A Disulfide bond example is a type of bond where the bonding is through sulfide atoms connected to each other. In this article we are going to have a closer approach towards some important disulfide bonds examples.
Disulfide bonds can be formed by the process of oxidation, reduction, or isomerization. Disulfide bonds are usually found in the proteins (secretory and extracellular domain). The main function of the disulfide bond is the stabilization of the tertiary/quaternary protein .
Below are some disulfide bonds examples :
Allicin
Allicin is found/obtained from garlic and it is said to be an organosulfur (compound). Allicin is not readily present in garlic it is produced in response to the damage to the tissue of the plant. This involves the action of the enzymes like allinase on the component Alliin (non-proteinogenic) amino acid.The structure of Allicin is said to be deduced by Seeback, stoll (in 1948).
The structure has six atoms of carbon, ten atoms of hydrogen, one atom of oxygen two atoms of sulfur. Talking about linkage it has one sulfur and oxygen bond (wherein sulfur has formed a positive charge and oxygen is seen to have a negative charge. The structure also has a disulfide bond (a sulfur-sulfur bond)). In appearance, Allicin is seen to be oily type, yellow (slight/faint) colored liquid. Its odor is the same as garlic.
Image credit : Wikipedia
The observed density is around 1.112 g cm-3. It can be produced/synthesized by carrying out the oxidation of polysulfides (analogues) or can be obtained from extract of garlic (purified). Allicin has got a lot of applications in the chemistry of medicine.
Talking about its antibacterial properties it can act against both gram-positive and negative bacteria and many more (even the multidrug-resistant). Can work against protozoan (intestinal) and acts as antiparasitic. The reason behind its antimicrobial activity is the reaction with various enzymes (with thiol group) such as thioredoxin reductase etc.
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Disulfur dichloride (S2Cl2)
Its synonym is dimeric sulfenic chloride and it is an inorganic compound made up of two sulfur atoms and two chlorine atoms. In appearance, it is light/faint amber or sometimes yellow-red colored liquid (oily type). The odor is quite characteristic pungent (can be nauseating). observed density is 1.688 g/cm3. It melts at a temperature of -80 degrees Celsius and boils at a temperature of 137 degrees Celsius.
Can dissolve in carbon tetrachloride and alcohol like ethanol. This particular compound can be prepared by carrying out the distillation of elemental sulfur ( should be in excess) . It can be also obtained by passing chlorine onto sulfur solution ( cooling should be done at around a temperature of 50-70 degrees Celsius).
Taking into account the important reaction of disulfur dichloride, on treating it with hydrogen sulfide it is hydrolyzed ( to sulfur dioxide). The reaction between S2Cl2 and benzene with the presence of AlCl3 ( Aluminium Chloride) yields diphenyl sulfide. It is involved in the preparation of thioindigo dye ( used in polyester fabric) an important component of the textile industry.
The reaction is carried out between disulfur dichloride which gives 1,2,3-benzodithiazolium salt which upon further reaction with sodium hydroxide and sodium bisulfite gives the precursor which is then involved in preparing thioindigo. Disulfur dichloride is used in vulcanizing(cold) rubber also used in industries for manufacturing various kinds of insecticides, dyes of sulfur etc.
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Cystine
It s synonym is 3,3’-disulfanediylbis ( 2-aminopropanoic acid ). It’s observed molecular weight is 240g. The observed melting point is 247 degrees Celsius. In appearance, it is solid (white) and soluble (slightly) in water. The main functions of cystine are it is a redox reaction site and the other one is said to be the linkage (mechanical ) which allows the three-dimensional structure of the protein to be retained.
Cystine is found in food like meat (also eggs and dairy products), whole grains, skin, hair, etc (there is around 9-14 % cystine in hair of human). Nowadays supplements of cystine are available ( as the anti-aging component). But if cystine (excess ) is deposited in the urine it can lead to the formation of calculus (hard mineral) which if grows bigger can pose to be dangerous.
Lipoic acid
It’s synonym is thioctic acid or alpha lipoic acid. It is a compound of organosulfur which is obtained from octanoic acid (caprylic acid ). Its molar mass is found to be around 206 g/mol. It is made up 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 degrees Celsius. It is slightly soluble in water. Lipoic acid is said to be an antioxidant which can be naturally found in body as well as food like carrot, potato etc. One important factor about it is that it can break down carbohydrates and release/make energy.
Diphenyl Disulfide (Ph2S2)
Its synonym is Disulfanyl dibenzene . It is an organic disulfide compound. It’s observed molar mass is around 218 g/mol. In appearance, it is colorless ( crystals). Its recorded melting point is around 62 degrees Celsius and boils at a temperature of 192 degrees Celsius. And its density is found to be 1.353. It is found to be insoluble in water but soluble in carbon disulfide, benzene, etc.
The structure contains two phenyls and two atoms of sulfur. It can be prepared by the process of oxidation of the chemical thiophenol. The important reaction of diphenyl disulfide include the reaction between chlorine and diphenyl disulfide to form the compound phenylsulfenyl chloride. It is a very important compound for various organic synthesis reactions.
Hydrogen Disulfide (H2S2)
It’s synonym is thiosulfenic acid. The structure has two hydrogen atoms and two atoms of sulfur. It’s observed molar mass is around 66.14 g/mol. In appearance, it is a liquid (yellow colored) and density is said to be 1.334 gcm-3. It’s observed melting point is-89 degrees Celsius and boils at a temperature of 70 degrees Celsius.
It can be prepared by mixing/dissolving metals (alkali) in water and then concentrated hydrochloric acid (at a temperature of-15 degrees Celsius) is added to it. Later the pure form of required product is obtained by carrying out fractional distillation of the yellow-colored oil formed after mixing the above solutions. It is used for preparing sulfuric acid, dyes and other pharmaceutical uses. While handling hydrogen sulfide special care should be taken as its odor can cause allergies like tears.
How disulfide bonds are broken?
The disulfide bonds are found in various inorganic compounds like disulfur dichloride, cystine, DNA, etc.
The disulfide bonds can be broken by addition of various substances or sometimes by simply heating the protein (we know that disulfide bonds are usually present in proteins). Upon adding reducing agents ( like beta-mercaptoethanol BME and dithiothreitol DTT ) the disulfide bonds are observed to break. If we heat the substance containing disulfur bonds it can break.
Consider the compound dibenzyl disulfide at a temperature of 200 degrees Celsius or more it causes its decomposition and gives sulfur and stilbene and also many other components
Are disulfide bonds weak?
The strength of the bond is governed by various factors but here we shall focus more on temperature.
Disulfide bonds can be weak or strong based upon temperature. It has been observed that when the protein (as discussed above) containing disulfide bonds are heated at higher temperature (around 200 degrees Celsius or more ) are broken or we can say they are decomposed.
But usually, the disulfide bonds are quite strong having a bond dissociation energy-60 kcal/mol. The strength of disulfide bridge/bond is responsible for stability of proteins.
What chemical breaks disulfide bonds?
Till date there are many studies conducted related to this topic but here we will study specifically about reducing agents.
Usually reducing agents are seen to break the disulfide bond ( beta-mercaptoethanol BME and dithiothreitol DTT). If disulfide bonds between or within molecules are needed to break dithiothreitol can be added to the buffer solution. Also in an alkaline pH atmosphere and excess disulfide reagent ( but thiol should be in catalytic amount) can cleave the disulfide bond.
Also, the sodium hydroxide relaxer can break the disulfide bond referring with respect to protein in hair. This compound of sodium hydroxide relaxer is used for straightening of hair.
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