A study on the binding of two water-soluble tetrapyridinoporphyrazinato copper(II) complexes to DNA

Interaction of N,N0,N00,N000-tetramethyltetra-2,3-pyridinoporphyrazinatocopper(II), ([Cu(2,3-tmtppa)]4C) and N,N0,N00,N000-tetramethyltetra- 3,4-pyridinoporphyrazinatocopper(II), ([Cu(3,4-tmtppa)]4C) with calf thymus DNA was studied in 1 mM phosphate buffer and low ionic strength (5 mM NaCl) at various temperatures by UV-visible and circular dichroism (CD) spectroscopies and viscometric method. The binding constants were determined from the changes in the visible part of porphyrazine complexes spectra using SQUAD software. The values of K have been obtained (7.9G0.4)!104 and (2.2G0.1)!105 MK1 for [Cu(2,3-tmtppa)]4C and [Cu(3,4-tmtppa)]4C, respectively at 27 8C. The higher affinity of 3,4-isomer of Cu complex towards DNA with respect to the 2,3-isomer was attributed to favorable external positioning of the cationic charges in former, which enables superior interaction with the DNA duplex. The thermodynamic parameters (DG8, DH8, DS8) were calculated by van’t Hoff equation. The enthalpy and entropy changes were determined, C34.2G3.6 kJ molK1 and C207.8G12.70 J molK1 KK1 for [Cu(2,3-tmtppa)]4C and C49.7G2.1 kJ molK1 and C267.8G7.9 J molK1 KK1 for [Cu(3,4-tmtppa)]4C. The existence of extensive hypochromicity, large red shift and negative CD in the visible part of [Cu(3,4-tmtppa)]4C spectra suggested an intercalation binding mode. Analysis of the moderate hypochromicity, red shift and bisignate CD in the Q-band absorption region of [Cu(2,3- tmtppa)]4C spectra possibly led us to the coexistence of intercalation and outside binding modes. The influence of the ionic strength on the binding parameters and binding modes was investigated. The results show that increase in ionic strength causes the decrease in the binding constants. It was also concluded that increase in ionic strength affects the binding characteristics of positively charged complexes with DNA. The increase in DNA viscosity in the presence of Cu–tmtppa complexes is attributed to the lengthening of DNA helix due to the intercalation. This result is consistent with conclusions obtained from the spectroscopic techniques. q 2005 Elsevier B.V. All rights reserved.