SYNTHETIC SPECTROSCOPIC MODELS RELATED TO COENZYMES AND BASE PAIRS .3. A 1,1'-TRIMETHYLENE-LINKED THYMINE PHOTODIMER OF CIS-SYN STRUCTURE

1,1′-Trimethylenebisthymine (1), Th-C3-Th, rapidly undergoes photoreaction when irradiated at 300 nm in dilute aqueous solution to form an intramolecular dimer. The shortness of the trimethylene bridge connecting the 1- and 1′-nitrogens of the thymine moieties precludes an anti structure and permits only cis-syn (2) or trans-syn (3) geometry. The conversion of the internal photodimer to a product having an additional o-xylylene bridge between the 3- and 3 ′-nitrogens (4) excluded the trans-syn possibility. X-Ray study reveals that the crystals of the internal photodimer of 1,1 ′-trimethylenebisthymine are monoclinic, space group P21/c, with a = 9.04 Å, b = 17.70 Å, c = 12.48 Å, and β = 138˚ 30′, and there are four molecules in the cell. The structure has been refined to an R factor of 0.046 on 1823 structure amplitudes collected on a diffractometer. The cis-syn stereochemistry (2) has been established for the photodimer. The virtual planarity of the cyclobutane ring results in almost complete overlap of the projections of the thymine rings in the dimer. About 10% of the molecules exist in the monomeric form after prolonged X irradiation. The complete structural analysis of crystalline [formula omitted] is of special interest because of its relationship, in cis-syn stereochemistry, with the major thymine photoproduct from native DNA. © 1969, American Chemical Society. All rights reserved.