Inhibition of OH Radical-induced Strand Break Formation of Poly(U) by Ru(bpy)2+3 or Ru(phen)2+3 Attached to the Polynucleotide

Reactions of OH radicals with poly(U) (polyuridylic acid) in the presence of Ru(bpy)32+ or Ru(phen)32+ in aqueous solutions were studied. OH radicals were produced by pulse radiolysis and their reactions with ruthenium complexes were measured spectrophotometrically under conditions were the complexes are attached to the polynucleotide. The OH radical adds to either the uracil moiety or the ruthenium complexes. The ratio of the radicals produced depends only on the ratio of their rate constants and the concentrations of poly(U) and ruthenium complexes. Similar results were obtained with uridine-5'-monosphosphate, where the ruthenium complexes are not attached to the nucleotide. Surprisingly, the yield of single-strand break formation from the OH adducts of uracil in poly(U) is much smaller than that expected on the basis of the yield measured in the absence of ruthenium complexes. Possible reasons for this behaviour are discussed.