Crystal structures of thecis-anti photodimer of uracil

Irradiation of uracil in ice produces a small quantity of thecis-anti cyclobutyl type photodimer. The molecule contains a pseudo two-fold axis and a cyclobutyl ring which is puckered, with dihedral angles near 150°. Torsion angles about the C(5)-C(6) and C(5)-C(6) bonds are 21°. Each uracil residue has seven atoms which are coplanar to within 0-07 Å, while C(6) and C(6) are 0·38 Å and 0·35 Å out of the planes of their respective residues. Each molecule participates in eight NH...O bonds which range from 2·83 to 2·97 Å. The material crystallizes in the triclinic space group P¯1 with unit cell parametersa = 8·594 Å,b = 7·478 Å,c = 6·915 Å, = 96·9 °, = 95·4 ° and = 85·5 °. The structure was solved by means of the symbolic addition procedure for phase determination.National Academy of Science-National Research Council Postdoctoral Resident Research Associate.