Interactions of copper(II) porphyrins with DNA

The interactions of three cationic water soluble copper(II) porphyrins, differing in peripheral substituents, with calf thymus DNA are described. Tetrakis(N-methylpyridinium-4-yl)porphinecopper(II) behaves as a simple intercalator under the conditions investigated, whereas tetrakis(4-N,N′,N″-trimethylanilinium)porphinecopper(II), binds externally, with some limited aggregation under high drug load conditions. In contrast, trans-bis(N-methylpyridinium-4-yl)diphenylporphinecopper(II) (t-CuPagg), like the free-base t-H₂Pagg from which it is derived, is capable of forming extended electronically coupled arrays while bound to the DNA template. These arrays have been investigated using a combination of extinction spectroscopy, circular dichroism, RLS and resonance Raman spectroscopy. They are found to contain 10⁵-10⁶ porphyrin units, arranged in long, narrow organized structures. The kinetics of assembly of t-CuPagg is reported on three DNAs: ct DNA, poly(dG-dC)₂ and poly(dA-dT)₂. A non-conventional autocatalytic model first proposed for t-H₂Pagg assembly formation is successful at fitting these data, permitting direct comparisons of kinetic parameters for the two porphyrins. It is found that the catalytic rate constant (k_c) is considerably smaller for t-CuPagg than for t-H₂Pagg under comparable conditions, and that the template rigidity fosters assembly formation. We also report the resonance Raman spectra of t-H₂Pagg/ct DNA and t-CuPagg/ct DNA complexes. Aggregation on the DNA template changes the intensity pattern of the porphyrin's resonance Raman spectra, with some low- and high-frequency bands becoming strongly enhanced upon aggregation. We conclude that aggregation-enhanced resonance Raman spectroscopy is a useful probe of aggregation in porphyrin-DNA complexes that also gives detailed information about structural changes that accompany the aggregation process.