Intercalation Of Cu Research Articles

Interaction of water-soluble Cu(II), Ni(II), and Co(III) porphyrins with polynucleotides

The interaction of the Cu(II), Ni(II) and Co(III) complexes of the following six water-soluble cationic porphyrins with calf thymus DNA, poly(dG-dC) 2 and poly(dA-dT) 2 was studied by UV-visible and resonance Raman spectroscopy: tetrakis(2-N-) and (3-N-methylpyridyl) porphyrin ( 1, 2); monophenyltris(4-N-methylpyridyl)porphyrin ( 4); cis- and trans-diphenyl-bis (4-N-methylpyridyl)porphyrin ( 5, 6). The binding to nucleic acids was compared with that of tetrakis(4-N-methylpyridyl)porphyrin ( 3). If the N +-CH 3 group is moved from the para ( 3) to the meta position ( 2), binding of the free porphyrin as well as that of the metal complexes is only gradually modified; thus, the square-planar Cu- and Ni- 2 are intercalated at the G-C site whereas Co- 2 is groove-bound at A-T. Additionally, Ni- 2 is probably also intercalated at the A-T site. When the N +-CH 3 group is located at ortho position ( 1), the high rotation barrier of the 2-N-methylpyridyl group prevents intercalation of Cu- and Ni- 1, resulting in weak outside binding. At ionic strength μ = 0.2, there is no evidence of significant interaction of Co- 1 with any of the polynucleotides. When the charged N-methylpyridyl groups in 3 are subsequently replaced by phenyl groups ( 4, 5/6), the tendency of the Cu(II) and Ni(II) complexes to bind to the outside of the helix or to intercalate only partially increases at the expense of full intercalation. The coulombic attraction remains strong, no significant differences can be detected between 3, 4, 5, and 6. Ni- 4 binds to poly(dA-dT) 2 in the same complicated manner as Ni- 3. The outside-binding in Co- 4, - 5 and - 6 differs slightly from that in Co- 2 and Co- 3.