Fluorescence and phosphorescence spectra of Au(III), Pt(II) and Pd(II) porphyrins with DNA at room temperature

The fluorescence and phosphorescence studies on the interactions of Au(III), Pt(II) and Pd(II) derivatives of tetrakis(1-methylpyridinium-4yl)porphyrin (H2(TMPyP)4+) with pBluescript II plasmid DNA have been conducted at 0.1 M NaCl, pH 7.5 and 25 °C. The free base porphyrin (H2(TMPyP)4+), Au(III)(TMPyP)5+ and Pt(II)(TMPyP)4+ emitted fluorescence. On the other hand, the Pd(II)(TMPyP)4+ emitted both fluorescence and phosphorescence at room temperature. Upon the addition of excess amounts of DNA the broad fluorescence spectra of H2(TMPyP)4+ and Au(III)(TMPyP)5+ changed to two clear peaks centred at 650 and 710 nm for H2(TMPyP)4+, and 659.2 and 719.6 nm for Au(III)(TMPyP)5+, respectively. However, the fluorescence intensity of the Pt(II)(TMPyP)4+ increased with addition of high amounts of DNA, giving a peak at 660 nm with a shoulder. Interestingly for Pd(II)(TMPyP)4+, it was observed that in the presence of increasing amounts of DNA, the phosphorescence intensity observed at 700 nm increased. In order to confirm the chemical species of the porphyrins (that is monomer or dimer) the fluorescence and phosphorescence spectra of these porphyrins were also measured at different concentrations of NaCl (0.001–0.1 M) and ethanol (0–40% v/v). Comparison of these current data to previous studies suggests that H2(TMPyP)4+ and Au(III)(TMPyP)5+ show aggregates in aqueous solution, Pt(II)(TMPyP)4+ partially aggregates, and they become converted to the monomer form by intercalation or outside binding to DNA. The weak phosphorescence intensity observed for Pd(II)(TMPyP)4+ that was increased by the addition of large amounts of DNA is due to shielding of the intercalated porphyrin by the DNA which prevents it from reacting with molecular oxygen dissolved in water. The UV–Vis and CD results indicate that, like H2(TMPyP)4+, Pt(II)(TMPyP)4+ and Pd(II)(TMPyP)4+ intercalate into the DNA bases while Au(III)(TMPyP)5+ interacts with DNA via the out-side binding mode but with partial intercalation at high concentrations.