Synthesis, characterization and biomolecule-binding properties of novel tetra-platinum(ii)-thiopyridylporphyrins

The new complexes tetra-platinum(ii)-thiopyridylporphyrin 3 and tetra-platinum(ii)-thiopyridylporphyrinato Zn(ii) 4 were obtained by coordination of the peripheral thiopyridyl units of the free-base 5,10,15,20-tetrakis[2,3,5,6-tetrafluoro-4-(4-pyridylsulfanyl)phenyl]porphyrin 1 or their corresponding zinc complex 2, respectively, with four chloro(2,2′-bipyridine)platinum(ii) [Pt(bpy)Cl]⁺ units. Both compounds were characterized by several spectroscopic techniques demonstrating a particular behaviour in the emission spectra due to the absence or presence of zinc. The tetra-platinum(ii)-thiopyridylporphyrins exhibited an increase in the emission quantum yield when compared with the starting thiopyridylporphyrins 1 and 2. Spectroscopic studies of both platinum derivatives reveal their ability to interact unequivocally with DNA from calf thymus and DNA of low molecular weight from salmon sperms, and also with the most abundant protein in human blood plasma, human serum albumin (HSA). Herein, both tetra-platinum(ii)-thiopyridylporphyrins 3 and 4 exhibit electrostatic surface binding with the negative phosphate groups of DNA. Similar to cationic-anionic binding with DNA, tetra-platinum(ii)-thiopyridylporphyrinato zinc(ii) demonstrates a particular binding intercalation mode with DNA. Photophysical studies demonstrated that both porphyrins are photostable and able to generate singlet oxygen (¹O₂) after light irradiation. Exposure of pMT123 plasmid DNA to tetra-platinum(ii)-thiopyridylporphyrins and irradiation with light lead to single-strand breakage as determined by the conversion of the supercoiled form of the plasmid (form I) into the nicked circular form (form II). The tetra-platinum(ii)-thiopyridylporphyrinato Zn(ii) demonstrates a particular intercalation binding mode with DNA and an ability to cleave DNA after photo-excitation.