Figure 1.

Sulfurous acid (bisulfite)-mediated deamination and degradation of DNA. (A) Protonation of cytosine followed by nucleophilic attack by bisulfite activates the cytosine ring for hydrolytic deamination and β-elimination to produce uracil. (B) A similar process deaminates 5-methylcytosine at a much slower rate than that of cytosine. (C) Protonated bases created at low pH are removed from the DNA chain by glycosyl bond hydrolysis, leading to chain breaks through aldose conversion and β-elimination.