and it is an inorganic compound composed of two sulfur atoms and two chlorine atoms. It is a light/faint amber or sometimes yellowish-reddish liquid (oily type) in appearance. The smell is quite characteristic pungent (nausea may occur). The observed density is 1.688 g/cm3. It melts at a temperature of -80 °C and boils at a temperature of 137 °C.

Can dissolve in alcohols such as carbon tetrachloride and ethanol. This particular compound can be prepared by distillation of elemental sulfur (should be in excess). It can also be obtained by adding chlorine to a solution of sulfur (cooling should be carried out at a temperature of about 50-70 ° C).

2H_2S~+~S_2Cl_2 \longrightarrow H_2S_4~+~2HCl

Considering the important reactivity of disulfur dichloride, it is hydrolyzed (to sulfur dioxide) when treated with hydrogen sulfide. Diphenyl sulfide is obtained from the reaction between S2Cl3 and benzene with the presence of AlCl2 (aluminum chloride). It is involved in the preparation of thioindigo dye (used in polyester fabrics) an important component of the textile industry.

The reaction is carried out between disulfur dichloride which gives the 1,2,3-benzodithiazolium salt which on further reaction with sodium hydroxide and sodium bisulfite gives the precursor which is then involved in the preparation of thioindigo. Disulfur dichloride is used in vulcanizing (cooling) rubber, which is also used in industries to manufacture a variety of insecticides, sulfur dyes, etc.

## cystine

Its synonym is 3,3′-disulfenedylbis (2-aminopropanoic acid). It is observed that the molecular weight is 240g. The observed melting point is 247 °C. It is solid (white) in appearance and soluble in water (slightly). The main function of cysteine ​​is that it is a redox reaction site and another called linkage (mechanical) that allows the three-dimensional structure of the protein to be maintained.

Cysteine ​​is found in food such as meat (eggs and dairy products also), whole grains, skin, hair, etc. (A human hair contains about 9-14% cysteine). There are cysteine ​​supplements available today (as an anti-aging ingredient). But if cysteine ​​(excess) accumulates in the urine, it can form a calculus (hard mineral) which can be dangerous if it gets bigger.

## lipoic acid

Its synonym is thioctic acid or alpha lipoic acid. It is a compound of organosulfur derived from octanoic acid (caprylic acid). Its molar mass is found to be about 206 g/mol. It is composed of eight carbon atoms, fourteen hydrogen atoms, two oxygen atoms and two sulfur atoms. In appearance, it is in the form of yellow colored crystals (needle-like).

Its observed melting point is 62 °C. It is slightly soluble in water. Lipoic acid is said to be an antioxidant that can be found naturally in the body as well as in food like carrots, potatoes etc. An important factor about it is that it can break down carbohydrates and release energy.

## Diphenyl disulfide (Ph2S2 .) )

Its synonym is disulfanyl dibenzene. It is an organic disulfide compound. It is observed that the molar mass is about 218 g/mol. It is colorless (crystal) in appearance. Its recorded melting point is around 62 °C and boils at a temperature of 192 °C. And its density is found to be 1.353. It is found insoluble in water but soluble in carbon disulfide, benzene etc.’

The structure consists of two phenyl and two sulfur atoms. It can be prepared by the process of chemical thiophenol oxidation. The important reaction of diphenyl disulfide involves the reaction between chlorine and diphenyl disulfide to form the compound phenylsulfenyl chloride. It is a very important compound for various organic synthesis reactions.

2PhSH~+~I_2~ \longrightarrow ~Ph_2S_2~+~2HI

## Hydrogen disulfide (H2S2 .) )

Its synonym is thiosulfenic acid. The structure consists of two hydrogen atoms and two sulfur atoms. It is observed that the molar mass is about 66.14 g/mol. It is a liquid (yellowish) in appearance and the density is said to be 1.334 gcm-3. Its melting point has been observed to be -89 °C and boils at a temperature of 70 °C.

It can be prepared by mixing/dissolving metals (alkalis) in water and then adding concentrated hydrochloric acid (at a temperature of -15 °C). Later, the pure form of the required product is obtained by fractional distillation of the yellow colored oil formed by mixing the above solutions. It is used to prepare sulfuric acid, dyes and other medicinal uses. Special care should be taken when handling hydrogen sulfide as its smell can cause allergic reactions such as tears.

## How do disulfide bonds break?

Disulfide bonds are found in various inorganic compounds such as disulfur dichloride, cysteine, DNA, etc.

Disulfide bonds can be broken by the addition of various substances or sometimes simply by heating proteins (we know that disulfide bonds are commonly present in proteins). Disulfide bond breaks are observed when reducing agents (such as beta-mercaptoethanol BME and dithiothreitol DTT) are added. If we heat a material containing disulfur bonds, it can break.

Consider the compound dibenzyl disulfide at a temperature of 200 °C or more, it causes its decomposition and gives sulfur and stilbene and many other components

## Are disulfide bonds weak?

Bond strength is governed by various factors but here we will focus more on temperature.

Disulfide bonds can be weak or strong depending on the temperature. It has been observed that when proteins containing disulfide bonds (as discussed above) are heated at high temperature (around 200 °C or more) they break down or we can say that they disintegrate. Huh.

But usually, disulfide bonds are quite strong, with a bond dissociation energy of -60 kcal/mol. The strength of the disulfide bridge/bond is responsible for the stability of the protein.

## Which chemical breaks disulfide bonds?

Till date many studies have been done related to this topic but here we will study specifically about reducing agents.

Reducing agents are commonly observed to break disulfide bonds (beta-mercaptoethanol BME and dithiothreitol DTT). If disulfide bonds between or within molecules are required to break dithiothreitol, buffer can be added to the solution. Also in an alkaline pH environment and additional disulfide reagent (but the thiol catalyst must be in quantity) can break the disulfide bond.

In addition, the sodium hydroxide relaxer can break the disulfide bond with respect to the proteins in the hair. This compound of sodium hydroxide relaxer is used for hair straightening.