Crystal Structures of Dimers of Uracil and 6-Methyluracil

Abstract

THERE is at present great interest in the chemical and biological effects of ultraviolet irradiation of nucleic acids. The possible correlation between the stereochemistry of the photoproducts and the biological consequences requires a detailed knowledge of the structures of these photo products. In addition to the methods of nuclear magnetic resonance^1,2, mass spectra³, topochemistry^4–7 and chemical analysis^8–10, X-ray analysis is most conclusive. Consequently, crystal structure analyses have been performed on an adduct of thymine^11–13 and a trimer of thymine (unpublished) in addition to the cis-syn^14–16, cis-anti¹⁷ and trans-anti^18,19 cyclobutyl dimers of uracil and thymine derivatives. The analyses of various dimers show remarkable agreement in bond lengths and angles of the molecules even though the packing and hydrogen bonding, if any, are quite different for the various materials. The cis-syn and cis-anti forms all have a puckered four-membered ring with a dihedral angle ∼155° except for the cis-syn photoproduct from 1,1′-trimethylene bis-thymine where the trimethylene bridge seems to force the cyclobutyl ring into a planar configuration¹⁶. The two structure determinations of the trans-anti form showed that the molecules themselves have a centre of symmetry and consequently the four-membered rings must be planar.