Interactions of chloroaluminium-tetramethyl-tetrapyridino-porphyrazine++ + with DNA.

The interaction of chloroaluminium-tetramethyl-tetrapyridinoporphyrazine (ClAlTMPyPa) with DNA was investigated by absorption/emission and circular-dichroic spectroscopy. Formation of a DNA-porphyrazine complex was evidenced by quenching of the ClAlTMPyPa fluorescence, a hypochromic red shift of the absorption band in the visible part of the spectrum and induction of a characteristic CD band with high positive ellipticity. Scatchard analysis of the spectral changes in low ionic strength buffer suggested an association constant of 9.7 +/- 0.6 x 10(7) M-1. Quenching of the fluorescence of a DNA-ethidium complex by added ClAlTMPyPa was shown to result from direct DNA-dye interaction rather than the displacement of ethidium from DNA. Our data suggest that porphyrazine binds to the outside of the DNA helix, involving interactions which are not limited to electrostatic forces only. The computer assisted modelling of ClAlTMPyPa-DNA interactions showed that the energy-minimized intermolecular orientation involves face-on binding of the ligand, preferably on the minor DNA groove.