Pteridine Cleavage Facilitates DNA Photocleavage by Ru(II) Polypyridyl Compounds

The synthesis, characterization,
binding to calf thymus DNA, and
plasmid DNA photocleavage studies of two ruthenium­(II) pteridinylphenanthroline
complexes are reported where the new pteridinylphenantholine ligands
in these complexes are additions to a larger family designed to resemble
DNA bases. [Ru­(bpy)2(L-keto)]­(PF6)2 1 is synthesized from ligand substitution of Ru­(bpy)2Cl2 by 4-keto-pteridino­[6,7-f]­phenanthroline
(L-keto). Increasing the reaction temperature during synthesis of 1 causes a ring scission of the L-keto ligand within the pyrimidine
ring yielding a second Ru complex, [Ru­(bpy)2(L-aap)]­(PF6)2 2 where L-aap is 2-amino-3-amidopyrazino­[5,6-f]­phenanthroline. The ring cleavage reaction is accompanied
by the loss of one carbon in the pyrimidine ring. Complexes 1 and 2 are characterized by 1H NMR,
UV/visible absorption and FT-IR spectroscopies and by cyclic voltammetry,
and these results are presented in comparison to the previously reported
related complexes [Ru­(bpy)2(L-allox)]­(PF6)2, [Ru­(bpy)2(L-amino)]­(PF6)2, and [Ru­(bpy)2(dppz)]­(PF6)2. In
addition, 2 has been structurally characterized by X-ray
diffraction. Both 1 and 2 are good intercalators
of calf thymus DNA as determined by viscometry and binding constants
obtained from absorption titrations. Only the ring-cleaved complex 2 exhibits a high degree of pBR322 plasmid photocleavage in
contrast to the other pteridinyl-phenanthroline complexes, which exhibit
no plasmid DNA photocleavage. Complex 1, however, decomposes
in buffer forming the photocleaver 2, demonstrating that
sample age and reactivity can affect observed photocleavage. Complex 2 appears to photocleave DNA through a singlet oxygen mechanism.