X-ray crystallographic identification of bisulfite-uracil adduct as sodium 5,6-dihydrouracil 6-sulfonate.

DNA methylation at position 5 of cytosine residues plays an important role in the gene function control. The analytical method for determining the sites of 5- methylcytosine residues utilizes bisulfite treatment of genomes. Cytosines in DNA are converted into uracils by this treatment, while 5-methylcytosines remain unaltered. The bisulfite treatment followed by amplification by polymerase chain reaction and by sequencing the resulting DNA allows determination of the 5-methylcytosine sites in the original. In this chemical modification, key intermediates are those formed by addition of bisulfite across the 5,6-double bond of pyrimidine ring. Their structures were proposed in 1970 as 5,6-dihydropyrimidine 6-sulfonates, but not its 6-sulfurous acid ester, on the basis of spectral data. X-ray analysis has now been performed for a single crystal of sodium bisulfite-uracil adduct and the results showed its structure as sodium 5,6-dihydrouracil 6-sulfonate monohydrate, thus providing definite evidence for the C(6)-sulfonate structure.