Sulfides

Figure 95.

The disulfide bond or disulfide bridge is important in proteins. The amino acid cysteine has a sulfhydryl group on it. This can make double bonds with other cysteine sulfhydryl groups to make a disulfide bridge that forms the three-dimensional structure of the protein. This is a redox reaction, in which the thiol is in the reduced state and the disulfide bond is in the oxidized state. The loss of the hydrogen from this reaction is why it is called “oxidized” even though no oxygen is involved.

In biochemistry, a complex coenzyme called glutathione is used to create and break down these proteins. It, too, has a sulfhydryl group on it and comes in a reduced “GSH” and an oxidized “GSSG” form. It comes into the reaction oxidized and is reduced to GSH in the redox reaction that oxidizes the cysteine SH group to create a double bond. Disulfide bridges are seen in proteins located outside the cell because glutathione reductase in the cell keeps GSSG in the reduced GSH state, driving the reaction toward the absence of disulfide bridges.

SULFIDES

Sulfur analogues of ethers, as you know, are referred to as sulfides or thioethers. These are similar in some ways but different in other ways than ethers. This is because of the acidity of thiols compared to alcohols and phenols. This means that thiolate conjugate bases more easily form to make nucleophiles in reactions between the thiolate and alkyl halides and tosylates. The basicity of ethers is a hundred times greater than sulfides, which means that the sulfur atom is more nucleophilic.

If the complex sulfide formula is referred to, it is not called a sulfide but the term “alkylthio” is used instead of “alkoxy”. This leads to more complex names like 3-