A study on the self assembly of Fe(II) and dual binding of Ni(II) porphyrazines on CT-DNA

Abstract

Interaction of N , N ′, N ″, N ‴-tetramethyltetra-2,3-pyridinoporphyrazinatonickel(II), ([ Ni (2,3- tmtppa )]4+) and N , N ′, N ″, N ‴-tetramethyltetra-2,3-pyridinoporphyrazinatoiron(II), ([ Fe (2,3- tmtppa )]4+) with calf thymus DNA (CT-DNA) have been studied in 1mM phosphate buffer and low ionic strength (5 mM NaCl ) using several spectroscopic techniques: absorbance, fluorescence and circular dichroism. The 2:1 porphyrazine/DNA mole ratio, existence of moderate hypochromicity, moderate and progressive red shift and conservative ICD in the visible part of [ Fe (2,3- tmtppa )]4+ spectra suggested an outside binding with self-stacking mode. Analysis of the dramatic hypochromicity, small blue shift and bisignate ICD in the Q-band absorption region of [ Ni (2,3- tmtppa )]4+ spectra and 1:1 porphyrazine/DNA mole ratio possibly led us to the dual binding mode of this complex, i.e. at the minor and major grooves of DNA. The influence of the ionic strength on the porphyrazine-DNA binding confirmed this suggestion. The fluorescence quenching of DNA-ethidium bromide complex by porphyrazines was investigated. The values of quenching constants ( K sv ) and the rate constants of the quenching ( K q ) were determined by Stern-Volmer equation. The values of K sv have been obtained (2.07 ± 0.11) × 106 and (0.48 ± 0.01) × 106 M−1 for [ Fe (2,3- tmtppa )]4+ and [ Ni (2,3- tmtppa )]4+, respectively, at 27°C. The higher affinity of [ Fe (2,3- tmtppa )]4+ for the quenching of EB-DNA fluorescence with respect to Ni complex was attributed to self-stacking of the former along DNA helix and favorable energy transfer between EB+ and cationic porphyrazine. ΔG25 (the difference between the free energy of the native ( N ) and the denatured (D) form of DNA at 25°C), ΔHm and ΔSm (difference between enthalpy and entropy at Tm) were extracted from thermal curves. The decline in the melting temperature of DNA following the addition of porphyrazine complexes confirms that the substantial mode of Ni and Fe interaction is not intercalation. The thermodynamic results imply that both Ni and Fe complexes have a slight destabilizing effect on the DNA. A comparison between [ Ni ( tmtppa )]4+ and [ Fe ( tmtppa )]4+ indicates that the latter is more effective than the former.