In step 1, the lone pair of electrons on the unionized amino group conducts an intramolecular nucleophilic attack at the β-carbon of the mustard, displacing chloride anion and forming the highly electrophilic aziridinium ion intermediate, a quaternary amine. The carbon atoms of this strained cyclic structure are highly electrophilic due to the strong negative inductive effect of the positively charged nitrogen atom.
In step 2, a DNA nucleophile conducts an intermolecular nucleophilic attack, which breaks the aziridine ring and alkylates DNA. Although guanine is the preferred nucleic acid base involved in the alkylation reaction.
Steps 3 and 4 are involving the second arm of the mustard and a second molecule of DNA. Ultimately, two molecules