Spectroscopic study on the binding of porphyrins to (G4T4G4)4 parallel G-quadruplex

The binding mode and stoichiometry of the cationic porphyrin TMPyP4 to G-quadruplex structure are still controversial to date, mainly due to the intricate polymorphism of G-rich sequences in the different conditions of solution. Here in the presence of the molecular crowding agent PEG, the binding interaction of TMPyP4 and another porphyrin derivative TPrPyP4 with four-stranded parallel (G4T4G4)4 G-quadruplex was studied systematically using circular dichroism, visible absorption titration, and steady-state and time-resolved fluorescence spectroscopies. The results show that each (G4T4G4)4 molecule is able to bind four TMPyP4 or TPrPyP4 molecules. Two types of independent and nonequivalent binding sites with the higher and lower binding affinity are confirmed, and the stronger and weaker binding constants are 2.74 × 108 and 8.21 × 105 M− 1 for (G4T4G4)4-TMPyP4, 2.05 × 108 and 1.05 × 106 M− 1 for (G4T4G4)4-TPrPyP4, respectively. The two porphyrin molecules stack on the two ends of G-quadruplex with the higher binding affinity, another two porphyrins bind weakly to the two external grooves.