Pyrazinoporphyrazines with Externally Appended Pyridine Rings. 13. Structure, UV–Visible Spectral Features, and Noncovalent Interaction with DNA of a Positively Charged Binuclear (ZnII/PtII) Macrocycle with Multimodal Anticancer Potentialities

We investigated with spectroscopic techniques the noncovalent interaction of a bimetallic water-soluble (ZnII/PtII) porphyrazine hexacation, [(PtCl2)(CH3)6LZn]6+, and its octacationic analogue [(CH3)8LZn]8+, lacking the cis-platin-like functionality, with a 21-mer double strand (ds) 5′-d[GGG(TTAGGG)3]-3′/3′-d[CCC(AATCCC)3]-5′, as model for B-DNA. Both hexacation and octacation tend to aggregate in water. The structure as well as the ground and excited-state electronic properties of the ZnII/PtII hexacation [(PtCl2)(CH3)6LZn]6+ in water solution were investigated using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. TDDFT calculations of the lowest excited states of [(PtCl2)(CH3)6LZn]6+ in water provided an accurate description of the Q-band spectral region. In particular, the calculated optical spectra were in agreement with the experimental ones, obtained in the presence of micelles favoring complete disruption of the aggregates. The model for dsDNA binding that emerges from the analysis of UV–vis absorption and time-resolved fluorescence data shows the presence of complexes of 1 dsDNA molecule with 1, 2, and 4 macrocycles. Comparing the results for the hexacation [(PtCl2)(CH3)6LZn]6+ with those for the [(CH3)8LZn]8+octacation, we observed a higher degree of monomerization for the [(PtCl2)(CH3)6LZn]6+ derivative.