Spectroscopic study of binding of a cationic Pheophorbide-a to an antiparallel quadruplex Tel22

Binding of water-soluble cationic Pheophorbide- a derivative (CatPheo- a ) to Na + -stabilized antiparallel quadruplex formed by 22-mer oligonucleotide d[AG 3 (T 2 AG 3 ) 3 ], a fragment of human telomeric DNA (Tel22, PDB ID: 143D), has been examined using experimental techniques of absorption and polarized fluorescent spectroscopy as well as absorption melting. The binding affinity of CatPheo- a to Tel22 was studied in titration experiments registering the dependence of the dye fluorescence intensity and polarization degree on molar phosphate-to-dye ratio ( P/D ). CatPheo- a was found to bind effectively to the quadruplex, and two competitive binding modes were detected. The first one predominates at the dye excess and results in the fluorescence quenching, whereas the second one is preferential at the biopolymer excess and results in the enhancement of pheophorbide emission. The effect of CatPheo- a on thermodynamic parameters of Tel22 quadruplex unfolding was estimated using a two-state model. It was found that CatPheo- a destabilizes the quadruplex structure of Tel22 slightly decreasing its 4→1 transition midpoint temperature, gives destabilizing increment into Gibbs standard free energy and 2-fold decrease in the equilibrium quadruplex folding constant at 37°C. In ethanol CatPheo- a exhibits 15% higher efficiency of singlet oxygen generation as compared to the parent Pheo- a compound that makes it a promising photosensitizer for photodynamic therapy.