Structure of a thymine-thymine adduct formed by menadione photosensitization

The structure of one of the dithymine adducts formed by near-UV photosensitization of aqueous oxygenated thymine in the presence of 2-methyl-1,4-naphthoquinone is reported. The compound, 6' beta-hydroxy-5' beta-1-[5-methylpyrimidinyl-2,4-dione]-5'- 6'-dihydrothymine, has one thymine ring (ring I) linked through the 1-nitrogen atom to the C5' atom of a second thymine, which has been hydroxylated at C6' (ring II). Crystals are monoclinic, space group I2/c, and the structure was refined to R(obs) = 0.036 for 2072 unique reflections with intensities I greater than 2.33 sigma(I). Ring I is planar, whereas ring II is not. Ring I and the OH group on ring II are cis to ring II. The planar ring I and the OH group are attached, respectively, in an equatorial and axial manner to ring II (which is in the sofa conformation). The planar rings I of close-lying pairs of molecules stack parallel to each other. The structure is held together by a hydrogen-bonding system consisting of the water molecules, the NH groups, two of the C = O groups and the OH group. The chemical formula and relative configurations at C5' and C6' are established by this analysis.