Formation of diastereomers of 5,6-dihydrothymine-6-sulfonate by deamination of 5-methylcytosine with bisulfite.

The bisulflte-mediated deamination of 5-methylcytosine yielded a diastereomeric mixture of 5, 6-dihydrothymine-6-sulfonate. One of the diastereomers was identical with the thymine-bisulfite adduct formed by treatment of thymine with bisulfite, and therefore was assigned the structure in which the hydrogen at position 5 and the sulfonate at position 6 are trans. The rates of degradation of this adduct into thymine in aqueous solutions were determined, and it was found that the adduct was considerably stable at pH 6.9, the half life being 13 hr at 37°, whereas at pH 8.9 it degrades with a half life of 70 min. The other diastereomer, in which the 5-H and 6-SO3- are cis, was stable in mildly alkaline conditions, as might be expected from the difficulty in undergoing elimination. This isomer rapidly generated thymine at pH 13.8.