The structure of crystals containing a hydrogen-bonded complex of 1-methylthymine and 9-methyladenine

Abstract

Crystals of a 1:1 hydrogen-bonded complex between 1-methylthymine and 9-methyladenine can be grown from an aqueous solution containing equimolecular quantities of the two compounds. The crystals are monoclinic, with a = 8.304, b = 6.552, c = 12.837 A, and beta=106° 50'. The space group is P21/m, with two base-pair complexes in the unit cell. The structure was refined with three-dimensional data taken with copper radiation. The positional coordinates and anisotropic temperature factors of the heavy atoms were obtained by least-squares analyses. The hydrogen atoms, except those of two methyl groups, were located from a three-dimensional difference Fourier synthesis. The 1-methylthymine and 9-methyladenine molecules form a planar base pair lying in a mirror plane and are connected to each other by two nearly linear hydrogen bonds, from the NH 2 group of 9-methyladenine to O(9) of 1-methylthymine (2.846 A) and from N(3) of 1-methylthymine to N(7) of 9-methyladenine (2.924 A). This structure differs from the adenine-thymine pairing proposed by Watson & Crick, where N(3) of thymine is hydrogen bonded to N(i) of adenine. The distance between the methyl group at N(1) of 1-methylthymine and the one at N(9) of 9-methyladenine is 8.645 A, whereas this distance is 11.1 angstroem in the pairing proposed by Watson & Crick.