Mechanism of Toxicity
The compound readily eliminates a chloride ion by intramolecular nucleophilic substitution to form a cyclic sulfonium ion. This very reactive intermediate tends to permanently alkylate the guanine nucleotide in DNA strands, which prevents cellular division and generally leads directly to programmed cell death, or, if cell death is not immediate, the damaged DNA may lead to the development of cancer. Sulfur mustard is not very soluble in water but is very soluble in fat, contributing to its rapid absorption into the skin.
In the wider sense, compounds with the structural element BCH2CH2X, where X is any leaving group and B is a Lewis base are known as mustards. Such compounds can form cyclic "onium" ions (sulfonium, ammoniums, etc.) that are good alkylating agents. Examples are bis(2-chloroethyl)ether, the (2-haloethyl)amines (nitrogen mustards), and sulfur sesquimustard, which has two α-chloroethyl thioether groups (ClH2C-CH2-S-) connected by an ethylene (-CH2CH2-) group. These compounds have a similar ability to alkylate DNA, but their physical properties, e.g. melting point, vary.
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