Model Compounds for Protein-Nucleic Acid Interactions. 5.la 5-S-Cysteinyluracil Monohydrate, a Photoaddition Product between an Amino Acid and Pyrimidine Base

The molecular and crystal structure of a pyrimidine-amino acid irradiation product, 5-S-cysteinyluridine, has been determined by x-ray crystallography. The cell dimensions are a = 14.59 (3) Å, b = 6.82 (1) Å, β = 95.1 (1)°, space group P2₁ with two formula units of C₇H₉N₃O₄S-H₂O per cell. The pyrimidine rings are linked together in ribbons by four hydrogen bonds. Seven other hydrogen bonds interconnect the other polar functions and indicate how the unusual conformation of the cysteinyl group is stabilized. The irradiation product in its observed conformation can be fit into a RNA helix and proposed structures of intermediates in the in vivo covalent coupling of cysteine to uracil are described.