Binding of the cationic 5-coordinate Zn(II)-5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin to DNA and model polynucleotides: Ionic-strength dependent intercalation in [poly(dG-dC)]2

Abstract

The localization of the water-soluble cationic porphyrin ZnTMpyP(4) [Zn(II) derivative of 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin] in its complex with [poly(dG-dC)]₂ is studied as a function of the solution ionic strength μ (μ = 0.20–0.03). It is shown that the position of the Soret band maximum of ZnTMpyP(4) shifts from 436 to 446 nm on complexation, while its intensity markedly decreases (∼40%) when μ decreases from 0.20 to 0.03. This suggests that the porphyrin is mainly intercalated in [poly(dG-dC)]₂ at μ = 0.03. Shifts of resonance Raman marker lines under ZnTMpyP(4) complexation to [poly(dG-dC)]₂ at μ = 0.03 also support porphyrin intercalation. At last, a time-resolved transient absorption study of the porphyrin triplet state quenching by molecular oxygen shows that, at μ = 0.03, biexponential kinetics of triplet state quenching is observed with time constants ∼12 μs (40%) and ∼35 μs (60%), that is, much longer than that of the free ZnTMpyP(4) species (∼3 μs). Such a strong “shielding effect” of [poly(dG-dC)]₂, resulting in the ∼35-μs component, is characteristic for porphyrin intercalation. The other component at ∼12 μs likely corresponds to a less protected, “partially intercalated” species in the polynucleotide. It is assumed that a decrease of the ionic strength μ (in terms of Na⁺ concentration) decreases the neutralization of the negative charges of the polynucleotide phosphate groups and, therefore, allows cationic ZnTMpyP(4) molecules to come closer to the nucleotides, this favoring, in a second step, full (or partial) intercalation. Since ZnTMpyP(4) is known to be axially ligated by a H₂O molecule, intercalation undoubtedly must result in a loss of its axial ligand. As far as we know, this is the first observation of such an axial ligand release in metalloporphyrin, induced by intercalation in a DNA sequence. © 1999 John Wiley & Sons, Inc. Biospectroscopy 5: 302–312, 1999